Re: [PATCH RESEND bpf-next] bpftool: Use sysfs vmlinux when dumping BTF by ID

[Daniel Borkmann <daniel@iogearbox.net>]

On 4/28/22 1:14 PM, Larysa Zaremba wrote:
> Currently, dumping almost all BTFs specified by id requires
> using the -B option to pass the base BTF. For most cases
> the vmlinux BTF sysfs path should work.
> 
> This patch simplifies dumping by ID usage by attempting to
> use vmlinux BTF from sysfs, if the first try of loading BTF by ID
> fails with certain conditions.
> 
> Signed-off-by: Larysa Zaremba <larysa.zaremba@intel.com>
> Reviewed-by: Alexander Lobakin <alexandr.lobakin@intel.com>
> ---
>   tools/bpf/bpftool/btf.c | 35 ++++++++++++++++++++++++++---------
>   1 file changed, 26 insertions(+), 9 deletions(-)
> 
> diff --git a/tools/bpf/bpftool/btf.c b/tools/bpf/bpftool/btf.c
> index a2c665beda87..557f65e2de5c 100644
> --- a/tools/bpf/bpftool/btf.c
> +++ b/tools/bpf/bpftool/btf.c
> @@ -459,6 +459,22 @@ static int dump_btf_c(const struct btf *btf,
>   	return err;
>   }
>   
> +static const char sysfs_vmlinux[] = "/sys/kernel/btf/vmlinux";
> +
> +static struct btf *get_vmlinux_btf_from_sysfs(void)
> +{
> +	struct btf *base;
> +
> +	base = btf__parse(sysfs_vmlinux, NULL);
> +	if (libbpf_get_error(base)) {
> +		p_err("failed to parse vmlinux BTF at '%s': %ld\n",
> +		      sysfs_vmlinux, libbpf_get_error(base));
> +		base = NULL;
> +	}

Could we reuse libbpf's btf__load_vmlinux_btf() which probes well-known
locations?

> +	return base;
> +}
> +
>   static int do_dump(int argc, char **argv)
>   {
>   	struct btf *btf = NULL, *base = NULL;
> @@ -536,18 +552,11 @@ static int do_dump(int argc, char **argv)
>   		NEXT_ARG();
>   	} else if (is_prefix(src, "file")) {
>   		const char sysfs_prefix[] = "/sys/kernel/btf/";
> -		const char sysfs_vmlinux[] = "/sys/kernel/btf/vmlinux";
>   
>   		if (!base_btf &&
>   		    strncmp(*argv, sysfs_prefix, sizeof(sysfs_prefix) - 1) == 0 &&
> -		    strcmp(*argv, sysfs_vmlinux) != 0) {
> -			base = btf__parse(sysfs_vmlinux, NULL);
> -			if (libbpf_get_error(base)) {
> -				p_err("failed to parse vmlinux BTF at '%s': %ld\n",
> -				      sysfs_vmlinux, libbpf_get_error(base));
> -				base = NULL;
> -			}
> -		}
> +		    strcmp(*argv, sysfs_vmlinux))
> +			base = get_vmlinux_btf_from_sysfs();
>   
>   		btf = btf__parse_split(*argv, base ?: base_btf);
>   		err = libbpf_get_error(btf);
> @@ -593,6 +602,14 @@ static int do_dump(int argc, char **argv)
>   	if (!btf) {
>   		btf = btf__load_from_kernel_by_id_split(btf_id, base_btf);
>   		err = libbpf_get_error(btf);
> +		if (err == -EINVAL && !base_btf) {
> +			btf__free(base);
> +			base = get_vmlinux_btf_from_sysfs();
> +			p_info("Warning: valid base BTF was not specified with -B option, falling back on standard base BTF (sysfs vmlinux)");
> +			btf = btf__load_from_kernel_by_id_split(btf_id, base);
> +			err = libbpf_get_error(btf);
> +		}
> +
>   		if (err) {
>   			p_err("get btf by id (%u): %s", btf_id, strerror(err));
>   			goto done;
>