[PATCH] D63280: [llvm-objdump] Use <first-symbol>-<offset> as the section start symbol

[Jim Wilson via Phabricator via llvm-commits]

jimw added a comment.

This one had me confused for a while, but after a bit of experimenting I figured out that the problem is strip.

hifiveu017:1075$ cat tmp.c
extern int sub2 (void);
int sub (void) { return sub2 (); }
hifiveu017:1076$ gcc --shared -fpic -O -o tmp.so tmp.c
hifiveu017:1077$ objdump -d tmp.so | head

tmp.so:     file format elf64-littleriscv

Disassembly of section .plt:

00000000000003d0 <.plt>:
 3d0:	00002397          	auipc	t2,0x2
 3d4:	41c30333          	sub	t1,t1,t3
 3d8:	c303be03          	ld	t3,-976(t2) # 2000 <__TMC_END__>
hifiveu017:1078$ strip tmp.so
hifiveu017:1079$ objdump -d tmp.so | head

tmp.so:     file format elf64-littleriscv

Disassembly of section .plt:

00000000000003d0 <sub2 at plt-0x20>:
 3d0:	00002397          	auipc	t2,0x2
 3d4:	41c30333          	sub	t1,t1,t3
 3d8:	c303be03          	ld	t3,-976(t2) # 2000 <sub@@Base+0x1b6e>
hifiveu017:1080$

MaskRay is looking at system libraries which have all been stripped.  I've been looking at libraries built as part of the toolchain build, which are not stripped.  The difference between the stripped and unstripped file is that the unstripped file has a section symbol for each section.  Readelf --syms shows it as

  9: 00000000000003d0     0 SECTION LOCAL  DEFAULT    9 

and nm shows it as
00000000000003d0 l    d  .plt	0000000000000000              .plt
The stripped file doesn't have the section symbols.

Objdump always uses the nearest symbol, so if you have the section symbols, then the output starts with that symbol name.  This was mentioned near the top, when discussing
Disassembly of section .text:
0000000000001000 .text:
The first .text is the section, the second one is the section symbol.  And that section symbol disappears when you strip the file, if it isn't needed by something else like a dynamic reloc.

So the issue here is that the objdump output is a little confusing when run on stripped files.  I don't think that is a bug.  If you want good objdump output, don't strip the files.

There is a secondary issue that the first plt in the plt section does not have a symbol of its own, and certainly not one that will survive strip, but I don't think that is a bug either.

Perhaps objdump could be extended to create synthetic section symbols if they don't exist, to get better output for stripped files, but that would be an enhancement not a bug fix.


Repository:
  rG LLVM Github Monorepo

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  https://reviews.llvm.org/D63280/new/

https://reviews.llvm.org/D63280