Re: [RFC PATCH v5 07/30] bpf tools: Check endianess and make libbpf fail early

[Arnaldo Carvalho de Melo <acme@kernel.org>]

Em Mon, Jun 01, 2015 at 07:37:53AM +0000, Wang Nan escreveu:
> Check endianess according to EHDR. Code is taken from
> tools/perf/util/symbol-elf.c.
> 
> Libbpf doesn't magically convert missmatched endianess. See discussion
> on https://lkml.org/lkml/2015/5/18/650 that, even if we swap

Please don't use URLs like this, I tried accessing this link and it
seems lkml.org is not accessible now :-\

Simply avoid having URLs like this, but if you insist, then follow the
Link: standard and use:

 http://lkml.kernel.org/r/MESSAGE-ID

This way I can click on it to redirect me to some archive and if it
still doesn't work, then I can google for that message-id, but the way
you did I can't even google for the subject of that message :-\

BTW: this is what I get for lkml.org now:

504 Gateway Time-out
nginx/1.4.6 (Ubuntu)

- Arnaldo

> eBPF instructions to correct byte order, we are unable to deal with
> endianess in code logical generated by LLVM.
> 
> Therefore, libbpf should simply reject missmatched ELF object, and let
> LLVM to create good code.
> 
> Signed-off-by: Wang Nan <wangnan0@huawei.com>
> Acked-by: Alexei Starovoitov <ast@plumgrid.com>
> ---
>  tools/lib/bpf/libbpf.c | 30 ++++++++++++++++++++++++++++++
>  1 file changed, 30 insertions(+)
> 
> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> index 0824822..5d56c40 100644
> --- a/tools/lib/bpf/libbpf.c
> +++ b/tools/lib/bpf/libbpf.c
> @@ -166,6 +166,34 @@ errout:
>  	return err;
>  }
>  
> +static int
> +bpf_object__check_endianess(struct bpf_object *obj)
> +{
> +	static unsigned int const endian = 1;
> +
> +	switch (obj->efile.ehdr.e_ident[EI_DATA]) {
> +	case ELFDATA2LSB:
> +		/* We are big endian, BPF obj is little endian. */
> +		if (*(unsigned char const *)&endian != 1)
> +			goto mismatch;
> +		break;
> +
> +	case ELFDATA2MSB:
> +		/* We are little endian, BPF obj is big endian. */
> +		if (*(unsigned char const *)&endian != 0)
> +			goto mismatch;
> +		break;
> +	default:
> +		return -EINVAL;
> +	}
> +
> +	return 0;
> +
> +mismatch:
> +	pr_warning("Error: endianess mismatch.\n");
> +	return -EINVAL;
> +}
> +
>  struct bpf_object *bpf_object__open(const char *path)
>  {
>  	struct bpf_object *obj;
> @@ -187,6 +215,8 @@ struct bpf_object *bpf_object__open(const char *path)
>  
>  	if (bpf_object__elf_init(obj))
>  		goto out;
> +	if (bpf_object__check_endianess(obj))
> +		goto out;
>  
>  	bpf_object__elf_finish(obj);
>  	return obj;
> -- 
> 1.8.3.4