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[llvm-xray] Support for PIE

Authored by phosek on Dec 10 2018, 7:30 PM.



When the instrumented binary is linked as PIE, we need to apply the
relative relocations to sleds. This is handled by the dynamic linker
at runtime, but when processing the file we have to do it ourselves.

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phosek created this revision.Dec 10 2018, 7:30 PM
dberris accepted this revision.Dec 10 2018, 8:22 PM

LGTM -- Thanks @phosek!

This revision is now accepted and ready to land.Dec 10 2018, 8:22 PM
MaskRay added inline comments.Dec 11 2018, 9:38 AM
90 ↗(On Diff #177648)

Section.relocations() applies on either SHT_REL or SHT_RELA. I think the code can be applied on just SHT_RELA.

if (Section.isBSS() || Section.isVirtual()) is not necessary.

MaskRay added inline comments.Dec 11 2018, 9:40 AM
97 ↗(On Diff #177648)

The code only works on x86_64 and ppc64le. So the relative relocations can just be enumerated here.

joerg added a subscriber: joerg.Dec 11 2018, 12:55 PM
joerg added inline comments.
97 ↗(On Diff #177648)

Please explicitly check for the relocation types you care about. They are not the only relocations you can find in a PIE binary, even with static linking.

phosek updated this revision to Diff 178162.Dec 13 2018, 5:02 PM
phosek marked 3 inline comments as done.
dberris accepted this revision.Dec 13 2018, 5:19 PM

LGTM still, thanks @phosek!

MaskRay accepted this revision.Dec 13 2018, 5:22 PM
MaskRay added inline comments.
105 ↗(On Diff #178162)

It can be simplified with
if (auto AddendOrErr = object::ELFRelocationRef(Reloc).getAddend())

phosek updated this revision to Diff 178164.Dec 13 2018, 5:39 PM
phosek marked an inline comment as done.
Closed by commit rL349120: [llvm-xray] Support for PIE (authored by phosek, committed by ). · Explain WhyDec 13 2018, 5:41 PM
This revision was automatically updated to reflect the committed changes.