Re: [PATCH v4 bpf-next 1/3] libbpf: Fix event name too long error

From: Hengqi Chen
Date: Tue Apr 15 2025 - 21:44:40 EST

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In reply to: Andrii Nakryiko: "Re: [PATCH v4 bpf-next 1/3] libbpf: Fix event name too long error"
Next in thread: Feng Yang: "[PATCH v4 bpf-next 3/3] selftests/bpf: Add test for attaching kprobe with long event names"
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Hi Andrii,

On Wed, Apr 16, 2025 at 7:05 AM Andrii Nakryiko
<andrii.nakryiko@xxxxxxxxx> wrote:
>
> 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.
>

This has been used in bpf_object__new() like:

/* Using basename() GNU version which doesn't modify arg. */
libbpf_strlcpy(obj->name, basename((void *)path), sizeof(obj->name));

> 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
> >

Next message: David Airlie: "Re: MGA G200 issue in 6.12 and up"
Previous message: Cliff Liu: "[PATCH 5.15.y] smb: client: fix potential deadlock when releasing mids"
In reply to: Andrii Nakryiko: "Re: [PATCH v4 bpf-next 1/3] libbpf: Fix event name too long error"
Next in thread: Feng Yang: "[PATCH v4 bpf-next 3/3] selftests/bpf: Add test for attaching kprobe with long event names"
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