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@xxxxxxxxxx>
---
tools/lib/bpf/libbpf.c | 26 +++++++++++++++++++++++++-
1 file changed, 25 insertions(+), 1 deletion(-)
diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index 6c262d0152f8..194809da5172 100644
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -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;
--
2.43.5
