[PATCH bpf-next] libbpf: improve LINUX_VERSION_CODE detection

[Andrii Nakryiko <andrii@kernel.org>]

Ubuntu reports incorrect kernel version through uname(), which on older
kernels leads to kprobe BPF programs failing to load due to the version
check mismatch.

Accommodate Ubuntu's quirks with LINUX_VERSION_CODE by using
Ubuntu-specific /proc/version_code to fetch major/minor/patch versions
to form LINUX_VERSION_CODE.

While at it, consolide libbpf's kernel version detection code between
libbpf.c and libbpf_probes.c.

  [0] Closes: https://github.com/libbpf/libbpf/issues/421

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
---
 tools/lib/bpf/libbpf.c          | 27 ++++++++++++++++++++++++++-
 tools/lib/bpf/libbpf_internal.h |  2 ++
 tools/lib/bpf/libbpf_probes.c   | 16 ----------------
 3 files changed, 28 insertions(+), 17 deletions(-)

diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index cf862a19222b..9cb99d1e2385 100644
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -795,11 +795,36 @@ bpf_object__add_programs(struct bpf_object *obj, Elf_Data *sec_data,
 	return 0;
 }
 
-static __u32 get_kernel_version(void)
+__u32 get_kernel_version(void)
 {
+	/* On Ubuntu LINUX_VERSION_CODE doesn't correspond to info.release,
+	 * but Ubuntu provides /proc/version_signature file, as described at
+	 * https://ubuntu.com/kernel, with an example contents below, which we
+	 * can use to get a proper LINUX_VERSION_CODE.
+	 *
+	 *   Ubuntu 5.4.0-12.15-generic 5.4.8
+	 *
+	 * In the above, 5.4.8 is what kernel is actually expecting, while
+	 * uname() call will return 5.4.0 in info.release.
+	 */
+	const char *ubuntu_kver_file = "/proc/version_signature";
 	__u32 major, minor, patch;
 	struct utsname info;
 
+	if (access(ubuntu_kver_file, R_OK) == 0) {
+		FILE *f;
+
+		f = fopen(ubuntu_kver_file, "r");
+		if (f) {
+			if (fscanf(f, "%*s %*s %d.%d.%d\n", &major, &minor, &patch) == 3) {
+				fclose(f);
+				return KERNEL_VERSION(major, minor, patch);
+			}
+			fclose(f);
+		}
+		/* something went wrong, fall back to uname() approach */
+	}
+
 	uname(&info);
 	if (sscanf(info.release, "%u.%u.%u", &major, &minor, &patch) != 3)
 		return 0;
diff --git a/tools/lib/bpf/libbpf_internal.h b/tools/lib/bpf/libbpf_internal.h
index 5dbe4f463880..1565679eb432 100644
--- a/tools/lib/bpf/libbpf_internal.h
+++ b/tools/lib/bpf/libbpf_internal.h
@@ -188,6 +188,8 @@ static inline void libbpf_strlcpy(char *dst, const char *src, size_t sz)
 	dst[i] = '\0';
 }
 
+__u32 get_kernel_version(void);
+
 struct btf;
 struct btf_type;
 
diff --git a/tools/lib/bpf/libbpf_probes.c b/tools/lib/bpf/libbpf_probes.c
index 9d3985283dfc..97b06cede56f 100644
--- a/tools/lib/bpf/libbpf_probes.c
+++ b/tools/lib/bpf/libbpf_probes.c
@@ -48,22 +48,6 @@ static int get_vendor_id(int ifindex)
 	return strtol(buf, NULL, 0);
 }
 
-static int get_kernel_version(void)
-{
-	int version, subversion, patchlevel;
-	struct utsname utsn;
-
-	/* Return 0 on failure, and attempt to probe with empty kversion */
-	if (uname(&utsn))
-		return 0;
-
-	if (sscanf(utsn.release, "%d.%d.%d",
-		   &version, &subversion, &patchlevel) != 3)
-		return 0;
-
-	return (version << 16) + (subversion << 8) + patchlevel;
-}
-
 static int probe_prog_load(enum bpf_prog_type prog_type,
 			   const struct bpf_insn *insns, size_t insns_cnt,
 			   char *log_buf, size_t log_buf_sz,
-- 
2.30.2