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

[Andrii Nakryiko <andrii.nakryiko@gmail.com>]

On Thu, Mar 18, 2021 at 12:31 PM Rafael David Tinoco
<rafaeldtinoco@ubuntu.com> wrote:
>
> 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>
> ---

Libbpf currently provides a way to specify custom kernel version using
a special "version" ELF section:

int _version SEC("version") = 123;

But it seems like you would want to set it at runtime, so this
compile-time approach might be problematic. But instead of hijacking
get_kernel_version(), it's better to add a simple setter counterpart
to bpf_object__kversion() that would just override obj->kern_version.

>  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
>