diff --git a/lib/std/Progress.zig b/lib/std/Progress.zig
index 24f58e1c685b..2213bd9b1166 100644
--- a/lib/std/Progress.zig
+++ b/lib/std/Progress.zig
@@ -410,9 +410,9 @@ pub fn start(options: Options) Node {
             }
 
             if (have_sigwinch) {
-                var act: posix.Sigaction = .{
+                const act: posix.Sigaction = .{
                     .handler = .{ .sigaction = handleSigWinch },
-                    .mask = posix.empty_sigset,
+                    .mask = posix.sigemptyset(),
                     .flags = (posix.SA.SIGINFO | posix.SA.RESTART),
                 };
                 posix.sigaction(posix.SIG.WINCH, &act, null);
diff --git a/lib/std/c.zig b/lib/std/c.zig
index 35d175f0b9aa..d5560c3f6fa2 100644
--- a/lib/std/c.zig
+++ b/lib/std/c.zig
@@ -3115,6 +3115,21 @@ pub const SYS = switch (native_os) {
     .linux => linux.SYS,
     else => void,
 };
+
+/// A common format for the Sigaction struct across a variety of Linux flavors.
+const common_linux_Sigaction = extern struct {
+    pub const handler_fn = *align(1) const fn (i32) callconv(.c) void;
+    pub const sigaction_fn = *const fn (i32, *const siginfo_t, ?*anyopaque) callconv(.c) void;
+
+    handler: extern union {
+        handler: ?handler_fn,
+        sigaction: ?sigaction_fn,
+    },
+    mask: sigset_t,
+    flags: c_uint,
+    restorer: ?*const fn () callconv(.c) void = null, // C library will fill this in
+};
+
 /// Renamed from `sigaction` to `Sigaction` to avoid conflict with function name.
 pub const Sigaction = switch (native_os) {
     .linux => switch (native_arch) {
@@ -3123,7 +3138,7 @@ pub const Sigaction = switch (native_os) {
         .mips64,
         .mips64el,
         => if (builtin.target.abi.isMusl())
-            linux.Sigaction
+            common_linux_Sigaction
         else if (builtin.target.ptrBitWidth() == 64) extern struct {
             pub const handler_fn = *align(1) const fn (i32) callconv(.c) void;
             pub const sigaction_fn = *const fn (i32, *const siginfo_t, ?*anyopaque) callconv(.c) void;
@@ -3160,8 +3175,8 @@ pub const Sigaction = switch (native_os) {
             flags: c_uint,
             restorer: ?*const fn () callconv(.c) void = null,
             mask: sigset_t,
-        } else linux.Sigaction,
-        else => linux.Sigaction,
+        } else common_linux_Sigaction,
+        else => common_linux_Sigaction,
     },
     .emscripten => emscripten.Sigaction,
     .netbsd, .macos, .ios, .tvos, .watchos, .visionos => extern struct {
@@ -4518,27 +4533,18 @@ pub const siginfo_t = switch (native_os) {
     else => void,
 };
 pub const sigset_t = switch (native_os) {
-    .linux => linux.sigset_t,
+    .linux => [1024 / @bitSizeOf(c_ulong)]c_ulong, // glibc and musl present a 1024-bit sigset_t, while kernel's is 128-bit or less.
     .emscripten => emscripten.sigset_t,
     // https://github.com/SerenityOS/serenity/blob/ec492a1a0819e6239ea44156825c4ee7234ca3db/Kernel/API/POSIX/signal.h#L19
-    .openbsd, .macos, .ios, .tvos, .watchos, .visionos, .serenity => u32,
+    .openbsd, .serenity => u32,
+    .macos, .ios, .tvos, .watchos, .visionos => darwin.sigset_t,
     .dragonfly, .netbsd, .solaris, .illumos, .freebsd => extern struct {
         __bits: [SIG.WORDS]u32,
     },
     .haiku => u64,
     else => u0,
 };
-pub const empty_sigset: sigset_t = switch (native_os) {
-    .linux => linux.empty_sigset,
-    .emscripten => emscripten.empty_sigset,
-    .dragonfly, .netbsd, .solaris, .illumos, .freebsd => .{ .__bits = [_]u32{0} ** SIG.WORDS },
-    else => 0,
-};
-pub const filled_sigset = switch (native_os) {
-    .linux => linux.filled_sigset,
-    .haiku => ~@as(sigset_t, 0),
-    else => 0,
-};
+
 pub const sigval = switch (native_os) {
     .linux => linux.sigval,
     // https://github.com/SerenityOS/serenity/blob/ec492a1a0819e6239ea44156825c4ee7234ca3db/Kernel/API/POSIX/signal.h#L22-L25
@@ -6665,7 +6671,7 @@ pub const timezone = switch (native_os) {
 };
 
 pub const ucontext_t = switch (native_os) {
-    .linux => linux.ucontext_t,
+    .linux => linux.ucontext_t, // std.os.linux.ucontext_t is currently glibc-compatible, but it should probably not be.
     .emscripten => emscripten.ucontext_t,
     .macos, .ios, .tvos, .watchos, .visionos => extern struct {
         onstack: c_int,
@@ -9596,6 +9602,7 @@ pub const NSIG = switch (native_os) {
     .windows => 23,
     .haiku => 65,
     .netbsd, .freebsd => 32,
+    .macos => darwin.NSIG,
     .solaris, .illumos => 75,
     // https://github.com/SerenityOS/serenity/blob/046c23f567a17758d762a33bdf04bacbfd088f9f/Kernel/API/POSIX/signal_numbers.h#L42
     .openbsd, .serenity => 33,
@@ -10345,6 +10352,11 @@ pub const sigfillset = switch (native_os) {
     else => private.sigfillset,
 };
 
+pub const sigaddset = private.sigaddset;
+pub const sigemptyset = private.sigemptyset;
+pub const sigdelset = private.sigdelset;
+pub const sigismember = private.sigismember;
+
 pub const sigprocmask = switch (native_os) {
     .netbsd => private.__sigprocmask14,
     else => private.sigprocmask,
@@ -11025,7 +11037,6 @@ pub const pthread_attr_set_qos_class_np = darwin.pthread_attr_set_qos_class_np;
 pub const pthread_get_qos_class_np = darwin.pthread_get_qos_class_np;
 pub const pthread_set_qos_class_self_np = darwin.pthread_set_qos_class_self_np;
 pub const ptrace = darwin.ptrace;
-pub const sigaddset = darwin.sigaddset;
 pub const task_for_pid = darwin.task_for_pid;
 pub const task_get_exception_ports = darwin.task_get_exception_ports;
 pub const task_info = darwin.task_info;
@@ -11148,7 +11159,11 @@ const private = struct {
     extern "c" fn sched_yield() c_int;
     extern "c" fn sendfile(out_fd: fd_t, in_fd: fd_t, offset: ?*off_t, count: usize) isize;
     extern "c" fn sigaction(sig: c_int, noalias act: ?*const Sigaction, noalias oact: ?*Sigaction) c_int;
-    extern "c" fn sigfillset(set: ?*sigset_t) void;
+    extern "c" fn sigdelset(set: ?*sigset_t, signo: c_int) c_int;
+    extern "c" fn sigaddset(set: ?*sigset_t, signo: c_int) c_int;
+    extern "c" fn sigfillset(set: ?*sigset_t) c_int;
+    extern "c" fn sigemptyset(set: ?*sigset_t) c_int;
+    extern "c" fn sigismember(set: ?*const sigset_t, signo: c_int) c_int;
     extern "c" fn sigprocmask(how: c_int, noalias set: ?*const sigset_t, noalias oset: ?*sigset_t) c_int;
     extern "c" fn socket(domain: c_uint, sock_type: c_uint, protocol: c_uint) c_int;
     extern "c" fn stat(noalias path: [*:0]const u8, noalias buf: *Stat) c_int;
diff --git a/lib/std/c/darwin.zig b/lib/std/c/darwin.zig
index ac083ec9f2d9..74d79842be25 100644
--- a/lib/std/c/darwin.zig
+++ b/lib/std/c/darwin.zig
@@ -10,7 +10,6 @@ const mode_t = std.c.mode_t;
 const off_t = std.c.off_t;
 const pid_t = std.c.pid_t;
 const pthread_attr_t = std.c.pthread_attr_t;
-const sigset_t = std.c.sigset_t;
 const timespec = std.c.timespec;
 const sf_hdtr = std.c.sf_hdtr;
 
@@ -840,9 +839,11 @@ pub extern "c" fn sendfile(
     flags: u32,
 ) c_int;
 
-pub fn sigaddset(set: *sigset_t, signo: u5) void {
-    set.* |= @as(u32, 1) << (signo - 1);
-}
+// https://github.com/apple/darwin-xnu/blob/2ff845c2e033bd0ff64b5b6aa6063a1f8f65aa32/bsd/sys/_types.h#L74
+pub const sigset_t = u32;
+
+// https://github.com/apple/darwin-xnu/blob/2ff845c2e033bd0ff64b5b6aa6063a1f8f65aa32/bsd/sys/signal.h#L76
+pub const NSIG = 32;
 
 pub const qos_class_t = enum(c_uint) {
     /// highest priority QOS class for critical tasks
diff --git a/lib/std/debug.zig b/lib/std/debug.zig
index d2b17ed30dbe..eb7cf793f2d7 100644
--- a/lib/std/debug.zig
+++ b/lib/std/debug.zig
@@ -1387,12 +1387,11 @@ pub fn attachSegfaultHandler() void {
         windows_segfault_handle = windows.kernel32.AddVectoredExceptionHandler(0, handleSegfaultWindows);
         return;
     }
-    var act = posix.Sigaction{
+    const act = posix.Sigaction{
         .handler = .{ .sigaction = handleSegfaultPosix },
-        .mask = posix.empty_sigset,
+        .mask = posix.sigemptyset(),
         .flags = (posix.SA.SIGINFO | posix.SA.RESTART | posix.SA.RESETHAND),
     };
-
     updateSegfaultHandler(&act);
 }
 
@@ -1404,9 +1403,9 @@ fn resetSegfaultHandler() void {
         }
         return;
     }
-    var act = posix.Sigaction{
+    const act = posix.Sigaction{
         .handler = .{ .handler = posix.SIG.DFL },
-        .mask = posix.empty_sigset,
+        .mask = posix.sigemptyset(),
         .flags = 0,
     };
     updateSegfaultHandler(&act);
diff --git a/lib/std/os/emscripten.zig b/lib/std/os/emscripten.zig
index ad7a3f65c1ce..0e07a8afb77f 100644
--- a/lib/std/os/emscripten.zig
+++ b/lib/std/os/emscripten.zig
@@ -560,7 +560,9 @@ pub const Sigaction = extern struct {
 };
 
 pub const sigset_t = [1024 / 32]u32;
-pub const empty_sigset = [_]u32{0} ** @typeInfo(sigset_t).array.len;
+pub fn sigemptyset() sigset_t {
+    return [_]u32{0} ** @typeInfo(sigset_t).array.len;
+}
 pub const siginfo_t = extern struct {
     signo: i32,
     errno: i32,
diff --git a/lib/std/os/linux.zig b/lib/std/os/linux.zig
index 00390c0800d7..83bf8e101346 100644
--- a/lib/std/os/linux.zig
+++ b/lib/std/os/linux.zig
@@ -1745,8 +1745,9 @@ pub fn sigprocmask(flags: u32, noalias set: ?*const sigset_t, noalias oldset: ?*
     return syscall4(.rt_sigprocmask, flags, @intFromPtr(set), @intFromPtr(oldset), NSIG / 8);
 }
 
-pub fn sigaction(sig: u6, noalias act: ?*const Sigaction, noalias oact: ?*Sigaction) usize {
-    assert(sig >= 1);
+pub fn sigaction(sig: u8, noalias act: ?*const Sigaction, noalias oact: ?*Sigaction) usize {
+    assert(sig > 0);
+    assert(sig < NSIG);
     assert(sig != SIG.KILL);
     assert(sig != SIG.STOP);
 
@@ -1755,14 +1756,15 @@ pub fn sigaction(sig: u6, noalias act: ?*const Sigaction, noalias oact: ?*Sigact
     const mask_size = @sizeOf(@TypeOf(ksa.mask));
 
     if (act) |new| {
+        // Zig needs to install our arch restorer function with any signal handler, so
+        // must copy the Sigaction struct
         const restorer_fn = if ((new.flags & SA.SIGINFO) != 0) &restore_rt else &restore;
         ksa = k_sigaction{
             .handler = new.handler.handler,
             .flags = new.flags | SA.RESTORER,
-            .mask = undefined,
+            .mask = new.mask,
             .restorer = @ptrCast(restorer_fn),
         };
-        @memcpy(@as([*]u8, @ptrCast(&ksa.mask))[0..mask_size], @as([*]const u8, @ptrCast(&new.mask)));
     }
 
     const ksa_arg = if (act != null) @intFromPtr(&ksa) else 0;
@@ -1777,8 +1779,8 @@ pub fn sigaction(sig: u6, noalias act: ?*const Sigaction, noalias oact: ?*Sigact
 
     if (oact) |old| {
         old.handler.handler = oldksa.handler;
-        old.flags = @as(c_uint, @truncate(oldksa.flags));
-        @memcpy(@as([*]u8, @ptrCast(&old.mask))[0..mask_size], @as([*]const u8, @ptrCast(&oldksa.mask)));
+        old.flags = oldksa.flags;
+        old.mask = oldksa.mask;
     }
 
     return 0;
@@ -1786,25 +1788,35 @@ pub fn sigaction(sig: u6, noalias act: ?*const Sigaction, noalias oact: ?*Sigact
 
 const usize_bits = @typeInfo(usize).int.bits;
 
-pub const sigset_t = [1024 / 32]u32;
+/// Defined as one greater than the largest defined signal number.
+pub const NSIG = if (is_mips) 128 else 65;
 
-const sigset_len = @typeInfo(sigset_t).array.len;
+/// Linux kernel's sigset_t.  This is logically 64-bit on most
+/// architectures, but 128-bit on MIPS.  Contrast with the 1024-bit
+/// sigset_t exported by the glibc and musl library ABIs.
+pub const sigset_t = [(NSIG - 1 + 7) / @bitSizeOf(SigsetElement)]SigsetElement;
 
-/// Empty set to initialize sigset_t instances from.
-pub const empty_sigset: sigset_t = [_]u32{0} ** sigset_len;
+const SigsetElement = c_ulong;
 
-pub const filled_sigset: sigset_t = [_]u32{0x7fff_ffff} ++ [_]u32{0} ** (sigset_len - 1);
+const sigset_len = @typeInfo(sigset_t).array.len;
 
-pub const all_mask: sigset_t = [_]u32{0xffff_ffff} ** sigset_len;
+/// Zig's version of sigemptyset.  Returns initialized sigset_t.
+pub fn sigemptyset() sigset_t {
+    return [_]SigsetElement{0} ** sigset_len;
+}
 
-fn sigset_bit_index(sig: usize) struct { word: usize, mask: u32 } {
+/// Zig's version of sigfillset.  Returns initalized sigset_t.
+pub fn sigfillset() sigset_t {
+    return [_]SigsetElement{~@as(SigsetElement, 0)} ** sigset_len;
+}
+
+fn sigset_bit_index(sig: usize) struct { word: usize, mask: SigsetElement } {
     assert(sig > 0);
     assert(sig < NSIG);
     const bit = sig - 1;
-    const shift = @as(u5, @truncate(bit % 32));
     return .{
-        .word = bit / 32,
-        .mask = @as(u32, 1) << shift,
+        .word = bit / @bitSizeOf(SigsetElement),
+        .mask = @as(SigsetElement, 1) << @truncate(bit % @bitSizeOf(SigsetElement)),
     };
 }
 
@@ -3487,6 +3499,7 @@ pub const SIG = if (is_mips) struct {
     pub const UNBLOCK = 2;
     pub const SETMASK = 3;
 
+    // https://github.com/torvalds/linux/blob/ca91b9500108d4cf083a635c2e11c884d5dd20ea/arch/mips/include/uapi/asm/signal.h#L25
     pub const HUP = 1;
     pub const INT = 2;
     pub const QUIT = 3;
@@ -3494,33 +3507,32 @@ pub const SIG = if (is_mips) struct {
     pub const TRAP = 5;
     pub const ABRT = 6;
     pub const IOT = ABRT;
-    pub const BUS = 7;
+    pub const EMT = 7;
     pub const FPE = 8;
     pub const KILL = 9;
-    pub const USR1 = 10;
+    pub const BUS = 10;
     pub const SEGV = 11;
-    pub const USR2 = 12;
+    pub const SYS = 12;
     pub const PIPE = 13;
     pub const ALRM = 14;
     pub const TERM = 15;
-    pub const STKFLT = 16;
-    pub const CHLD = 17;
-    pub const CONT = 18;
-    pub const STOP = 19;
-    pub const TSTP = 20;
-    pub const TTIN = 21;
-    pub const TTOU = 22;
-    pub const URG = 23;
-    pub const XCPU = 24;
-    pub const XFSZ = 25;
-    pub const VTALRM = 26;
-    pub const PROF = 27;
-    pub const WINCH = 28;
-    pub const IO = 29;
-    pub const POLL = 29;
-    pub const PWR = 30;
-    pub const SYS = 31;
-    pub const UNUSED = SIG.SYS;
+    pub const USR1 = 16;
+    pub const USR2 = 17;
+    pub const CHLD = 18;
+    pub const PWR = 19;
+    pub const WINCH = 20;
+    pub const URG = 21;
+    pub const IO = 22;
+    pub const POLL = IO;
+    pub const STOP = 23;
+    pub const TSTP = 24;
+    pub const CONT = 25;
+    pub const TTIN = 26;
+    pub const TTOU = 27;
+    pub const VTALRM = 28;
+    pub const PROF = 29;
+    pub const XCPU = 30;
+    pub const XFZ = 31;
 
     pub const ERR: ?Sigaction.handler_fn = @ptrFromInt(maxInt(usize));
     pub const DFL: ?Sigaction.handler_fn = @ptrFromInt(0);
@@ -5479,38 +5491,33 @@ pub const TFD = switch (native_arch) {
     },
 };
 
-/// NSIG is the total number of signals defined.
-/// As signal numbers are sequential, NSIG is one greater than the largest defined signal number.
-pub const NSIG = if (is_mips) 128 else 65;
-
 const k_sigaction_funcs = struct {
     const handler = ?*align(1) const fn (i32) callconv(.c) void;
     const restorer = *const fn () callconv(.c) void;
 };
 
+/// Kernel sigaction struct, as expected by the `rt_sigaction` syscall.  Includes restorer.
 pub const k_sigaction = switch (native_arch) {
-    .mips, .mipsel => extern struct {
-        flags: c_uint,
-        handler: k_sigaction_funcs.handler,
-        mask: [4]c_ulong,
-        restorer: k_sigaction_funcs.restorer,
-    },
-    .mips64, .mips64el => extern struct {
+    .mips, .mipsel, .mips64, .mips64el => extern struct {
         flags: c_uint,
         handler: k_sigaction_funcs.handler,
-        mask: [2]c_ulong,
+        mask: sigset_t,
         restorer: k_sigaction_funcs.restorer,
     },
     else => extern struct {
         handler: k_sigaction_funcs.handler,
         flags: c_ulong,
         restorer: k_sigaction_funcs.restorer,
-        mask: [2]c_uint,
+        mask: sigset_t,
     },
 };
 
+/// Kernel Sigaction wrapper for the actual ABI `k_sigaction`.  The Zig
+/// linux.zig wrapper library still does some pre-processing on
+/// sigaction() calls (to add the `restorer` field).
+///
 /// Renamed from `sigaction` to `Sigaction` to avoid conflict with the syscall.
-pub const Sigaction = extern struct {
+pub const Sigaction = struct {
     pub const handler_fn = *align(1) const fn (i32) callconv(.c) void;
     pub const sigaction_fn = *const fn (i32, *const siginfo_t, ?*anyopaque) callconv(.c) void;
 
@@ -5519,8 +5526,10 @@ pub const Sigaction = extern struct {
         sigaction: ?sigaction_fn,
     },
     mask: sigset_t,
-    flags: c_uint,
-    restorer: ?*const fn () callconv(.c) void = null,
+    flags: switch (native_arch) {
+        .mips, .mipsel, .mips64, .mips64el => c_uint,
+        else => c_ulong,
+    },
 };
 
 pub const SFD = struct {
diff --git a/lib/std/os/linux/aarch64.zig b/lib/std/os/linux/aarch64.zig
index 649e474310fc..ed40145e1bbb 100644
--- a/lib/std/os/linux/aarch64.zig
+++ b/lib/std/os/linux/aarch64.zig
@@ -289,7 +289,7 @@ pub const ucontext_t = extern struct {
     flags: usize,
     link: ?*ucontext_t,
     stack: stack_t,
-    sigmask: sigset_t,
+    sigmask: [1024 / @bitSizeOf(c_ulong)]c_ulong, // Currently a libc-compatible (1024-bit) sigmask
     mcontext: mcontext_t,
 };
 
diff --git a/lib/std/os/linux/arm.zig b/lib/std/os/linux/arm.zig
index 8f83bb60a1ba..8fea30a5da8b 100644
--- a/lib/std/os/linux/arm.zig
+++ b/lib/std/os/linux/arm.zig
@@ -337,7 +337,7 @@ pub const ucontext_t = extern struct {
     link: ?*ucontext_t,
     stack: stack_t,
     mcontext: mcontext_t,
-    sigmask: sigset_t,
+    sigmask: [1024 / @bitSizeOf(c_ulong)]c_ulong, // Currently a libc-compatible (1024-bit) sigmask
     regspace: [64]u64,
 };
 
diff --git a/lib/std/os/linux/loongarch64.zig b/lib/std/os/linux/loongarch64.zig
index 9ee8fe229c2a..50c4664ac821 100644
--- a/lib/std/os/linux/loongarch64.zig
+++ b/lib/std/os/linux/loongarch64.zig
@@ -264,8 +264,7 @@ pub const ucontext_t = extern struct {
     flags: c_ulong,
     link: ?*ucontext_t,
     stack: stack_t,
-    sigmask: sigset_t,
-    _pad: [1024 / 8 - @sizeOf(sigset_t)]u8,
+    sigmask: [1024 / @bitSizeOf(c_ulong)]c_ulong, // Currently a libc-compatible (1024-bit) sigmask
     mcontext: mcontext_t,
 };
 
diff --git a/lib/std/os/linux/powerpc.zig b/lib/std/os/linux/powerpc.zig
index 443a32769ab5..450cb27c7ec4 100644
--- a/lib/std/os/linux/powerpc.zig
+++ b/lib/std/os/linux/powerpc.zig
@@ -341,7 +341,7 @@ pub const ucontext_t = extern struct {
     stack: stack_t,
     pad: [7]i32,
     regs: *mcontext_t,
-    sigmask: sigset_t,
+    sigmask: [1024 / @bitSizeOf(c_ulong)]c_ulong, // Currently a libc-compatible (1024-bit) sigmask
     pad2: [3]i32,
     mcontext: mcontext_t,
 };
diff --git a/lib/std/os/linux/powerpc64.zig b/lib/std/os/linux/powerpc64.zig
index bb1032fb21ad..b0a14ff3cb6e 100644
--- a/lib/std/os/linux/powerpc64.zig
+++ b/lib/std/os/linux/powerpc64.zig
@@ -337,7 +337,7 @@ pub const ucontext_t = extern struct {
     flags: u32,
     link: ?*ucontext_t,
     stack: stack_t,
-    sigmask: sigset_t,
+    sigmask: [1024 / @bitSizeOf(c_ulong)]c_ulong, // Currently a libc-compatible (1024-bit) sigmask
     mcontext: mcontext_t,
 };
 
diff --git a/lib/std/os/linux/s390x.zig b/lib/std/os/linux/s390x.zig
index 70cf53f6fd08..614c33d076f5 100644
--- a/lib/std/os/linux/s390x.zig
+++ b/lib/std/os/linux/s390x.zig
@@ -273,7 +273,7 @@ pub const ucontext_t = extern struct {
     link: ?*ucontext_t,
     stack: stack_t,
     mcontext: mcontext_t,
-    sigmask: sigset_t,
+    sigmask: [1024 / @bitSizeOf(c_ulong)]c_ulong, // Currently a libc-compatible (1024-bit) sigmask
 };
 
 pub const mcontext_t = extern struct {
diff --git a/lib/std/os/linux/sparc64.zig b/lib/std/os/linux/sparc64.zig
index 594b11b7accc..34df73fcb160 100644
--- a/lib/std/os/linux/sparc64.zig
+++ b/lib/std/os/linux/sparc64.zig
@@ -454,7 +454,7 @@ pub const ucontext_t = extern struct {
     sigmask: u64,
     mcontext: mcontext_t,
     stack: stack_t,
-    sigset: sigset_t,
+    sigset: [1024 / @bitSizeOf(c_ulong)]c_ulong, // Currently a libc-compatible (1024-bit) sigmask
 };
 
 /// TODO
diff --git a/lib/std/os/linux/test.zig b/lib/std/os/linux/test.zig
index fa6f8223297b..7f5deff480d3 100644
--- a/lib/std/os/linux/test.zig
+++ b/lib/std/os/linux/test.zig
@@ -126,7 +126,9 @@ test "fadvise" {
 }
 
 test "sigset_t" {
-    var sigset = linux.empty_sigset;
+    std.debug.assert(@sizeOf(linux.sigset_t) == (linux.NSIG / 8));
+
+    var sigset = linux.sigemptyset();
 
     // See that none are set, then set each one, see that they're all set, then
     // remove them all, and then see that none are set.
@@ -138,7 +140,6 @@ test "sigset_t" {
     }
     for (1..linux.NSIG) |i| {
         try expectEqual(linux.sigismember(&sigset, @truncate(i)), true);
-        try expectEqual(linux.sigismember(&linux.empty_sigset, @truncate(i)), false);
     }
     for (1..linux.NSIG) |i| {
         linux.sigdelset(&sigset, @truncate(i));
@@ -147,22 +148,52 @@ test "sigset_t" {
         try expectEqual(linux.sigismember(&sigset, @truncate(i)), false);
     }
 
+    // Kernel sigset_t is either 2+ 32-bit values or 1+ 64-bit value(s).
+    const sigset_len = @typeInfo(linux.sigset_t).array.len;
+    const sigset_elemis64 = 64 == @bitSizeOf(@typeInfo(linux.sigset_t).array.child);
+
     linux.sigaddset(&sigset, 1);
     try expectEqual(sigset[0], 1);
-    try expectEqual(sigset[1], 0);
+    if (sigset_len > 1) {
+        try expectEqual(sigset[1], 0);
+    }
 
     linux.sigaddset(&sigset, 31);
     try expectEqual(sigset[0], 0x4000_0001);
-    try expectEqual(sigset[1], 0);
+    if (sigset_len > 1) {
+        try expectEqual(sigset[1], 0);
+    }
 
     linux.sigaddset(&sigset, 36);
-    try expectEqual(sigset[0], 0x4000_0001);
-    try expectEqual(sigset[1], 0x8);
+    if (sigset_elemis64) {
+        try expectEqual(sigset[0], 0x8_4000_0001);
+    } else {
+        try expectEqual(sigset[0], 0x4000_0001);
+        try expectEqual(sigset[1], 0x8);
+    }
 
     linux.sigaddset(&sigset, 64);
-    try expectEqual(sigset[0], 0x4000_0001);
-    try expectEqual(sigset[1], 0x8000_0008);
-    try expectEqual(sigset[2], 0);
+    if (sigset_elemis64) {
+        try expectEqual(sigset[0], 0x8000_0008_4000_0001);
+    } else {
+        try expectEqual(sigset[0], 0x4000_0001);
+        try expectEqual(sigset[1], 0x8000_0008);
+    }
+}
+
+test "sigfillset" {
+    // unlike the C library, all the signals are set in the kernel-level fillset
+    const sigset = linux.sigfillset();
+    for (1..linux.NSIG) |i| {
+        try expectEqual(linux.sigismember(&sigset, @truncate(i)), true);
+    }
+}
+
+test "sigemptyset" {
+    const sigset = linux.sigemptyset();
+    for (1..linux.NSIG) |i| {
+        try expectEqual(linux.sigismember(&sigset, @truncate(i)), false);
+    }
 }
 
 test "sysinfo" {
diff --git a/lib/std/os/linux/x86.zig b/lib/std/os/linux/x86.zig
index cef038820616..41e1ec7d997d 100644
--- a/lib/std/os/linux/x86.zig
+++ b/lib/std/os/linux/x86.zig
@@ -350,7 +350,7 @@ pub const ucontext_t = extern struct {
     link: ?*ucontext_t,
     stack: stack_t,
     mcontext: mcontext_t,
-    sigmask: sigset_t,
+    sigmask: [1024 / @bitSizeOf(c_ulong)]c_ulong, // Currently a libc-compatible (1024-bit) sigmask
     regspace: [64]u64,
 };
 
diff --git a/lib/std/os/linux/x86_64.zig b/lib/std/os/linux/x86_64.zig
index 90147876c817..7ef86fdcf888 100644
--- a/lib/std/os/linux/x86_64.zig
+++ b/lib/std/os/linux/x86_64.zig
@@ -369,13 +369,21 @@ pub const mcontext_t = extern struct {
     reserved1: [8]usize = undefined,
 };
 
+/// ucontext_t is part of the state pushed on the stack by the kernel for
+/// a signal handler.  And also a subset of the state returned from the
+/// makecontext/getcontext/swapcontext POSIX APIs.
+///
+/// Currently this structure matches the glibc/musl layout.  It contains a
+/// 1024-bit signal mask, and `fpregs_mem`.  This structure should be
+/// split into one for the kernel ABI and c.zig should define a glibc/musl
+/// compatible structure.
 pub const ucontext_t = extern struct {
     flags: usize,
     link: ?*ucontext_t,
     stack: stack_t,
     mcontext: mcontext_t,
-    sigmask: sigset_t,
-    fpregs_mem: [64]usize,
+    sigmask: [1024 / @bitSizeOf(c_ulong)]c_ulong, // Currently a glibc-compatible (1024-bit) sigmask.
+    fpregs_mem: [64]usize, // Not part of kernel ABI, only part of glibc ucontext_t
 };
 
 fn gpRegisterOffset(comptime reg_index: comptime_int) usize {
@@ -455,7 +463,7 @@ fn getContextInternal() callconv(.naked) usize {
           [stack_offset] "i" (@offsetOf(ucontext_t, "stack")),
           [sigprocmask] "i" (@intFromEnum(linux.SYS.rt_sigprocmask)),
           [sigmask_offset] "i" (@offsetOf(ucontext_t, "sigmask")),
-          [sigset_size] "i" (linux.NSIG / 8),
+          [sigset_size] "i" (@sizeOf(sigset_t)),
         : "cc", "memory", "rax", "rcx", "rdx", "rdi", "rsi", "r8", "r10", "r11"
     );
 }
diff --git a/lib/std/os/plan9.zig b/lib/std/os/plan9.zig
index 1882eda47696..b910e26c7134 100644
--- a/lib/std/os/plan9.zig
+++ b/lib/std/os/plan9.zig
@@ -182,7 +182,6 @@ pub const SIG = struct {
     pub const TTOU = 20;
 };
 pub const sigset_t = c_long;
-pub const empty_sigset = 0;
 pub const siginfo_t = c_long;
 // TODO plan9 doesn't have sigaction_fn. Sigaction is not a union, but we include it here to be compatible.
 pub const Sigaction = extern struct {
@@ -199,6 +198,10 @@ pub const Sigaction = extern struct {
 pub const AT = struct {
     pub const FDCWD = -100; // we just make up a constant; FDCWD and openat don't actually exist in plan9
 };
+// Plan 9 doesn't do signals.  This is just needed to get through start.zig.
+pub fn sigemptyset() sigset_t {
+    return 0;
+}
 // TODO implement sigaction
 // right now it is just a shim to allow using start.zig code
 pub fn sigaction(sig: u6, noalias act: ?*const Sigaction, noalias oact: ?*Sigaction) usize {
diff --git a/lib/std/posix.zig b/lib/std/posix.zig
index ad65cf205ded..f414aa801b96 100644
--- a/lib/std/posix.zig
+++ b/lib/std/posix.zig
@@ -86,6 +86,7 @@ pub const MREMAP = system.MREMAP;
 pub const MSF = system.MSF;
 pub const MSG = system.MSG;
 pub const NAME_MAX = system.NAME_MAX;
+pub const NSIG = system.NSIG;
 pub const O = system.O;
 pub const PATH_MAX = system.PATH_MAX;
 pub const POLL = system.POLL;
@@ -126,10 +127,8 @@ pub const timerfd_clockid_t = system.timerfd_clockid_t;
 pub const cpu_set_t = system.cpu_set_t;
 pub const dev_t = system.dev_t;
 pub const dl_phdr_info = system.dl_phdr_info;
-pub const empty_sigset = system.empty_sigset;
 pub const fd_t = system.fd_t;
 pub const file_obj = system.file_obj;
-pub const filled_sigset = system.filled_sigset;
 pub const gid_t = system.gid_t;
 pub const ifreq = system.ifreq;
 pub const ino_t = system.ino_t;
@@ -678,7 +677,8 @@ pub fn abort() noreturn {
         raise(SIG.ABRT) catch {};
 
         // Disable all signal handlers.
-        sigprocmask(SIG.BLOCK, &linux.all_mask, null);
+        const filledset = linux.sigfillset();
+        sigprocmask(SIG.BLOCK, &filledset, null);
 
         // Only one thread may proceed to the rest of abort().
         if (!builtin.single_threaded) {
@@ -691,14 +691,15 @@ pub fn abort() noreturn {
         // Install default handler so that the tkill below will terminate.
         const sigact = Sigaction{
             .handler = .{ .handler = SIG.DFL },
-            .mask = empty_sigset,
+            .mask = sigemptyset(),
             .flags = 0,
         };
         sigaction(SIG.ABRT, &sigact, null);
 
         _ = linux.tkill(linux.gettid(), SIG.ABRT);
 
-        const sigabrtmask: linux.sigset_t = [_]u32{0} ** 31 ++ [_]u32{1 << (SIG.ABRT - 1)};
+        var sigabrtmask = sigemptyset();
+        sigaddset(&sigabrtmask, SIG.ABRT);
         sigprocmask(SIG.UNBLOCK, &sigabrtmask, null);
 
         // Beyond this point should be unreachable.
@@ -723,18 +724,13 @@ pub fn raise(sig: u8) RaiseError!void {
     }
 
     if (native_os == .linux) {
-        // https://git.musl-libc.org/cgit/musl/commit/?id=0bed7e0acfd34e3fb63ca0e4d99b7592571355a9
-        //
-        // Unlike musl, libc-less Zig std does not have any internal signals for implementation purposes, so we
-        // need to block all signals on the assumption that any of them could potentially fork() in a handler.
-        var set: sigset_t = undefined;
-        sigprocmask(SIG.BLOCK, &linux.all_mask, &set);
-
-        const tid = linux.gettid();
-        const rc = linux.tkill(tid, sig);
-
-        // restore signal mask
-        sigprocmask(SIG.SETMASK, &set, null);
+        // Block all signals so a `fork` (from a signal handler) between the gettid() and kill() syscalls
+        // cannot trigger an extra, unexpected, inter-process signal.  Signal paranoia inherited from Musl.
+        const filled = linux.sigfillset();
+        var orig: sigset_t = undefined;
+        sigprocmask(SIG.BLOCK, &filled, &orig);
+        const rc = linux.tkill(linux.gettid(), sig);
+        sigprocmask(SIG.SETMASK, &orig, null);
 
         switch (errno(rc)) {
             .SUCCESS => return,
@@ -5818,8 +5814,63 @@ pub fn sigaltstack(ss: ?*stack_t, old_ss: ?*stack_t) SigaltstackError!void {
     }
 }
 
+/// Return a filled sigset_t.
+pub fn sigfillset() sigset_t {
+    if (builtin.link_libc) {
+        var set: sigset_t = undefined;
+        switch (errno(system.sigfillset(&set))) {
+            .SUCCESS => return set,
+            else => unreachable,
+        }
+    }
+    return system.sigfillset();
+}
+
+/// Return an empty sigset_t.
+pub fn sigemptyset() sigset_t {
+    if (builtin.link_libc) {
+        var set: sigset_t = undefined;
+        switch (errno(system.sigemptyset(&set))) {
+            .SUCCESS => return set,
+            else => unreachable,
+        }
+    }
+    return system.sigemptyset();
+}
+
+pub fn sigaddset(set: *sigset_t, sig: u8) void {
+    if (builtin.link_libc) {
+        switch (errno(system.sigaddset(set, sig))) {
+            .SUCCESS => return,
+            else => unreachable,
+        }
+    }
+    system.sigaddset(set, sig);
+}
+
+pub fn sigdelset(set: *sigset_t, sig: u8) void {
+    if (builtin.link_libc) {
+        switch (errno(system.sigdelset(set, sig))) {
+            .SUCCESS => return,
+            else => unreachable,
+        }
+    }
+    system.sigdelset(set, sig);
+}
+
+pub fn sigismember(set: *const sigset_t, sig: u8) bool {
+    if (builtin.link_libc) {
+        const rc = system.sigismember(set, sig);
+        switch (errno(rc)) {
+            .SUCCESS => return rc == 1,
+            else => unreachable,
+        }
+    }
+    return system.sigismember(set, sig);
+}
+
 /// Examine and change a signal action.
-pub fn sigaction(sig: u6, noalias act: ?*const Sigaction, noalias oact: ?*Sigaction) void {
+pub fn sigaction(sig: u8, noalias act: ?*const Sigaction, noalias oact: ?*Sigaction) void {
     switch (errno(system.sigaction(sig, act, oact))) {
         .SUCCESS => return,
         // EINVAL means the signal is either invalid or some signal that cannot have its action
diff --git a/lib/std/posix/test.zig b/lib/std/posix/test.zig
index e27f8971d7e6..c886ed6153fb 100644
--- a/lib/std/posix/test.zig
+++ b/lib/std/posix/test.zig
@@ -859,12 +859,61 @@ test "shutdown socket" {
     std.net.Stream.close(.{ .handle = sock });
 }
 
-test "sigaction" {
+test "sigset empty/full" {
+    if (native_os == .wasi or native_os == .windows)
+        return error.SkipZigTest;
+
+    var set: posix.sigset_t = posix.sigemptyset();
+    for (1..posix.NSIG) |i| {
+        try expectEqual(false, posix.sigismember(&set, @truncate(i)));
+    }
+
+    // The C library can reserve some (unnamed) signals, so can't check the full
+    // NSIG set is defined, but just test a couple:
+    set = posix.sigfillset();
+    try expectEqual(true, posix.sigismember(&set, @truncate(posix.SIG.CHLD)));
+    try expectEqual(true, posix.sigismember(&set, @truncate(posix.SIG.INT)));
+}
+
+// Some signals (32 - 34 on glibc/musl) are not allowed to be added to a
+// sigset by the C library, so avoid testing them.
+fn reserved_signo(i: usize) bool {
+    return builtin.link_libc and (i >= 32 and i <= 34);
+}
+
+test "sigset add/del" {
     if (native_os == .wasi or native_os == .windows)
         return error.SkipZigTest;
 
-    // https://github.com/ziglang/zig/issues/7427
-    if (native_os == .linux and builtin.target.cpu.arch == .x86)
+    var sigset: posix.sigset_t = posix.sigemptyset();
+
+    // See that none are set, then set each one, see that they're all set, then
+    // remove them all, and then see that none are set.
+    for (1..posix.NSIG) |i| {
+        try expectEqual(false, posix.sigismember(&sigset, @truncate(i)));
+    }
+    for (1..posix.NSIG) |i| {
+        if (!reserved_signo(i)) {
+            posix.sigaddset(&sigset, @truncate(i));
+        }
+    }
+    for (1..posix.NSIG) |i| {
+        if (!reserved_signo(i)) {
+            try expectEqual(true, posix.sigismember(&sigset, @truncate(i)));
+        }
+    }
+    for (1..posix.NSIG) |i| {
+        if (!reserved_signo(i)) {
+            posix.sigdelset(&sigset, @truncate(i));
+        }
+    }
+    for (1..posix.NSIG) |i| {
+        try expectEqual(false, posix.sigismember(&sigset, @truncate(i)));
+    }
+}
+
+test "sigaction" {
+    if (native_os == .wasi or native_os == .windows)
         return error.SkipZigTest;
 
     // https://github.com/ziglang/zig/issues/15381
@@ -872,66 +921,138 @@ test "sigaction" {
         return error.SkipZigTest;
     }
 
+    const test_signo = posix.SIG.URG; // URG only because it is ignored by default in debuggers
+
     const S = struct {
         var handler_called_count: u32 = 0;
 
         fn handler(sig: i32, info: *const posix.siginfo_t, ctx_ptr: ?*anyopaque) callconv(.c) void {
             _ = ctx_ptr;
             // Check that we received the correct signal.
-            switch (native_os) {
-                .netbsd => {
-                    if (sig == posix.SIG.USR1 and sig == info.info.signo)
-                        handler_called_count += 1;
-                },
-                else => {
-                    if (sig == posix.SIG.USR1 and sig == info.signo)
-                        handler_called_count += 1;
-                },
+            const info_sig = switch (native_os) {
+                .netbsd => info.info.signo,
+                else => info.signo,
+            };
+            if (sig == test_signo and sig == info_sig) {
+                handler_called_count += 1;
             }
         }
     };
 
     var sa: posix.Sigaction = .{
         .handler = .{ .sigaction = &S.handler },
-        .mask = posix.empty_sigset,
+        .mask = posix.sigemptyset(),
         .flags = posix.SA.SIGINFO | posix.SA.RESETHAND,
     };
+
     var old_sa: posix.Sigaction = undefined;
 
     // Install the new signal handler.
-    posix.sigaction(posix.SIG.USR1, &sa, null);
+    posix.sigaction(test_signo, &sa, null);
 
     // Check that we can read it back correctly.
-    posix.sigaction(posix.SIG.USR1, null, &old_sa);
+    posix.sigaction(test_signo, null, &old_sa);
     try testing.expectEqual(&S.handler, old_sa.handler.sigaction.?);
     try testing.expect((old_sa.flags & posix.SA.SIGINFO) != 0);
 
     // Invoke the handler.
-    try posix.raise(posix.SIG.USR1);
-    try testing.expect(S.handler_called_count == 1);
+    try posix.raise(test_signo);
+    try testing.expectEqual(1, S.handler_called_count);
 
     // Check if passing RESETHAND correctly reset the handler to SIG_DFL
-    posix.sigaction(posix.SIG.USR1, null, &old_sa);
+    posix.sigaction(test_signo, null, &old_sa);
     try testing.expectEqual(posix.SIG.DFL, old_sa.handler.handler);
 
     // Reinstall the signal w/o RESETHAND and re-raise
     sa.flags = posix.SA.SIGINFO;
-    posix.sigaction(posix.SIG.USR1, &sa, null);
-    try posix.raise(posix.SIG.USR1);
-    try testing.expect(S.handler_called_count == 2);
+    posix.sigaction(test_signo, &sa, null);
+    try posix.raise(test_signo);
+    try testing.expectEqual(2, S.handler_called_count);
 
     // Now set the signal to ignored
     sa.handler = .{ .handler = posix.SIG.IGN };
     sa.flags = 0;
-    posix.sigaction(posix.SIG.USR1, &sa, null);
+    posix.sigaction(test_signo, &sa, null);
 
     // Re-raise to ensure handler is actually ignored
-    try posix.raise(posix.SIG.USR1);
-    try testing.expect(S.handler_called_count == 2);
+    try posix.raise(test_signo);
+    try testing.expectEqual(2, S.handler_called_count);
 
     // Ensure that ignored state is returned when querying
-    posix.sigaction(posix.SIG.USR1, null, &old_sa);
-    try testing.expectEqual(posix.SIG.IGN, old_sa.handler.handler.?);
+    posix.sigaction(test_signo, null, &old_sa);
+    try testing.expectEqual(posix.SIG.IGN, old_sa.handler.handler);
+}
+
+test "sigset_t bits" {
+    if (native_os == .wasi or native_os == .windows)
+        return error.SkipZigTest;
+
+    const S = struct {
+        var expected_sig: i32 = undefined;
+        var handler_called_count: u32 = 0;
+
+        fn handler(sig: i32, info: *const posix.siginfo_t, ctx_ptr: ?*anyopaque) callconv(.c) void {
+            _ = ctx_ptr;
+
+            const info_sig = switch (native_os) {
+                .netbsd => info.info.signo,
+                else => info.signo,
+            };
+            if (sig == expected_sig and sig == info_sig) {
+                handler_called_count += 1;
+            }
+        }
+    };
+
+    const self_pid = posix.system.getpid();
+
+    // To check that sigset_t mapping matches kernel (think u32/u64 mismatches on
+    // big-endian), try sending a blocked signal to make sure the mask matches the
+    // signal.  (Send URG and CHLD because they're ignored by default in the
+    // debugger, vs. USR1 or other named signals)
+    inline for ([_]usize{ posix.SIG.URG, posix.SIG.CHLD, 62, 94, 126 }) |test_signo| {
+        if (test_signo >= posix.NSIG) continue;
+
+        S.expected_sig = test_signo;
+        S.handler_called_count = 0;
+
+        const sa: posix.Sigaction = .{
+            .handler = .{ .sigaction = &S.handler },
+            .mask = posix.sigemptyset(),
+            .flags = posix.SA.SIGINFO | posix.SA.RESETHAND,
+        };
+
+        var old_sa: posix.Sigaction = undefined;
+
+        // Install the new signal handler.
+        posix.sigaction(test_signo, &sa, &old_sa);
+
+        // block the signal and see that its delayed until unblocked
+        var block_one: posix.sigset_t = posix.sigemptyset();
+        posix.sigaddset(&block_one, test_signo);
+        posix.sigprocmask(posix.SIG.BLOCK, &block_one, null);
+
+        // qemu maps target signals to host signals 1-to-1, so targets
+        // with more signals than the host will fail to send the signal.
+        const rc = posix.system.kill(self_pid, test_signo);
+        switch (posix.errno(rc)) {
+            .SUCCESS => {
+                // See that the signal is blocked, then unblocked
+                try testing.expectEqual(0, S.handler_called_count);
+                posix.sigprocmask(posix.SIG.UNBLOCK, &block_one, null);
+                try testing.expectEqual(1, S.handler_called_count);
+            },
+            .INVAL => {
+                // Signal won't get delviered.  Just clean up.
+                posix.sigprocmask(posix.SIG.UNBLOCK, &block_one, null);
+                try testing.expectEqual(0, S.handler_called_count);
+            },
+            else => |errno| return posix.unexpectedErrno(errno),
+        }
+
+        // Restore original handler
+        posix.sigaction(test_signo, &old_sa, null);
+    }
 }
 
 test "dup & dup2" {
diff --git a/lib/std/start.zig b/lib/std/start.zig
index 9072c97c0254..6495db1c51a6 100644
--- a/lib/std/start.zig
+++ b/lib/std/start.zig
@@ -749,7 +749,7 @@ fn maybeIgnoreSigpipe() void {
             // Set handler to a noop function instead of `SIG.IGN` to prevent
             // leaking signal disposition to a child process.
             .handler = .{ .handler = noopSigHandler },
-            .mask = posix.empty_sigset,
+            .mask = posix.sigemptyset(),
             .flags = 0,
         };
         posix.sigaction(posix.SIG.PIPE, &act, null);
diff --git a/src/crash_report.zig b/src/crash_report.zig
index 431381fce4e5..6f043c3bfb79 100644
--- a/src/crash_report.zig
+++ b/src/crash_report.zig
@@ -175,12 +175,11 @@ pub fn attachSegfaultHandler() void {
         _ = windows.kernel32.AddVectoredExceptionHandler(0, handleSegfaultWindows);
         return;
     }
-    var act: posix.Sigaction = .{
+    const act: posix.Sigaction = .{
         .handler = .{ .sigaction = handleSegfaultPosix },
-        .mask = posix.empty_sigset,
+        .mask = posix.sigemptyset(),
         .flags = (posix.SA.SIGINFO | posix.SA.RESTART | posix.SA.RESETHAND),
     };
-
     debug.updateSegfaultHandler(&act);
 }
 
diff --git a/test/standalone/sigpipe/build.zig b/test/standalone/sigpipe/build.zig
index 8e1cb5e1227f..de06e1df1553 100644
--- a/test/standalone/sigpipe/build.zig
+++ b/test/standalone/sigpipe/build.zig
@@ -18,7 +18,7 @@ pub fn build(b: *std.build.Builder) !void {
     {
         const act = posix.Sigaction{
             .handler = .{ .handler = posix.SIG.DFL },
-            .mask = posix.empty_sigset,
+            .mask = posix.sigemptyset(),
             .flags = 0,
         };
         try posix.sigaction(posix.SIG.PIPE, &act, null);