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

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Message-ID: <d8620b04-346a-11eb-000f-34d0f9f0cd51@fb.com>
Date:   Fri, 18 Oct 2019 16:48:25 +0000
From:   Andrii Nakryiko <andriin@...com>
To:     Jiri Olsa <jolsa@...nel.org>, Alexei Starovoitov <ast@...nel.org>,
        "Daniel Borkmann" <daniel@...earbox.net>
CC:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "bpf@...r.kernel.org" <bpf@...r.kernel.org>,
        Yonghong Song <yhs@...com>, Martin Lau <kafai@...com>
Subject: Re: [PATCH] bpftool: Try to read btf as raw data if elf read fails

On 10/18/19 3:34 AM, Jiri Olsa wrote:
> The bpftool interface stays the same, but now it's possible
> to run it over BTF raw data, like:

Oh, great, I had similar patch laying around for a while, never got to 
cleaning it up, though, so thanks for picking this up!

> 
>    $ bpftool btf dump file /sys/kernel/btf/vmlinux
>    libbpf: failed to get EHDR from /sys/kernel/btf/vmlinux

We should implement this so that we don't get an extra log output with 
errors. I've been thinking about checking first few bytes of the file. 
If that matches BTF_MAGIC, then try to parse it as raw BTF, otherwise 
parse as ELF w/ BTF. Does it make sense?

>    [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;
> +	struct stat st;
> +	FILE *f;
> +
> +	if (stat(file, &st))
> +		return btf;
> +
> +	f = fopen(file, "rb");
> +	if (!f)
> +		return btf;
> +
> +	buf = malloc(st.st_size);
> +	if (!buf)
> +		goto err;
> +
> +	if ((size_t) st.st_size != fread(buf, 1, st.st_size, f))
> +		goto err;
> +
> +	btf = btf__new(buf, st.st_size);
> +
> +err:
> +	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;
>   
>   	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 {
>