Commit 51f6463a authored Aug 19, 2020 by Jiri Olsa Committed by Alexei Starovoitov Aug 20, 2020

tools/resolve_btfids: Fix sections with wrong alignment



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: fbbb68de ("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

parent 1e891e51

tools/bpf/resolve_btfids/main.c

+36 −0

Original line number	Diff line number	Diff line
		@@ -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;