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btf_encoder: fix and complete filtering out zero-sized per-CPU variables 

btf_encoder is ignoring zero-sized per-CPU ELF symbols, but the same has to be
done for DWARF variables when matching them with ELF symbols. This is due to
zero-sized DWARF variables matching unrelated (non-zero-sized) variable that
happens to be allocated at the exact same address, leading to a lot of
confusion in BTF.

See [0] for when this causes big problems.

  [0] https://lore.kernel.org/bpf/CAEf4BzZ0-sihSL-UAm21JcaCCY92CqfNxycHRZYXcoj8OYb=wA@mail.gmail.com/

Committer notes:

Kept the {} around the if block with more than one line, which
simplifies the original patch by just removing that assignment
to the 'dwarf_name' variable.

Reported-by: Michal Suchánek <msuchanek@suse.de>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: bpf@vger.kernel.org
Cc: dwarves@vger.kernel.org
Cc: kernel-team@fb.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
This commit is contained in:
Andrii Nakryiko 2021-05-24 16:42:22 -07:00 committed by Arnaldo Carvalho de Melo
parent fb418f9d83
commit 0d17503db0
1 changed files with 9 additions and 1 deletions
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10
btf_encoder.c
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@ -538,6 +538,7 @@ int cu__encode_btf(struct cu *cu, int verbose, bool force,
cu__for_each_variable(cu, core_id, pos) {
uint32_t size, type, linkage;
const char *name, *dwarf_name;
const struct tag *tag;
uint64_t addr;
int id;
@ -550,6 +551,7 @@ int cu__encode_btf(struct cu *cu, int verbose, bool force,
/* addr has to be recorded before we follow spec */
addr = var->ip.addr;
dwarf_name = variable__name(var, cu);
/* DWARF takes into account .data..percpu section offset
* within its segment, which for vmlinux is 0, but for kernel
@ -583,7 +585,6 @@ int cu__encode_btf(struct cu *cu, int verbose, bool force,
* per-CPU symbols have non-zero values.
*/
if (var->ip.addr == 0) {
dwarf_name = variable__name(var, cu);
if (!dwarf_name || strcmp(dwarf_name, name))
continue;
}
@ -600,6 +601,13 @@ int cu__encode_btf(struct cu *cu, int verbose, bool force,
break;
}
tag = cu__type(cu, var->ip.tag.type);
if (tag__size(tag, cu) == 0) {
if (btf_elf__verbose)
fprintf(stderr, "Ignoring zero-sized per-CPU variable '%s'...\n", dwarf_name ?: "<missing name>");
continue;
}
type = var->ip.tag.type + type_id_off;
linkage = var->external ? BTF_VAR_GLOBAL_ALLOCATED : BTF_VAR_STATIC;