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

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

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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
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48 changes: 40 additions & 8 deletions arch/X86/X86DisassemblerDecoder.c
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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
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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 ]
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