linux-kernel - [PATCH] libbpf: correct the macro KERNEL

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Message-Id: <20230417084449.99848-1-songrui.771@bytedance.com>
Date:   Mon, 17 Apr 2023 16:44:49 +0800
From:   "songrui.771" <songrui.771@...edance.com>
To:     Andrii Nakryiko <andrii@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>
Cc:     bpf@...r.kernel.org, linux-kernel@...r.kernel.org,
        "songrui.771" <songrui.771@...edance.com>
Subject: [PATCH] libbpf: correct the macro KERNEL_VERSION for old kernel

The introduced header file linux/version.h in libbpf_probes.c may have a
wrong macro KERNEL_VERSION for calculating LINUX_VERSION_CODE in some old
kernel (Debian9, 10). Below is a version info example from Debian 10.

release: 4.19.0-22-amd64
version: #1 SMP Debian 4.19.260-1 (2022-09-29)

The macro KERNEL_VERSION is defined to (((a) << 16) + ((b) << 8)) + (c)),
which a, b, and c stand for major, minor and patch version. So in example here,
the major is 4, minor is 19, patch is 260, the LINUX_VERSION(4, 19, 260) which
is 267268 should be matched to LINUX_VERSION_CODE. However, the KERNEL_VERSION_CODE
in linux/version.h is defined to 267263.

I noticed that the macro KERNEL_VERSION in linux/version.h of some new kernel is
defined to (((a) << 16) + ((b) << 8) + ((c) > 255 ? 255 : (c))). And
KERNEL_VERSION(4, 19, 260) is equal to 267263 which is the right LINUX_VERSION_CODE.

The mismatched LINUX_VERSION_CODE which will cause failing to load kprobe BPF
programs in the version check of BPF syscall.

The return value of get_kernel_version in libbpf_probes.c should be matched to
LINUX_VERSION_CODE by correcting the macro KERNEL_VERSION.

Signed-off-by: songrui.771 <songrui.771@...edance.com>
---
 tools/lib/bpf/libbpf_probes.c | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/tools/lib/bpf/libbpf_probes.c b/tools/lib/bpf/libbpf_probes.c
index 4f3bc968ff8e..5b22a880c7e7 100644
--- a/tools/lib/bpf/libbpf_probes.c
+++ b/tools/lib/bpf/libbpf_probes.c
@@ -18,6 +18,10 @@
 #include "libbpf.h"
 #include "libbpf_internal.h"
 
+#ifndef LIBBPF_KERNEL_VERSION
+#define LIBBPF_KERNEL_VERSION(a, b, c) (((a) << 16) + ((b) << 8) + ((c) > 255 ? 255 : (c)))
+#endif
+
 /* 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
@@ -47,7 +51,7 @@ static __u32 get_ubuntu_kernel_version(void)
 	if (ret != 3)
 		return 0;
 
-	return KERNEL_VERSION(major, minor, patch);
+	return LIBBPF_KERNEL_VERSION(major, minor, patch);
 }
 
 /* On Debian LINUX_VERSION_CODE doesn't correspond to info.release.
@@ -74,7 +78,7 @@ static __u32 get_debian_kernel_version(struct utsname *info)
 	if (sscanf(p, "Debian %u.%u.%u", &major, &minor, &patch) != 3)
 		return 0;
 
-	return KERNEL_VERSION(major, minor, patch);
+	return LIBBPF_KERNEL_VERSION(major, minor, patch);
 }
 
 __u32 get_kernel_version(void)
@@ -97,7 +101,7 @@ __u32 get_kernel_version(void)
 	if (sscanf(info.release, "%u.%u.%u", &major, &minor, &patch) != 3)
 		return 0;
 
-	return KERNEL_VERSION(major, minor, patch);
+	return LIBBPF_KERNEL_VERSION(major, minor, patch);
 }
 
 static int probe_prog_load(enum bpf_prog_type prog_type,
-- 
2.39.2 (Apple Git-143)