[PATCH] libbpf: allow bpf object kern_version to be overridden

[Rafael David Tinoco <rafaeldtinoco@ubuntu.com>]

Unfortunately some distros don't have their accurate kernel version
defined correctly in version.h due to long term support decisions. This
makes LINUX_VERSION_CODE to be defined as the original upstream version
in header, while the running in-kernel version is different.

Older kernels might still check kern_version during bpf_prog_load(),
making it impossible to load a program that could still be portable.
This patch allows one to override bpf objects kern_version attributes,
so program can bypass this in-kernel check during load.

Example:

A kernel 4.15.0-129.132, for example, might have 4.15.18 version defined
in its headers, which is the kernel it is based on, but have a 4.15.0
version defined within the kernel due to other factors.

$ export LIBBPF_KERN_VERSION=4.15.18
$ sudo -E ./portablebpf -v
...
libbpf: bpf object: kernel_version enforced by env variable: 266002
...

This will also help portable binaries within similar older kernels, as
long as they have their BTF data converted from DWARVES (for example).

Signed-off-by: Rafael David Tinoco <rafaeldtinoco@ubuntu.com>
---
 src/libbpf.c | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/src/libbpf.c b/src/libbpf.c
index 4dc09d3..cbc6e61 100644
--- a/src/libbpf.c
+++ b/src/libbpf.c
@@ -708,13 +708,19 @@ bpf_object__add_programs(struct bpf_object *obj, Elf_Data *sec_data,
 
 static __u32 get_kernel_version(void)
 {
-	__u32 major, minor, patch;
 	struct utsname info;
+	__u32 major, minor, patch, ver;
+	char *ptr, *e = getenv("LIBBPF_KERN_VERSION");
 
 	uname(&info);
-	if (sscanf(info.release, "%u.%u.%u", &major, &minor, &patch) != 3)
+	ptr = (e != NULL) ? e : (char *) &info.release;
+	if (sscanf(ptr, "%u.%u.%u", &major, &minor, &patch) != 3)
 		return 0;
-	return KERNEL_VERSION(major, minor, patch);
+	ver = KERNEL_VERSION(major, minor, patch);
+	if (e)
+		pr_debug("bpf object: kernel_version enforced by env variable: %u\n", ver);
+
+	return ver;
 }
 
 static const struct btf_member *
-- 
2.17.1