Re: [PATCH v4 bpf-next 1/3] libbpf: Fix event name too long error
From: Andrii Nakryiko
Date: Tue Apr 15 2025 - 19:05:57 EST
On Tue, Apr 15, 2025 at 2:40 AM Feng Yang <yangfeng59949@xxxxxxx> wrote:
>
> From: Feng Yang <yangfeng@xxxxxxxxxx>
>
> 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.
>
> The fix reorders the fields to place the unique ID before the name.
> This ensures that even if truncation occurs via snprintf, the unique ID
> remains intact, preserving event name uniqueness. Additionally, explicit
> checks with MAX_EVENT_NAME_LEN are added to enforce length constraints.
>
> Before Fix:
> ./test_progs -t attach_probe/kprobe-long_name
> ......
> libbpf: failed to add legacy kprobe event for 'bpf_testmod_looooooooooooooooooooooooooooooong_name+0x0': -EINVAL
> libbpf: prog 'handle_kprobe': failed to create kprobe 'bpf_testmod_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@xxxxxxxxx>
> Signed-off-by: Feng Yang <yangfeng@xxxxxxxxxx>
> ---
> tools/lib/bpf/libbpf.c | 41 +++++++++++++++--------------------------
> 1 file changed, 15 insertions(+), 26 deletions(-)
>
> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> index b2591f5cab65..b7fc57ac16a6 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))
> @@ -11136,16 +11138,16 @@ static const char *tracefs_available_filter_functions_addrs(void)
> : TRACEFS"/available_filter_functions_addrs";
> }
>
> -static void gen_kprobe_legacy_event_name(char *buf, size_t buf_sz,
> - const char *kfunc_name, size_t offset)
> +static void gen_probe_legacy_event_name(char *buf, size_t buf_sz,
> + const char *name, size_t offset)
> {
> 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), name, offset);
>
> - /* sanitize binary_path in the probe name */
> + /* sanitize name in the probe name */
> for (i = 0; buf[i]; i++) {
> if (!isalnum(buf[i]))
> buf[i] = '_';
> @@ -11270,9 +11272,9 @@ 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);
> + gen_probe_legacy_event_name(probe_name, sizeof(probe_name), syscall_name, 0);
> if (add_kprobe_event_legacy(probe_name, false, syscall_name, 0) < 0)
> return 0;
>
> @@ -11328,9 +11330,9 @@ 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),
> + gen_probe_legacy_event_name(probe_name, sizeof(probe_name),
> func_name, offset);
>
> legacy_probe = strdup(probe_name);
> @@ -11875,20 +11877,6 @@ static int attach_uprobe_multi(const struct bpf_program *prog, long cookie, stru
> return ret;
> }
>
> -static void gen_uprobe_legacy_event_name(char *buf, size_t buf_sz,
> - const char *binary_path, uint64_t offset)
> -{
> - int i;
> -
> - snprintf(buf, buf_sz, "libbpf_%u_%s_0x%zx", getpid(), binary_path, (size_t)offset);
> -
> - /* sanitize binary_path in the probe name */
> - for (i = 0; buf[i]; i++) {
> - if (!isalnum(buf[i]))
> - buf[i] = '_';
> - }
> -}
> -
> static inline int add_uprobe_event_legacy(const char *probe_name, bool retprobe,
> const char *binary_path, size_t offset)
> {
> @@ -12312,13 +12300,14 @@ 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);
>
> - gen_uprobe_legacy_event_name(probe_name, sizeof(probe_name),
> - binary_path, func_offset);
> + gen_probe_legacy_event_name(probe_name, sizeof(probe_name),
> + basename((void *)binary_path),
This patch is a nice refactoring overall and I like it. But this (void
*) cast on binary_path I'm not so fond of. Yes, if you read
smallprint, you'll see that with _GNU_SOURCE basename won't *really*
modify input argument, but meh.
Let's instead do a simple `strrchr(binary_path, '/') ?: binary_path`?
pw-bot: cr
> + func_offset);
>
> legacy_probe = strdup(probe_name);
> if (!legacy_probe)
> --
> 2.43.0
>