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* [PATCH bpf-next] tools/resolve_btfids: Fix sections with wrong alignment
@ 2020-08-19  9:23 Jiri Olsa
  2020-08-19 15:31 ` Yonghong Song
                   ` (2 more replies)
  0 siblings, 3 replies; 15+ messages in thread
From: Jiri Olsa @ 2020-08-19  9:23 UTC (permalink / raw)
  To: Alexei Starovoitov, Daniel Borkmann
  Cc: Mark Wielaard, Nick Clifton, Jesper Dangaard Brouer, netdev, bpf,
	Martin KaFai Lau, Song Liu, Yonghong Song, Andrii Nakryiko,
	John Fastabend, KP Singh

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")
Cc: Mark Wielaard <mjw@redhat.com>
Cc: Nick Clifton <nickc@redhat.com>
Reported-by: Jesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
---
 tools/bpf/resolve_btfids/main.c | 36 +++++++++++++++++++++++++++++++++
 1 file changed, 36 insertions(+)

diff --git a/tools/bpf/resolve_btfids/main.c b/tools/bpf/resolve_btfids/main.c
index 4d9ecb975862..0def0bb1f783 100644
--- a/tools/bpf/resolve_btfids/main.c
+++ b/tools/bpf/resolve_btfids/main.c
@@ -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;
-- 
2.25.4


^ permalink raw reply related	[flat|nested] 15+ messages in thread
end of thread, other threads:[~2020-08-20 21:24 UTC | newest]

Thread overview: 15+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2020-08-19  9:23 [PATCH bpf-next] tools/resolve_btfids: Fix sections with wrong alignment Jiri Olsa
2020-08-19 15:31 ` Yonghong Song
2020-08-19 17:36   ` Jiri Olsa
2020-08-19 18:16     ` Nick Desaulniers
2020-08-19 21:30       ` Yonghong Song
2020-08-20  2:27         ` Fāng-ruì Sòng
2020-08-20  3:23           ` Yonghong Song
2020-08-20 10:18             ` Jiri Olsa
2020-08-20 10:18             ` Mark Wielaard
2020-08-20 15:51               ` Yonghong Song
2020-08-20 17:36                 ` Mark Wielaard
2020-08-20 17:54                   ` Yonghong Song
2020-08-20 21:24                     ` Alexei Starovoitov
2020-08-19 17:02 ` Jesper Dangaard Brouer
2020-08-19 21:32 ` Yonghong Song

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