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: fbbb68de80a4 ("bpf: Add resolve_btfids tool to resolve BTF IDs in ELF object")
Reported-by: Jesper Dangaard Brouer <[e-mail]>
Signed-off-by: Jiri Olsa <[e-mail]>
Signed-off-by: Alexei Starovoitov <[e-mail]>
Acked-by: Jesper Dangaard Brouer <[e-mail]>
Acked-by: Yonghong Song <[e-mail]>
Cc: Mark Wielaard <[e-mail]>
Cc: Nick Clifton <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

tools/bpf/resolve_btfids/main.c

···
 	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;
···
 			obj->efile.idlist_shndx = idx;
 			obj->efile.idlist_addr  = sh.sh_addr;
 		}
+
+		if (compressed_section_fix(elf, scn, &sh))
+			return -1;
 	}
 
 	return 0;