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[X86] Check if signed value is too large for fixup under some conditions #150976

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[X86] Check if signed value is too large for fixup under some conditions #150976

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48 changes: 45 additions & 3 deletions llvm/lib/Target/X86/MCTargetDesc/X86AsmBackend.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -21,6 +21,7 @@
#include "llvm/MC/MCELFObjectWriter.h"
#include "llvm/MC/MCELFStreamer.h"
#include "llvm/MC/MCExpr.h"
#include "llvm/MC/MCFixup.h"
#include "llvm/MC/MCInst.h"
#include "llvm/MC/MCInstrInfo.h"
#include "llvm/MC/MCObjectStreamer.h"
Expand Down Expand Up @@ -721,15 +722,56 @@ void X86AsmBackend::applyFixup(const MCFragment &F, const MCFixup &Fixup,
assert(Fixup.getOffset() + Size <= Data.size() && "Invalid fixup offset!");

int64_t SignedValue = static_cast<int64_t>(Value);
if (IsResolved && Fixup.isPCRel()) {
// check that PC relative fixup fits into the fixup size.

// If the fixup is resolved to an absolute value, and is explictly signed
// (either by its type or by being marked as PC-relative), we should check
// that it fits.
// Otherwise, be conservative because some users may rely on overflow, or
// being able to use a signed value where an unsigned value would normally be
// expected, so strict checks may break things.
bool CheckSignedVal = false;
if (IsResolved) {
switch (Fixup.getKind()) {
default:
llvm_unreachable("invalid fixup kind!");
case FK_NONE:
case FK_Data_1:
case FK_Data_2:
case FK_Data_4:
case FK_Data_8:
case FK_SecRel_1:
case FK_SecRel_2:
case FK_SecRel_4:
case FK_SecRel_8:
CheckSignedVal = false;
break;
case X86::reloc_riprel_4byte:
case X86::reloc_riprel_4byte_relax:
case X86::reloc_riprel_4byte_relax_rex:
case X86::reloc_riprel_4byte_relax_rex2:
case X86::reloc_riprel_4byte_movq_load:
case X86::reloc_riprel_4byte_movq_load_rex2:
case X86::reloc_riprel_4byte_relax_evex:
case X86::reloc_signed_4byte:
case X86::reloc_signed_4byte_relax:
case X86::reloc_global_offset_table:
case X86::reloc_branch_4byte_pcrel:
CheckSignedVal = true;
break;
}

if (Fixup.isPCRel())
CheckSignedVal = true;
Comment on lines +763 to +764
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@phoebewang phoebewang Aug 6, 2025 •
edited
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Why checking isPCRel since we listed all kinds above?

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Sometimes a PC-relative fixup uses one of the generic fixup types. I think this is because all of the architecture-specific ones are 4-byte, so for example a 1-byte PC-relative fixup will be emitted as a generic fixup, but with the PC-relative flag set to true

}

if (CheckSignedVal) {
if (Size > 0 && !isIntN(Size * 8, SignedValue))
getContext().reportError(Fixup.getLoc(),
"value of " + Twine(SignedValue) +
" is too large for field of " + Twine(Size) +
((Size == 1) ? " byte." : " bytes."));
} else {
// Check that uppper bits are either all zeros or all ones.
// Check that upper bits are either all zeros or all ones.
// Specifically ignore overflow/underflow as long as the leakage is
// limited to the lower bits. This is to remain compatible with
// other assemblers.
Expand Down
8 changes: 8 additions & 0 deletions llvm/test/MC/X86/fixup-range-check.s
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Original file line number Diff line number Diff line change
@@ -0,0 +1,8 @@
// RUN: not llvm-mc -filetype=obj -o /dev/null -triple x86_64-unknown-unknown %s 2>&1 | FileCheck %s

.intel_syntax noprefix
.code64
test_case:
// CHECK: error: value of -9223372036854775808 is too large for field of 4 bytes.
mov rcx, EXAMPLE
.set EXAMPLE, (1ULL<<63ULL)