netdev - Re: [PATCH v2 bpf] libbpf: detect supported kernel BTF features and sanitize BTF

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Message-ID: <20190510201315.jrzt2yo2pchbckda@ast-mbp>
Date:   Fri, 10 May 2019 13:13:17 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Andrii Nakryiko <andriin@...com>
Cc:     andrii.nakryiko@...il.com, netdev@...r.kernel.org, ast@...com,
        yhs@...com, daniel@...earbox.net
Subject: Re: [PATCH v2 bpf] libbpf: detect supported kernel BTF features and
 sanitize BTF

On Fri, May 10, 2019 at 11:41:50AM -0700, Andrii Nakryiko wrote:
> Depending on used versions of libbpf, Clang, and kernel, it's possible to
> have valid BPF object files with valid BTF information, that still won't
> load successfully due to Clang emitting newer BTF features (e.g.,
> BTF_KIND_FUNC, .BTF.ext's line_info/func_info, BTF_KIND_DATASEC, etc), that
> are not yet supported by older kernel.
> 
> This patch adds detection of BTF features and sanitizes BPF object's BTF
> by substituting various supported BTF kinds, which have compatible layout:
>   - BTF_KIND_FUNC -> BTF_KIND_TYPEDEF
>   - BTF_KIND_FUNC_PROTO -> BTF_KIND_ENUM
>   - BTF_KIND_VAR -> BTF_KIND_INT
>   - BTF_KIND_DATASEC -> BTF_KIND_STRUCT
> 
> Replacement is done in such a way as to preserve as much information as
> possible (names, sizes, etc) where possible without violating kernel's
> validation rules.
> 
> v1->v2:
>   - add internal libbpf_util.h w/ common stuff
>   - switch SK storage BTF to use new libbpf__probe_raw_btf()
> 
> Reported-by: Alexei Starovoitov <ast@...com>
> Signed-off-by: Andrii Nakryiko <andriin@...com>
...
> diff --git a/tools/lib/bpf/libbpf_util.h b/tools/lib/bpf/libbpf_util.h
> index da94c4cb2e4d..319e744eb33a 100644
> --- a/tools/lib/bpf/libbpf_util.h
> +++ b/tools/lib/bpf/libbpf_util.h
> @@ -57,4 +57,16 @@ do {				\
>  } /* extern "C" */
>  #endif
>  
> +#define BTF_INFO_ENC(kind, kind_flag, vlen) \
> +	((!!(kind_flag) << 31) | ((kind) << 24) | ((vlen) & BTF_MAX_VLEN))
> +#define BTF_TYPE_ENC(name, info, size_or_type) (name), (info), (size_or_type)
> +#define BTF_INT_ENC(encoding, bits_offset, nr_bits) \
> +	((encoding) << 24 | (bits_offset) << 16 | (nr_bits))
> +#define BTF_TYPE_INT_ENC(name, encoding, bits_offset, bits, sz) \
> +	BTF_TYPE_ENC(name, BTF_INFO_ENC(BTF_KIND_INT, 0, 0), sz), \
> +	BTF_INT_ENC(encoding, bits_offset, bits)
> +#define BTF_MEMBER_ENC(name, type, bits_offset) (name), (type), (bits_offset)
> +#define BTF_PARAM_ENC(name, type) (name), (type)
> +#define BTF_VAR_SECINFO_ENC(type, offset, size) (type), (offset), (size)

hmm. why are those needed in libbpf_util.h ?
I thought the goal is move them into libbpf_internal.h
and not to expose to users?