Skip to content

Ignore ES/CS/SS/DS segment overrides in x64 mode #2819

New issue

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

Sign up for GitHub

By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement. We’ll occasionally send you account related emails.

Already on GitHub? Sign in to your account

Open
jxors wants to merge 10 commits into
base: next
Choose a base branch
from
+345 −12
Open

Ignore ES/CS/SS/DS segment overrides in x64 mode #2819

Show file tree
Hide file tree
Changes from 5 commits
Commits
Show all changes
10 commits
Select commit Hold shift + click to select a range
b22f4ea
Ignore ES/CS/SS/DS segment overrides in x64 mode
Nov 20, 2025
0cf6d87
Add tests for x86 segment override prefixes
Dec 9, 2025
187d104
Only update prefix1 if needed
Jan 6, 2026
e84be54
Also check prefix details in tests
Jan 6, 2026
1e8c11b
Fix formatting
Jan 6, 2026
251208d
Update release notes
Feb 2, 2026
2eec5a8
Refactor segment override handling into separate function
Feb 4, 2026
2451c4b
Make sure function name matches documentation
Feb 4, 2026
e658f7a
Rename function so it passes clang-format and doesn't need to be spli…
Feb 4, 2026
36fe2f1
Run clang-format again
Feb 4, 2026
File filter

Filter by extension

Filter by extension
Conversations
Failed to load comments.
Loading
Jump to
Jump to file
Failed to load files.
Loading
Diff view
Diff view
48 changes: 40 additions & 8 deletions arch/X86/X86DisassemblerDecoder.c
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -523,20 +523,52 @@ static int readPrefixes(struct InternalInstruction *insn)
case 0x65: /* GS segment override */
switch (byte) {
case 0x2e:
insn->segmentOverride = SEG_OVERRIDE_CS;
insn->prefix1 = byte;
if (insn->mode != MODE_64BIT ||
(insn->prefix1 != 0x64 &&
insn->prefix1 != 0x65)) {
if (insn->mode != MODE_64BIT) {
insn->segmentOverride =
SEG_OVERRIDE_CS;
}

insn->prefix1 = byte;
}
break;
case 0x36:
insn->segmentOverride = SEG_OVERRIDE_SS;
insn->prefix1 = byte;
if (insn->mode != MODE_64BIT ||
(insn->prefix1 != 0x64 &&
insn->prefix1 != 0x65)) {
if (insn->mode != MODE_64BIT) {
insn->segmentOverride =
SEG_OVERRIDE_SS;
}

insn->prefix1 = byte;
}
break;
case 0x3e:
insn->segmentOverride = SEG_OVERRIDE_DS;
insn->prefix1 = byte;
if (insn->mode != MODE_64BIT ||
(insn->prefix1 != 0x64 &&
insn->prefix1 != 0x65)) {
if (insn->mode != MODE_64BIT) {
insn->segmentOverride =
SEG_OVERRIDE_DS;
}

insn->prefix1 = byte;
}
break;
case 0x26:
insn->segmentOverride = SEG_OVERRIDE_ES;
insn->prefix1 = byte;
if (insn->mode != MODE_64BIT ||
(insn->prefix1 != 0x64 &&
insn->prefix1 != 0x65)) {
if (insn->mode != MODE_64BIT) {
insn->segmentOverride =
SEG_OVERRIDE_ES;
}

insn->prefix1 = byte;
}
break;
case 0x64:
insn->segmentOverride = SEG_OVERRIDE_FS;
Expand Down
293 changes: 293 additions & 0 deletions tests/issues/x86-prefixes.yaml
View file
Open in desktop
Original file line number Diff line number Diff line change
@@ -0,0 +1,293 @@
test_cases:
# Test segment override priority
-
input:
name: "x86-16: rightmost segment override should take priority"
bytes: [ 0x26, 0x65, 0x64, 0x3E, 0x65, 0x2E, 0x00, 0x00 ]
arch: "CS_ARCH_X86"
options: [ CS_OPT_DETAIL, CS_MODE_16 ]
expected:
insns:
-
asm_text: "add byte ptr cs:[bx + si], al"
details:
x86:
prefix: [ X86_PREFIX_0, X86_PREFIX_CS, X86_PREFIX_0, X86_PREFIX_0 ]
-
input:
name: "x86-32: rightmost segment override should take priority"
bytes: [ 0x26, 0x65, 0x64, 0x3E, 0x65, 0x2E, 0x00, 0x00 ]
arch: "CS_ARCH_X86"
options: [ CS_OPT_DETAIL, CS_MODE_32 ]
expected:
insns:
-
asm_text: "add byte ptr cs:[eax], al"
details:
x86:
prefix: [ X86_PREFIX_0, X86_PREFIX_CS, X86_PREFIX_0, X86_PREFIX_0 ]
-
input:
name: "x86-64: rightmost segment override should take priority"
bytes: [ 0x26, 0x65, 0x64, 0x3E, 0x65, 0x2E, 0x00, 0x00 ]
arch: "CS_ARCH_X86"
options: [ CS_OPT_DETAIL, CS_MODE_64 ]
expected:
insns:
-
asm_text: "add byte ptr gs:[rax], al"
details:
x86:
prefix: [ X86_PREFIX_0, X86_PREFIX_GS, X86_PREFIX_0, X86_PREFIX_0 ]
-
input:
name: "x86-16: rightmost segment override should take priority"
bytes: [ 0x3E, 0x3E, 0x26, 0x36, 0x64, 0x36, 0x00, 0x00 ]
arch: "CS_ARCH_X86"
options: [ CS_OPT_DETAIL, CS_MODE_16 ]
expected:
insns:
-
asm_text: "add byte ptr ss:[bx + si], al"
details:
x86:
prefix: [ X86_PREFIX_0, X86_PREFIX_SS, X86_PREFIX_0, X86_PREFIX_0 ]
-
input:
name: "x86-32: rightmost segment override should take priority"
bytes: [ 0x3E, 0x3E, 0x26, 0x36, 0x64, 0x36, 0x00, 0x00 ]
arch: "CS_ARCH_X86"
options: [ CS_OPT_DETAIL, CS_MODE_32 ]
expected:
insns:
-
asm_text: "add byte ptr ss:[eax], al"
details:
x86:
prefix: [ X86_PREFIX_0, X86_PREFIX_SS, X86_PREFIX_0, X86_PREFIX_0 ]
-
input:
name: "x86-64: rightmost segment override should take priority"
bytes: [ 0x3E, 0x3E, 0x26, 0x36, 0x64, 0x36, 0x00, 0x00 ]
arch: "CS_ARCH_X86"
options: [ CS_OPT_DETAIL, CS_MODE_64 ]
expected:
insns:
-
asm_text: "add byte ptr fs:[rax], al"
details:
x86:
prefix: [ X86_PREFIX_0, X86_PREFIX_FS, X86_PREFIX_0, X86_PREFIX_0 ]
# Test segment override differences between 16/32 and 64-bit mode with ECDS and FS/GS
-
input:
name: "x86-16: ECSD segment overrides should override FS/GS segment overrides"
bytes: [ 0x64, 0x3E, 0x00, 0x00 ]
arch: "CS_ARCH_X86"
options: [ CS_OPT_DETAIL, CS_MODE_16 ]
expected:
insns:
-
asm_text: "add byte ptr ds:[bx + si], al"
details:
x86:
prefix: [ X86_PREFIX_0, X86_PREFIX_DS, X86_PREFIX_0, X86_PREFIX_0 ]
-
input:
name: "x86-32: ECSD segment overrides should override FS/GS segment overrides"
bytes: [ 0x64, 0x3E, 0x00, 0x00 ]
arch: "CS_ARCH_X86"
options: [ CS_OPT_DETAIL, CS_MODE_32 ]
expected:
insns:
-
asm_text: "add byte ptr ds:[eax], al"
details:
x86:
prefix: [ X86_PREFIX_0, X86_PREFIX_DS, X86_PREFIX_0, X86_PREFIX_0 ]
-
input:
name: "x86-64: ECSD segment overrides should be ignored and leave FS override intact"
bytes: [ 0x64, 0x3E, 0x00, 0x00 ]
arch: "CS_ARCH_X86"
options: [ CS_OPT_DETAIL, CS_MODE_64 ]
expected:
insns:
-
asm_text: "add byte ptr fs:[rax], al"
details:
x86:
prefix: [ X86_PREFIX_0, X86_PREFIX_FS, X86_PREFIX_0, X86_PREFIX_0 ]
-
input:
name: "x86-64: ECSD segment overrides should be ignored"
bytes: [ 0x3E, 0x00, 0x00 ]
arch: "CS_ARCH_X86"
options: [ CS_OPT_DETAIL, CS_MODE_64 ]
expected:
insns:
-
asm_text: "add byte ptr [rax], al"
details:
x86:
prefix: [ X86_PREFIX_0, X86_PREFIX_DS, X86_PREFIX_0, X86_PREFIX_0 ]
# Test duplicate segment override prefixes
-
input:
name: "x86-16: Duplicate ES override prefixes should decode successfully"
bytes: [ 0x26, 0x26, 0x00, 0x00 ]
arch: "CS_ARCH_X86"
options: [ CS_OPT_DETAIL, CS_MODE_16 ]
expected:
insns:
-
asm_text: "add byte ptr es:[bx + si], al"
details:
x86:
prefix: [ X86_PREFIX_0, X86_PREFIX_ES, X86_PREFIX_0, X86_PREFIX_0 ]
-
input:
name: "x86-32: Duplicate ES override prefixes should decode successfully"
bytes: [ 0x26, 0x26, 0x00, 0x00 ]
arch: "CS_ARCH_X86"
options: [ CS_OPT_DETAIL, CS_MODE_32 ]
expected:
insns:
-
asm_text: "add byte ptr es:[eax], al"
details:
x86:
prefix: [ X86_PREFIX_0, X86_PREFIX_ES, X86_PREFIX_0, X86_PREFIX_0 ]
-
input:
name: "x86-64: Duplicate FS override prefixes should decode successfully"
bytes: [ 0x64, 0x64, 0x00, 0x00 ]
arch: "CS_ARCH_X86"
options: [ CS_OPT_DETAIL, CS_MODE_64 ]
expected:
insns:
-
asm_text: "add byte ptr fs:[rax], al"
details:
x86:
prefix: [ X86_PREFIX_0, X86_PREFIX_FS, X86_PREFIX_0, X86_PREFIX_0 ]
# Test invalid REX prefix
-
input:
name: "x86-64: Invalid REX prefix should preserve previous segment override"
bytes: [ 0x64, 0x40, 0x2E, 0x00, 0x00 ]
arch: "CS_ARCH_X86"
options: [ CS_OPT_DETAIL, CS_MODE_64 ]
expected:
insns:
-
asm_text: "add byte ptr fs:[rax], al"
details:
x86:
prefix: [ X86_PREFIX_0, X86_PREFIX_FS, X86_PREFIX_0, X86_PREFIX_0 ]
-
input:
name: "x86-64: Invalid REX prefix should not add ECDS segment override"
bytes: [ 0x2E, 0x40, 0x2E, 0x00, 0x00 ]
arch: "CS_ARCH_X86"
options: [ CS_OPT_DETAIL, CS_MODE_64 ]
expected:
insns:
-
asm_text: "add byte ptr [rax], al"
details:
x86:
prefix: [ X86_PREFIX_0, X86_PREFIX_CS, X86_PREFIX_0, X86_PREFIX_0 ]
# Test whether `notrack` is correctly decoded
-
input:
name: "x86-16: notrack should decode correctly"
bytes: [ 0x3E, 0xE8, 0x00, 0x00 ]
arch: "CS_ARCH_X86"
options: [ CS_OPT_DETAIL, CS_MODE_16 ]
expected:
insns:
-
asm_text: "notrack call 4"
details:
x86:
prefix: [ X86_PREFIX_0, X86_PREFIX_DS, X86_PREFIX_0, X86_PREFIX_0 ]
-
input:
name: "x86-32: notrack should decode correctly"
bytes: [ 0x3E, 0xE8, 0x00, 0x00, 0x00, 0x00 ]
arch: "CS_ARCH_X86"
options: [ CS_OPT_DETAIL, CS_MODE_32 ]
expected:
insns:
-
asm_text: "notrack call 6"
details:
x86:
prefix: [ X86_PREFIX_0, X86_PREFIX_DS, X86_PREFIX_0, X86_PREFIX_0 ]
-
input:
name: "x86-64: notrack should decode correctly"
bytes: [ 0x3E, 0xE8, 0x00, 0x00, 0x00, 0x00 ]
arch: "CS_ARCH_X86"
options: [ CS_OPT_DETAIL, CS_MODE_64 ]
expected:
insns:
-
asm_text: "notrack call 6"
details:
x86:
prefix: [ X86_PREFIX_0, X86_PREFIX_DS, X86_PREFIX_0, X86_PREFIX_0 ]

-
input:
name: "x86-16: notrack should be applied when 0x3E is last segment override prefix"
bytes: [ 0x26, 0x64, 0x3E, 0xE8, 0x00, 0x00 ]
arch: "CS_ARCH_X86"
options: [ CS_OPT_DETAIL, CS_MODE_16 ]
expected:
insns:
-
asm_text: "notrack call 6"
details:
x86:
prefix: [ X86_PREFIX_0, X86_PREFIX_DS, X86_PREFIX_0, X86_PREFIX_0 ]
-
input:
name: "x86-32: notrack should be applied when 0x3E is last segment override prefix"
bytes: [ 0x26, 0x64, 0x3E, 0xE8, 0x00, 0x00, 0x00, 0x00 ]
arch: "CS_ARCH_X86"
options: [ CS_OPT_DETAIL, CS_MODE_32 ]
expected:
insns:
-
asm_text: "notrack call 8"
details:
x86:
prefix: [ X86_PREFIX_0, X86_PREFIX_DS, X86_PREFIX_0, X86_PREFIX_0 ]
-
input:
name: "x86-64: workaround: notrack should only be applied when 0x3E is last segment override prefix and no FS/GS segment override prefix is active"
bytes: [ 0x26, 0x64, 0x3E, 0xE8, 0x00, 0x00, 0x00, 0x00 ]
arch: "CS_ARCH_X86"
options: [ CS_OPT_DETAIL, CS_MODE_64 ]
expected:
insns:
-
asm_text: "call 8"
details:
x86:
prefix: [ X86_PREFIX_0, X86_PREFIX_FS, X86_PREFIX_0, X86_PREFIX_0 ]
-
input:
name: "x86-64: workaround: notrack should only be applied when 0x3E is last segment override prefix and no FS/GS segment override prefix is active"
bytes: [ 0x26, 0x2E, 0x3E, 0xE8, 0x00, 0x00, 0x00, 0x00 ]
arch: "CS_ARCH_X86"
options: [ CS_OPT_DETAIL, CS_MODE_64 ]
expected:
insns:
-
asm_text: "notrack call 8"
details:
x86:
prefix: [ X86_PREFIX_0, X86_PREFIX_DS, X86_PREFIX_0, X86_PREFIX_0 ]
Loading