[PATCH bpf-next] libbpf: Prefer mounted tracefs over debugfs

[Eduard Zateev <hackerowskiy@gmail.com>]

The tracefs path helpers look up /sys/kernel/debug/tracing first, even
when tracefs is already mounted at /sys/kernel/tracing. This can trigger
an unnecessary legacy tracefs mount and selects a path beneath debugfs,
which is commonly restricted.

Prefer the direct tracefs mount when statfs() reports TRACEFS_MAGIC.
Checking the filesystem type avoids selecting an existing but unmounted
sysfs mountpoint. Preserve the existing debugfs fallback and the default
tracefs path when neither location is available.

Fixes: a1ac9fd6c650 ("libbpf: fallback to tracefs mount point if debugfs is not mounted")
Closes: https://github.com/libbpf/libbpf/issues/966
Assisted-by: LLM
Signed-off-by: Eduard Zateev <hackerowskiy@gmail.com>
---

Validation on Fedora 44 (aarch64):
- Built static and shared libbpf with the default warning settings.
- Exercised all five path helpers using wrapped syscalls for both mounts,
  tracefs only, debugfs only, an unmounted tracefs directory, and neither
  path available. The original code fails the both-mounts scenario.
- Confirmed the direct tracefs paths on the host's mounted tracefs.
Privileged BPF attach/mount tests were not run.

 tools/lib/bpf/libbpf.c | 17 +++++++++++++----
 1 file changed, 13 insertions(+), 4 deletions(-)

diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index cb09ded90..9db0a8bde 100644
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -59,6 +59,10 @@
 #define BPF_FS_MAGIC		0xcafe4a11
 #endif
 
+#ifndef TRACEFS_MAGIC
+#define TRACEFS_MAGIC		0x74726163
+#endif
+
 #define MAX_EVENT_NAME_LEN	64
 
 #define BPF_FS_DEFAULT_PATH "/sys/fs/bpf"
@@ -12713,12 +12717,17 @@ static int append_to_file(const char *file, const char *fmt, ...)
 
 static bool use_debugfs(void)
 {
-	static int has_debugfs = -1;
+	static int needs_debugfs = -1;
+	struct statfs st_fs;
 
-	if (has_debugfs < 0)
-		has_debugfs = faccessat(AT_FDCWD, DEBUGFS, F_OK, AT_EACCESS) == 0;
+	if (needs_debugfs < 0) {
+		if (statfs(TRACEFS, &st_fs) == 0 && st_fs.f_type == TRACEFS_MAGIC)
+			needs_debugfs = 0;
+		else
+			needs_debugfs = faccessat(AT_FDCWD, DEBUGFS, F_OK, AT_EACCESS) == 0;
+	}
 
-	return has_debugfs == 1;
+	return needs_debugfs == 1;
 }
 
 static const char *tracefs_path(void)

base-commit: a6303535c2ad9d49598d52556183911f02287791
-- 
2.55.0