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

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Date:   Fri, 25 Oct 2019 09:31:16 -0700
From:   Jakub Kicinski <jakub.kicinski@...ronome.com>
To:     Andrii Nakryiko <andriin@...com>
Cc:     Jiri Olsa <jolsa@...nel.org>, Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "bpf@...r.kernel.org" <bpf@...r.kernel.org>,
        Yonghong Song <yhs@...com>, Martin Lau <kafai@...com>,
        "andrii.nakryiko@...il.com" <andrii.nakryiko@...il.com>
Subject: Re: [PATCHv2] bpftool: Try to read btf as raw data if elf read
 fails

On Fri, 25 Oct 2019 05:01:17 +0000, Andrii Nakryiko wrote:
> >> +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..  
> 
> Right now we support only loading BTF in native endianness, so I think 
> this should do. If we ever add ability to load non-native endianness, 
> then we'll have to adjust this.

This doesn't do native endianness, this does LE-only. It will not work
on BE machines.