From: John Fastabend <john.fastabend@gmail.com>
Date: Fri, 18 Oct 2019 07:41:26 -0700
Subject: bpf, libbpf: Add kernel version section parsing back
Patch-mainline: v5.5-rc1
Git-commit: 54b8625cd940b6baace0bd9b1cf26b2de68ba307
References: bsc#1155518

With commit "libbpf: stop enforcing kern_version,..." we removed the
kernel version section parsing in favor of querying for the kernel
using uname() and populating the version using the result of the
query. After this any version sections were simply ignored.

Unfortunately, the world of kernels is not so friendly. I've found some
customized kernels where uname() does not match the in kernel version.
To fix this so programs can load in this environment this patch adds
back parsing the section and if it exists uses the user specified
kernel version to override the uname() result. However, keep most the
kernel uname() discovery bits so users are not required to insert the
version except in these odd cases.

Fixes: 5e61f27070292 ("libbpf: stop enforcing kern_version, populate it for users")
Signed-off-by: John Fastabend <john.fastabend@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Link: https://lore.kernel.org/bpf/157140968634.9073.6407090804163937103.stgit@john-XPS-13-9370
Acked-by: Gary Lin <glin@suse.com>
---
 tools/lib/bpf/libbpf.c |   21 ++++++++++++++++++++-
 1 file changed, 20 insertions(+), 1 deletion(-)

--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -656,6 +656,21 @@ bpf_object__init_license(struct bpf_obje
 	return 0;
 }
 
+static int
+bpf_object__init_kversion(struct bpf_object *obj, void *data, size_t size)
+{
+	__u32 kver;
+
+	if (size != sizeof(kver)) {
+		pr_warning("invalid kver section in %s\n", obj->path);
+		return -LIBBPF_ERRNO__FORMAT;
+	}
+	memcpy(&kver, data, sizeof(kver));
+	obj->kern_version = kver;
+	pr_debug("kernel version of %s is %x\n", obj->path, obj->kern_version);
+	return 0;
+}
+
 static int compare_bpf_map(const void *_a, const void *_b)
 {
 	const struct bpf_map *a = _a;
@@ -1573,7 +1588,11 @@ static int bpf_object__elf_collect(struc
 			if (err)
 				return err;
 		} else if (strcmp(name, "version") == 0) {
-			/* skip, we don't need it anymore */
+			err = bpf_object__init_kversion(obj,
+							data->d_buf,
+							data->d_size);
+			if (err)
+				return err;
 		} else if (strcmp(name, "maps") == 0) {
 			obj->efile.maps_shndx = idx;
 		} else if (strcmp(name, MAPS_ELF_SEC) == 0) {