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Fix UB in DwarfExpression::emitLegacyZExt() 

A shift-left > 63 triggers a UBSAN failure. This patch kicks the can
down the road (to the consumer) by emitting a more compact
representation of the shift computation in DWARF expressions.

Differential Revision: https://reviews.llvm.org/D118183
This commit is contained in:
Adrian Prantl 2022-01-25 13:29:22 -08:00
parent e6564f39c7
commit 216002c4bb
2 changed files with 28 additions and 3 deletions
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22
llvm/lib/CodeGen/AsmPrinter/DwarfExpression.cpp
View File

@ -681,9 +681,25 @@ void DwarfExpression::emitLegacySExt(unsigned FromBits) {
}
void DwarfExpression::emitLegacyZExt(unsigned FromBits) {
// (X & (1 << FromBits - 1))
emitOp(dwarf::DW_OP_constu);
emitUnsigned((1ULL << FromBits) - 1);
// Heuristic to decide the most efficient encoding.
// A ULEB can encode 7 1-bits per byte.
if (FromBits / 7 < 1+1+1+1+1) {
// (X & (1 << FromBits - 1))
emitOp(dwarf::DW_OP_constu);
emitUnsigned((1ULL << FromBits) - 1);
} else {
// Note that the DWARF 4 stack consists of pointer-sized elements,
// so technically it doesn't make sense to shift left more than 64
// bits. We leave that for the consumer to decide though. LLDB for
// example uses APInt for the stack elements and can still deal
// with this.
emitOp(dwarf::DW_OP_lit1);
emitOp(dwarf::DW_OP_constu);
emitUnsigned(FromBits);
emitOp(dwarf::DW_OP_shl);
emitOp(dwarf::DW_OP_lit1);
emitOp(dwarf::DW_OP_minus);
}
emitOp(dwarf::DW_OP_and);
}

9
llvm/test/DebugInfo/X86/convert-debugloc.ll
View File

@ -64,11 +64,17 @@
; NOCONV: DW_AT_location (
; NOCONV: {{.*}}, DW_OP_dup, DW_OP_constu 0x7, DW_OP_shr, DW_OP_lit0, DW_OP_not, DW_OP_mul, DW_OP_constu 0x8, DW_OP_shl, DW_OP_or, DW_OP_stack_value)
; NOCONV: DW_AT_name ("y")
; NOCONV: DW_TAG_variable
; NOCONV: DW_AT_location (
; NOCONV: DW_OP_constu 0x40, DW_OP_lit0, DW_OP_plus, DW_OP_lit1, DW_OP_constu 0x40, DW_OP_shl, DW_OP_lit1, DW_OP_minus, DW_OP_and, DW_OP_stack_value)
; NOCONV: DW_AT_name ("z")
; NOCONV: NULL
; NOCONV: DW_TAG_base_type
; NOCONV: DW_AT_name ("signed char")
; NOCONV: DW_TAG_base_type
; NOCONV: DW_AT_name ("int")
; NOCONV: DW_TAG_base_type
; NOCONV: DW_AT_name ("unsigned long long")
; NOCONV: NULL
@ -81,6 +87,7 @@ entry:
;; will not attempt to eliminate. At the moment, only "convert" ops are folded.
;; If you have to change the expression, the expected DWO_id also changes.
call void @llvm.dbg.value(metadata i8 32, metadata !13, metadata !DIExpression(DW_OP_lit0, DW_OP_plus, DW_OP_LLVM_convert, 8, DW_ATE_signed, DW_OP_LLVM_convert, 32, DW_ATE_signed, DW_OP_stack_value)), !dbg !15
call void @llvm.dbg.value(metadata i8 64, metadata !17, metadata !DIExpression(DW_OP_lit0, DW_OP_plus, DW_OP_LLVM_convert, 64, DW_ATE_unsigned, DW_OP_LLVM_convert, 128, DW_ATE_unsigned, DW_OP_LLVM_convert, 64, DW_ATE_unsigned, DW_OP_stack_value)), !dbg !15
ret i8 %x, !dbg !16
}
@ -111,3 +118,5 @@ declare void @llvm.dbg.value(metadata, metadata, metadata)
!14 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
!15 = !DILocation(line: 3, column: 14, scope: !7)
!16 = !DILocation(line: 4, column: 3, scope: !7)
!17 = !DILocalVariable(name: "z", scope: !7, file: !1, line: 3, type: !18)
!18 = !DIBasicType(name: "unsigned long long", size: 64, encoding: DW_ATE_unsigned)