netdev - Re: [PATCHv2] bpftool: Try to read btf as raw data if elf read fails

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Date:   Thu, 24 Oct 2019 10:54:14 -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: [PATCHv2] bpftool: Try to read btf as raw data if elf read
 fails

On Thu, 24 Oct 2019 15:23:41 +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
>   [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

My knee jerk reaction would be to implement a new keyword, like:

$ bpftool btf dump rawfile /sys/kernel/btf/vmlinux

Or such. But perhaps the auto-detection is the standard way of dealing
with different formats in the compiler world. Regardless if anyone has
an opinion one way or the other please share!!

> Signed-off-by: Jiri Olsa <jolsa@...nel.org>
> ---
> v2 changes:
>  - added is_btf_raw to find out which btf__parse_* function to call
>  - changed labels and error propagation in btf__parse_raw 
>  - drop the err initialization, which is not needed under this change

The code looks good, thanks for the changes! One question below..

> diff --git a/tools/bpf/bpftool/btf.c b/tools/bpf/bpftool/btf.c
> index 9a9376d1d3df..a7b8bf233cf5 100644
> --- a/tools/bpf/bpftool/btf.c
> +++ b/tools/bpf/bpftool/btf.c

> +static bool is_btf_raw(const char *file)
> +{
> +	__u16 magic = 0;
> +	int fd;
> +
> +	fd = open(file, O_RDONLY);
> +	if (fd < 0)
> +		return false;
> +
> +	read(fd, &magic, sizeof(magic));
> +	close(fd);
> +	return magic == BTF_MAGIC;

Isn't it suspicious to read() 2 bytes into an u16 and compare to a
constant like endianness doesn't matter? Quick grep doesn't reveal
BTF_MAGIC being endian-aware..

> +}
> +
>  static int do_dump(int argc, char **argv)
>  {
>  	struct btf *btf = NULL;
> @@ -465,7 +516,11 @@ static int do_dump(int argc, char **argv)
>  		}
>  		NEXT_ARG();
>  	} else if (is_prefix(src, "file")) {
> -		btf = btf__parse_elf(*argv, NULL);
> +		if (is_btf_raw(*argv))
> +			btf = btf__parse_raw(*argv);
> +		else
> +			btf = btf__parse_elf(*argv, NULL);
>  		if (IS_ERR(btf)) {
>  			err = PTR_ERR(btf);
>  			btf = NULL;