Re: [PATCH v3 bpf-next 1/3] libbpf: Fix event name too long error

Feng Yang
2025-04-15T02:02:11Z

On Mon, 14 Apr 2025 13:43:38 +0200 Jiri Olsa <olsajiri@gmail.com> wrote:
> On Mon, Apr 14, 2025 at 05:34:00PM +0800, Feng Yang wrote:
> > From: Feng Yang <yangfeng@kylinos.cn>
> >
> > When the binary path is excessively long, the generated probe_name in libbpf
> > exceeds the kernel's MAX_EVENT_NAME_LEN limit (64 bytes).
> > This causes legacy uprobe event attachment to fail with error code -22.
> >
> > Before Fix:
> > ./test_progs -t attach_probe/kprobe-long_name
> > ......
> > libbpf: failed to add legacy kprobe event for 'bpf_kfunc_looooooooooooooooooooooooooooooong_name+0x0': -EINVAL
> > libbpf: prog 'handle_kprobe': failed to create kprobe 'bpf_kfunc_looooooooooooooooooooooooooooooong_name+0x0' perf event: -EINVAL
> > test_attach_kprobe_long_event_name:FAIL:attach_kprobe_long_event_name unexpected error: -22
> > test_attach_probe:PASS:uprobe_ref_ctr_cleanup 0 nsec
> > #13/11 attach_probe/kprobe-long_name:FAIL
> > #13 attach_probe:FAIL
> >
> > ./test_progs -t attach_probe/uprobe-long_name
> > ......
> > libbpf: failed to add legacy uprobe event for /root/linux-bpf/bpf-next/tools/testing/selftests/bpf/test_progs:0x13efd9: -EINVAL
> > libbpf: prog 'handle_uprobe': failed to create uprobe '/root/linux-bpf/bpf-next/tools/testing/selftests/bpf/test_progs:0x13efd9' perf event: -EINVAL
> > test_attach_uprobe_long_event_name:FAIL:attach_uprobe_long_event_name unexpected error: -22
> > #13/10 attach_probe/uprobe-long_name:FAIL
> > #13 attach_probe:FAIL
> > After Fix:
> > ./test_progs -t attach_probe/uprobe-long_name
> > #13/10 attach_probe/uprobe-long_name:OK
> > #13 attach_probe:OK
> > Summary: 1/1 PASSED, 0 SKIPPED, 0 FAILED
> >
> > ./test_progs -t attach_probe/kprobe-long_name
> > #13/11 attach_probe/kprobe-long_name:OK
> > #13 attach_probe:OK
> > Summary: 1/1 PASSED, 0 SKIPPED, 0 FAILED
> >
> > Fixes: 46ed5fc33db9 ("libbpf: Refactor and simplify legacy kprobe code")
> > Fixes: cc10623c6810 ("libbpf: Add legacy uprobe attaching support")
> > Signed-off-by: Hengqi Chen <hengqi.chen@gmail.com>
> > Signed-off-by: Feng Yang <yangfeng@kylinos.cn>
> > ---
> > tools/lib/bpf/libbpf.c | 19 ++++++++++++-------
> > 1 file changed, 12 insertions(+), 7 deletions(-)
> >
> > diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> > index b2591f5cab65..9e047641e001 100644
> > --- a/tools/lib/bpf/libbpf.c
> > +++ b/tools/lib/bpf/libbpf.c
> > @@ -60,6 +60,8 @@
> > #define BPF_FS_MAGIC 0xcafe4a11
> > #endif
> >
> > +#define MAX_EVENT_NAME_LEN 64
> > +
> > #define BPF_FS_DEFAULT_PATH "/sys/fs/bpf"
> >
> > #define BPF_INSN_SZ (sizeof(struct bpf_insn))
> > @@ -11142,10 +11144,10 @@ static void gen_kprobe_legacy_event_name(char *buf, size_t buf_sz,
> > static int index = 0;
> > int i;
> >
> > - snprintf(buf, buf_sz, "libbpf_%u_%s_0x%zx_%d", getpid(), kfunc_name, offset,
> > - __sync_fetch_and_add(&index, 1));
> > + snprintf(buf, buf_sz, "libbpf_%u_%d_%s_0x%zx", getpid(),
> > + __sync_fetch_and_add(&index, 1), kfunc_name, offset);
>
> so the fix is to move unique id before kfunc_name to make sure it gets
> to the event name right? would be great to have it in changelog
>

Yes, defining MAX_EVENT_NAME_LEN ensures event names are truncated via snprintf
to prevent exceeding the maximum length limit.
Moving the unique id before kfunc_name avoids truncating the id.
Regarding the changelog: Should this information go into the commit message of the patch, or somewhere else?

>
> >
> > - /* sanitize binary_path in the probe name */
> > + /* sanitize kfunc_name in the probe name */
> > for (i = 0; buf[i]; i++) {
> > if (!isalnum(buf[i]))
> > buf[i] = '_';
> > @@ -11270,7 +11272,7 @@ int probe_kern_syscall_wrapper(int token_fd)
> >
> > return pfd >= 0 ? 1 : 0;
> > } else { /* legacy mode */
> > - char probe_name[128];
> > + char probe_name[MAX_EVENT_NAME_LEN];
> >
> > gen_kprobe_legacy_event_name(probe_name, sizeof(probe_name), syscall_name, 0);
> > if (add_kprobe_event_legacy(probe_name, false, syscall_name, 0) < 0)
> > @@ -11328,7 +11330,7 @@ bpf_program__attach_kprobe_opts(const struct bpf_program *prog,
> > func_name, offset,
> > -1 /* pid */, 0 /* ref_ctr_off */);
> > } else {
> > - char probe_name[256];
> > + char probe_name[MAX_EVENT_NAME_LEN];
> >
> > gen_kprobe_legacy_event_name(probe_name, sizeof(probe_name),
> > func_name, offset);
> > @@ -11878,9 +11880,12 @@ static int attach_uprobe_multi(const struct bpf_program *prog, long cookie, stru
> > static void gen_uprobe_legacy_event_name(char *buf, size_t buf_sz,
> > const char *binary_path, uint64_t offset)
> > {
> > + static int index = 0;
> > int i;
> >
> > - snprintf(buf, buf_sz, "libbpf_%u_%s_0x%zx", getpid(), binary_path, (size_t)offset);
> > + snprintf(buf, buf_sz, "libbpf_%u_%d_%s_0x%zx", getpid(),
> > + __sync_fetch_and_add(&index, 1),
> > + basename((void *)binary_path), (size_t)offset);
>
> gen_kprobe_legacy_event_name and gen_uprobe_legacy_event_name seem to
> be identical now, maybe we can have just one ?
>
> thanks,
> jirka
>

The gen_uprobe_legacy_event_name function includes an extra basename compared to gen_kprobe_legacy_event_name,
as the prefixes of binary_path are often too similar to distinguish easily.
When merging these two into a single function, is it acceptable to pass basename((void *)binary_path)
directly during the uprobe invocation, or should we remove the addition of basename? Thank you!

> >
> > /* sanitize binary_path in the probe name */
> > for (i = 0; buf[i]; i++) {
> > @@ -12312,7 +12317,7 @@ bpf_program__attach_uprobe_opts(const struct bpf_program *prog, pid_t pid,
> > pfd = perf_event_open_probe(true /* uprobe */, retprobe, binary_path,
> > func_offset, pid, ref_ctr_off);
> > } else {
> > - char probe_name[PATH_MAX + 64];
> > + char probe_name[MAX_EVENT_NAME_LEN];
> >
> > if (ref_ctr_off)
> > return libbpf_err_ptr(-EINVAL);
> > --
> > 2.43.0
> >