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Bugfix for CompPhdr::operator() that could cause sort to fail. #139

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merged 2 commits into from
Jan 8, 2018
Merged

Bugfix for CompPhdr::operator() that could cause sort to fail. #139

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a1eab1c
Modified CompPhdr::operator() so that it provides a strict
kmillar Dec 7, 2017
71e99c5
Simplify ordering code as suggested in PR comments.
kmillar Jan 8, 2018
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5 changes: 4 additions & 1 deletion src/patchelf.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -135,7 +135,10 @@ class ElfFile
ElfFile * elfFile;
bool operator ()(const Elf_Phdr & x, const Elf_Phdr & y)
{
if (x.p_type == PT_PHDR) return true;
if (x.p_type == PT_PHDR) {
if (y.p_type == PT_PHDR) return false;
return true;
}
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This could be simplified a bit (mostly removing the duplicated check of y.p_type == PT_PHDR:

// Nothing is less than a PHDR:
if (y.p_type == PT_PHDR) return false;

// PHDR's are less than everything (that's not a PHDR)
if (x.p_type == PT_PHDR) return true;

// Sort non-PHDR's by address:
...

Comments are just suggestions.

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While it does seem worth fixing this, according to documents such as [1][2] it seems that only a single PT_PHDR entry may legally exist in a file.

Thinking about what PT_PHDR actually means, this makes sense :). Also my comments are indeed a bit silly, I didn't have my "ELF-knowledge hat" on.

So this is probably harmless as-is: for any legal set of segments this operator will define a valid (strict weak) ordering.

Might be worth ensuring we reject files that attempt to have multiple PT_PHDR segments?

[1] http://www.sco.com/developers/gabi/latest/ch5.pheader.html
[2] http://www.skyfree.org/linux/references/ELF_Format.pdf

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I've simplified the code now. Thanks.

Yes, this was found using a debug version of std::sort, not by obviously incorrect results. Since there's only a single PT_PHDR entry, It probably doesn't any difference on any sane implementation of std::sort, but it is undefined behavior, so there's no guarantees about that.

if (y.p_type == PT_PHDR) return false;
return elfFile->rdi(x.p_paddr) < elfFile->rdi(y.p_paddr);
}
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