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libbpf: Work around kernel inconsistently stripping '.llvm.' suffix

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Some versions of kernel were stripping out '.llvm.<hash>' suffix from
kerne symbols (produced by Clang LTO compilation) from function names
reported in available_filter_functions, while kallsyms reported full
original name. This confuses libbpf's multi-kprobe logic of finding all
matching kernel functions for specified user glob pattern by joining
available_filter_functions and kallsyms contents, because joining by
full symbol name won't work for symbols containing '.llvm.<hash>' suffix.

This was eventually fixed by [0] in the kernel, but we'd like to not
regress multi-kprobe experience and add a work around for this bug on
libbpf side, stripping kallsym's name if it matches user pattern and
contains '.llvm.' suffix.

  [0] fb6a421fb615 ("kallsyms: Match symbols exactly with CONFIG_LTO_CLANG")

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/bpf/20250117003957.179331-1-andrii@kernel.org
This commit is contained in:
Andrii Nakryiko
2025-01-16 16:39:57 -08:00
committed by Andrii Nakryiko
parent 719aeb7a6e
commit 444f3c0e7a
1 changed files with 25 additions and 1 deletions
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26
src/libbpf.c
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@@ -11387,9 +11387,33 @@ static int avail_kallsyms_cb(unsigned long long sym_addr, char sym_type,
struct kprobe_multi_resolve *res = data->res;
int err;
if (!bsearch(&sym_name, data->syms, data->cnt, sizeof(*data->syms), avail_func_cmp))
if (!glob_match(sym_name, res->pattern))
return 0;
if (!bsearch(&sym_name, data->syms, data->cnt, sizeof(*data->syms), avail_func_cmp)) {
/* Some versions of kernel strip out .llvm.<hash> suffix from
* function names reported in available_filter_functions, but
* don't do so for kallsyms. While this is clearly a kernel
* bug (fixed by [0]) we try to accommodate that in libbpf to
* make multi-kprobe usability a bit better: if no match is
* found, we will strip .llvm. suffix and try one more time.
*
* [0] fb6a421fb615 ("kallsyms: Match symbols exactly with CONFIG_LTO_CLANG")
*/
char sym_trim[256], *psym_trim = sym_trim, *sym_sfx;
if (!(sym_sfx = strstr(sym_name, ".llvm.")))
return 0;
/* psym_trim vs sym_trim dance is done to avoid pointer vs array
* coercion differences and get proper `const char **` pointer
* which avail_func_cmp() expects
*/
snprintf(sym_trim, sizeof(sym_trim), "%.*s", (int)(sym_sfx - sym_name), sym_name);
if (!bsearch(&psym_trim, data->syms, data->cnt, sizeof(*data->syms), avail_func_cmp))
return 0;
}
err = libbpf_ensure_mem((void **)&res->addrs, &res->cap, sizeof(*res->addrs), res->cnt + 1);
if (err)
return err;