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Differential D1826

libdwarf: Add symbol value when processing .rela relocations
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Authored by emaste on Feb 11 2015, 7:03 PM.
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kib
rpaulo
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rS278593: libdwarf: Add symbol value when processing .rela relocations
Summary

In practice .rela debug relocations often use the the .debug_* sections as the symbol, which has the value 0:

Relocation section '.rela.debug_info' at offset 0xd18 contains 205 entries:
  Offset          Info           Type           Sym. Value    Sym. Name + Addend
000000000006  001a0000000a R_X86_64_32       0000000000000000 .debug_abbrev + 0
00000000000c  001e0000000a R_X86_64_32       0000000000000000 .debug_str + 0
000000000012  001e0000000a R_X86_64_32       0000000000000000 .debug_str + 45

so in those cases there is no difference if the symbol value is used in the relocation or not.

It is not always the case though:

000000000039  002700000001 R_X86_64_64       0000000000000010 xen_pci_driver + 0

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emaste updated this revision to Diff 3737.Feb 11 2015, 7:03 PM
emaste retitled this revision from to libdwarf: Add symbol value when processing .rela relocations.
emaste updated this object.
emaste edited the test plan for this revision. (Show Details)
emaste added a subscriber: Unknown Object (MLST).

See also D1819

emaste added reviewers: kib, rpaulo.Feb 11 2015, 7:05 PM
kib accepted this revision.Feb 11 2015, 7:18 PM
kib edited edge metadata.

It looks fine, but definitely is extremely subtle. The dwarf_get_reloc_size() returns zero for non-abs relocations, so dwarf_write_{m.l}sb() does nothing for other relocation types.

This at least deserves explanation in a comment.

This revision is now accepted and ready to land.Feb 11 2015, 7:18 PM
rpaulo accepted this revision.Feb 11 2015, 7:21 PM
rpaulo edited edge metadata.

I agree with kib that this deserves a comment. And perhaps add another comment in D1819 explaining why it isn't needed for .rel relocations.

emaste updated this revision to Diff 3739.Feb 11 2015, 7:37 PM
emaste edited edge metadata.

explicitly skip _dwarf_write_* if size==0 and add comment explaining that means unknown or non-absolute relocation

This revision now requires review to proceed.Feb 11 2015, 7:37 PM
kib accepted this revision.Feb 11 2015, 7:49 PM
kib edited edge metadata.
This revision is now accepted and ready to land.Feb 11 2015, 7:49 PM
emaste closed this revision.Feb 11 2015, 7:54 PM
emaste updated this revision to Diff 3740.

Closed by commit rS278593 (authored by @emaste).

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contrib/
elftoolchain/
libdwarf/
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Diff 3739

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contrib/elftoolchain/libdwarf/libdwarf_elf_init.c

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