[ObjYAML] Pass 11 instead of 16 for strncmp

[gAlfonso-bit]

Summary

Passing a value of 16 is over-reading the string literal, which is undefined behavior

[MaskRay]

Thanks for noticing this issue. A better fix is to use strcmp since the trailing bytes should be zeroes.

[gAlfonso-bit]

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[gAlfonso-bit]

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[gAlfonso-bit]

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[MaskRay]

llvm/lib/ObjectYAML/MachOEmitter.cpp

304

strcmp(LC.Data.segment_command_data.segname, ...

[gAlfonso-bit]

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[gAlfonso-bit]

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[MaskRay]

Sorry, this is wrong. segname may not be NUl-terminated. Using 10 should be fine but 16 is fine, too. No need to change.

[gAlfonso-bit]

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[gAlfonso-bit]

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[MaskRay]

10 is correct but there seems no value to change this.

[gAlfonso-bit]

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[gAlfonso-bit]

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[MaskRay]

No. strcmp must take strings (NUL-terminated) as arguments. strncmp can work on possibly NUL-terminated arrays. 16 is correct and there is little point changing it since segname has 16 bytes.

[gAlfonso-bit]

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[MaskRay]

segname has 16 elements. It may contain a segname of exactly 16 bytes. It's UB to call strcmp with segname as an argument.

[gAlfonso-bit]

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[gAlfonso-bit]

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[gAlfonso-bit]

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[gAlfonso-bit]

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[gAlfonso-bit]

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llvm/lib/ObjectYAML/MachOEmitter.cpp

       uint64_t segOff = is64Bit ? LC.Data.segment_command_64_data.fileoff
                                 : LC.Data.segment_command_data.fileoff;
       if (0 ==
-          strncmp(&LC.Data.segment_command_data.segname[0], "__LINKEDIT", 16)) {
+          strncmp(LC.Data.segment_command_data.segname, "__LINKEDIT", 11)) {
         FoundLinkEditSeg = true;
         LinkEditOff = segOff;
         if (Obj.RawLinkEditSegment)