libc/powerpc64: Fix swapcontext(3)

On PowerPC platforms a valid link to the Table of Contents (TOC) is
required for PLT lookups to function.  This TOC pointer is stored in
a dedicated register, and is used along with the stack pointer by both
C prologue and PLT lookup code.

When calling swapcontext() with uc_link != NULL, a PLT lookup to
setcontext(3) is attempted from within the _ctx_done context.  The
exiting process has usually trashed both r1 and r2 at this point,
leading to a crash within the PLT lookup before setcontext(2) is
reached to restore the linked context.

Save and restore r1 and r2, using r16 as a scratch register to bypass
the prologue trampling r2.  This ensures the subsequent PLT lookup to
setcontext(3) succeeds.

Signed-off-by: Timothy Pearson <[email protected]>

Author: madscientist159

lib/libc/powerpc64/gen/_ctx_start.S

@@ -36,11 +36,20 @@
 #else
 	/* Load global entry point */
 	mr	%r12,%r14
+
+	/* Create minimal stack frame and save TOC */
+	stdu	%r1,-32(%r1)
+	std	%r16,24(%r1)
 #endif
 	mtlr	%r14
 	blrl			/* branch to start function */
 	mr	%r3,%r15	/* pass pointer to ucontext as argument */
 	nop
+#if defined(_CALL_ELF) && _CALL_ELF != 1
+	/* Restore TOC and pop stack frame */
+	ld	%r2,24(%r1)
+	ld	%r1,0(%r1)
+#endif
 	bl	CNAME(_ctx_done) /* branch to ctxt completion func */
 	/*
 	 * we should never return from the

lib/libc/powerpc64/gen/makecontext.c

@@ -111,14 +111,15 @@ __makecontext(ucontext_t *ucp, void (*start)(void), int argc, ...)
 	va_end(ap);
 
 	/*
-	 * Use caller-saved regs 14/15 to hold params that _ctx_start
+	 * Use caller-saved regs 14/15/16 to hold params that _ctx_start
 	 * will use to invoke the user-supplied func
 	 */
 #if !defined(_CALL_ELF) || _CALL_ELF == 1
 	/* Cast to ensure this is treated as a function descriptor. */
 	mc->mc_srr0 = *(uintptr_t *)_ctx_start;
 #else
 	mc->mc_srr0 = (uintptr_t) _ctx_start;
+	mc->mc_gpr[16] = mc->mc_gpr[2];		/* r16 <- TOC */
 #endif
 	mc->mc_gpr[1] = (uintptr_t) sp;		/* new stack pointer */
 	mc->mc_gpr[14] = (uintptr_t) start;	/* r14 <- start */