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

[Jiri Olsa <olsajiri@gmail.com>]

On Tue, Apr 15, 2025 at 10:01:15AM +0800, Feng Yang wrote:
> 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?

having this in changelog would help

> 
> > 
> > >  
> > > -	/* 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!

I think basename is fine, perhaps just pass it as argument
like below (on top of your change, untested)

jirka


---
diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index 9e047641e001..93e804b25da1 100644
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -11138,14 +11138,13 @@ 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_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_%d_%s_0x%zx", getpid(),
-		 __sync_fetch_and_add(&index, 1), kfunc_name, offset);
+		 __sync_fetch_and_add(&index, 1), name, offset);
 
 	/* sanitize kfunc_name in the probe name */
 	for (i = 0; buf[i]; i++) {
@@ -11274,7 +11273,7 @@ int probe_kern_syscall_wrapper(int token_fd)
 	} else { /* legacy mode */
 		char probe_name[MAX_EVENT_NAME_LEN];
 
-		gen_kprobe_legacy_event_name(probe_name, sizeof(probe_name), syscall_name, 0);
+		gen_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;
 
@@ -11332,8 +11331,7 @@ bpf_program__attach_kprobe_opts(const struct bpf_program *prog,
 	} else {
 		char probe_name[MAX_EVENT_NAME_LEN];
 
-		gen_kprobe_legacy_event_name(probe_name, sizeof(probe_name),
-					     func_name, offset);
+		gen_legacy_event_name(probe_name, sizeof(probe_name), func_name, offset);
 
 		legacy_probe = strdup(probe_name);
 		if (!legacy_probe)
@@ -11877,23 +11875,6 @@ 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)
-{
-	static int index = 0;
-	int i;
-
-	snprintf(buf, buf_sz, "libbpf_%u_%d_%s_0x%zx", getpid(),
-		 __sync_fetch_and_add(&index, 1),
-		 basename((void *)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)
 {
@@ -12322,8 +12303,8 @@ bpf_program__attach_uprobe_opts(const struct bpf_program *prog, pid_t pid,
 		if (ref_ctr_off)
 			return libbpf_err_ptr(-EINVAL);
 
-		gen_uprobe_legacy_event_name(probe_name, sizeof(probe_name),
-					     binary_path, func_offset);
+		gen_legacy_event_name(probe_name, sizeof(probe_name),
+				      basename((char *) binary_path), func_offset);
 
 		legacy_probe = strdup(probe_name);
 		if (!legacy_probe)