netdev - Re: [PATCH] bpftool: Try to read btf as raw data if elf read fails

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Date:   Fri, 18 Oct 2019 15:39:05 -0700
From:   Jakub Kicinski <jakub.kicinski@...ronome.com>
To:     Jiri Olsa <jolsa@...nel.org>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>, netdev@...r.kernel.org,
        bpf@...r.kernel.org, Andrii Nakryiko <andriin@...com>,
        Yonghong Song <yhs@...com>, Martin KaFai Lau <kafai@...com>
Subject: Re: [PATCH] bpftool: Try to read btf as raw data if elf read fails

On Fri, 18 Oct 2019 12:34:04 +0200, Jiri Olsa wrote:
> The bpftool interface stays the same, but now it's possible
> to run it over BTF raw data, like:
> 
>   $ bpftool btf dump file /sys/kernel/btf/vmlinux
>   libbpf: failed to get EHDR from /sys/kernel/btf/vmlinux
>   [1] INT '(anon)' size=4 bits_offset=0 nr_bits=32 encoding=(none)
>   [2] INT 'long unsigned int' size=8 bits_offset=0 nr_bits=64 encoding=(none)
>   [3] CONST '(anon)' type_id=2
> 
> I'm also adding err init to 0 because I was getting uninitialized
> warnings from gcc.
> 
> Signed-off-by: Jiri Olsa <jolsa@...nel.org>
> ---
>  tools/bpf/bpftool/btf.c | 47 ++++++++++++++++++++++++++++++++++++-----
>  1 file changed, 42 insertions(+), 5 deletions(-)
> 
> diff --git a/tools/bpf/bpftool/btf.c b/tools/bpf/bpftool/btf.c
> index 9a9376d1d3df..100fb7e02329 100644
> --- a/tools/bpf/bpftool/btf.c
> +++ b/tools/bpf/bpftool/btf.c
> @@ -12,6 +12,9 @@
>  #include <libbpf.h>
>  #include <linux/btf.h>
>  #include <linux/hashtable.h>
> +#include <sys/types.h>
> +#include <sys/stat.h>
> +#include <unistd.h>
>  
>  #include "btf.h"
>  #include "json_writer.h"
> @@ -388,6 +391,35 @@ static int dump_btf_c(const struct btf *btf,
>  	return err;
>  }
>  
> +static struct btf *btf__parse_raw(const char *file)
> +{
> +	struct btf *btf = ERR_PTR(-EINVAL);
> +	__u8 *buf = NULL;

Please drop the inits

> +	struct stat st;
> +	FILE *f;
> +
> +	if (stat(file, &st))
> +		return btf;

And return constants here

> +	f = fopen(file, "rb");
> +	if (!f)
> +		return btf;

and here

> +	buf = malloc(st.st_size);
> +	if (!buf)
> +		goto err;

and jump to the right place here.

> +	if ((size_t) st.st_size != fread(buf, 1, st.st_size, f))
> +		goto err;
> +
> +	btf = btf__new(buf, st.st_size);
> +
> +err:

The prefix for error labels which is shared with non-error path is exit_

> +	free(buf);
> +	fclose(f);
> +	return btf;
> +}
> +
>  static int do_dump(int argc, char **argv)
>  {
>  	struct btf *btf = NULL;
> @@ -397,7 +429,7 @@ static int do_dump(int argc, char **argv)
>  	__u32 btf_id = -1;
>  	const char *src;
>  	int fd = -1;
> -	int err;
> +	int err = 0;

This change looks unnecessary.

>  	if (!REQ_ARGS(2)) {
>  		usage();
> @@ -468,10 +500,15 @@ static int do_dump(int argc, char **argv)
>  		btf = btf__parse_elf(*argv, NULL);
>  		if (IS_ERR(btf)) {
>  			err = PTR_ERR(btf);
> -			btf = NULL;
> -			p_err("failed to load BTF from %s: %s", 
> -			      *argv, strerror(err));
> -			goto done;
> +			if (err == -LIBBPF_ERRNO__FORMAT)
> +				btf = btf__parse_raw(*argv);
> +			if (IS_ERR(btf)) {
> +				btf = NULL;
> +				/* Display the original error value. */
> +				p_err("failed to load BTF from %s: %s",
> +				      *argv, strerror(err));
> +				goto done;
> +			}
>  		}
>  		NEXT_ARG();
>  	} else {