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tools/resolve_btfids: Fix sections with wrong alignment

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The data of compressed section should be aligned to 4
(for 32bit) or 8 (for 64 bit) bytes.

The binutils ld sets sh_addralign to 1, which makes libelf
fail with misaligned section error during the update as
reported by Jesper:

   FAILED elf_update(WRITE): invalid section alignment

While waiting for ld fix, we can fix compressed sections
sh_addralign value manually.

Adding warning in -vv mode when the fix is triggered:

  $ ./tools/bpf/resolve_btfids/resolve_btfids -vv vmlinux
  ...
  section(36) .comment, size 44, link 0, flags 30, type=1
  section(37) .debug_aranges, size 45684, link 0, flags 800, type=1
   - fixing wrong alignment sh_addralign 16, expected 8
  section(38) .debug_info, size 129104957, link 0, flags 800, type=1
   - fixing wrong alignment sh_addralign 1, expected 8
  section(39) .debug_abbrev, size 1152583, link 0, flags 800, type=1
   - fixing wrong alignment sh_addralign 1, expected 8
  section(40) .debug_line, size 7374522, link 0, flags 800, type=1
   - fixing wrong alignment sh_addralign 1, expected 8
  section(41) .debug_frame, size 702463, link 0, flags 800, type=1
  section(42) .debug_str, size 1017571, link 0, flags 830, type=1
   - fixing wrong alignment sh_addralign 1, expected 8
  section(43) .debug_loc, size 3019453, link 0, flags 800, type=1
   - fixing wrong alignment sh_addralign 1, expected 8
  section(44) .debug_ranges, size 1744583, link 0, flags 800, type=1
   - fixing wrong alignment sh_addralign 16, expected 8
  section(45) .symtab, size 2955888, link 46, flags 0, type=2
  section(46) .strtab, size 2613072, link 0, flags 0, type=3
  ...
  update ok for vmlinux

Another workaround is to disable compressed debug info data
CONFIG_DEBUG_INFO_COMPRESSED kernel option.

Fixes: fbbb68de80a4 ("bpf: Add resolve_btfids tool to resolve BTF IDs in ELF object")
Reported-by: Jesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Jesper Dangaard Brouer <brouer@redhat.com>
Acked-by: Yonghong Song <yhs@fb.com>
Cc: Mark Wielaard <mjw@redhat.com>
Cc: Nick Clifton <nickc@redhat.com>
Link: https://lore.kernel.org/bpf/20200819092342.259004-1-jolsa@kernel.org
This commit is contained in:
Jiri Olsa 2020-08-19 11:23:42 +02:00 committed by Alexei Starovoitov
parent 1e891e513e
commit 51f6463aac
1 changed files with 36 additions and 0 deletions
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36
tools/bpf/resolve_btfids/main.c
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@ -233,6 +233,39 @@ static struct btf_id *add_symbol(struct rb_root *root, char *name, size_t size)
return btf_id__add(root, id, false);
}
/*
* The data of compressed section should be aligned to 4
* (for 32bit) or 8 (for 64 bit) bytes. The binutils ld
* sets sh_addralign to 1, which makes libelf fail with
* misaligned section error during the update:
* FAILED elf_update(WRITE): invalid section alignment
*
* While waiting for ld fix, we fix the compressed sections
* sh_addralign value manualy.
*/
static int compressed_section_fix(Elf *elf, Elf_Scn *scn, GElf_Shdr *sh)
{
int expected = gelf_getclass(elf) == ELFCLASS32 ? 4 : 8;
if (!(sh->sh_flags & SHF_COMPRESSED))
return 0;
if (sh->sh_addralign == expected)
return 0;
pr_debug2(" - fixing wrong alignment sh_addralign %u, expected %u\n",
sh->sh_addralign, expected);
sh->sh_addralign = expected;
if (gelf_update_shdr(scn, sh) == 0) {
printf("FAILED cannot update section header: %s\n",
elf_errmsg(-1));
return -1;
}
return 0;
}
static int elf_collect(struct object *obj)
{
Elf_Scn *scn = NULL;
@ -309,6 +342,9 @@ static int elf_collect(struct object *obj)
obj->efile.idlist_shndx = idx;
obj->efile.idlist_addr = sh.sh_addr;
}
if (compressed_section_fix(elf, scn, &sh))
return -1;
}
return 0;