linux-kernel - Re: [PATCH] libbpf: extending BTF_KIND_INIT to accommodate some unusual types

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Message-ID: <20240424065200.736080-1-liuxin350@huawei.com>
Date: Wed, 24 Apr 2024 14:52:00 +0800
From: Xin Liu <liuxin350@...wei.com>
To: <alan.maguire@...cle.com>
CC: <andrii@...nel.org>, <ast@...nel.org>, <bpf@...r.kernel.org>,
	<daniel@...earbox.net>, <haoluo@...gle.com>, <john.fastabend@...il.com>,
	<jolsa@...nel.org>, <kongweibin2@...wei.com>, <kpsingh@...nel.org>,
	<linux-kernel@...r.kernel.org>, <liuxin350@...wei.com>,
	<liwei883@...wei.com>, <martin.lau@...ux.dev>, <sdf@...gle.com>,
	<song@...nel.org>, <wuchangye@...wei.com>, <xiesongyang@...wei.com>,
	<yanan@...wei.com>, <yhs@...com>, <zhangmingyi5@...wei.com>
Subject: Re: [PATCH] libbpf: extending BTF_KIND_INIT to accommodate some unusual types

On Tue, 23 Apr 2024 15:30:03 +0100 Alan Maguire <alan.maguire@...cle.com> wrote:
> On 23/04/2024 14:15, Xin Liu wrote:
> > On Mon, 22 Apr 2024 10:43:38 -0700 Andrii Nakryiko <andrii.nakryiko@...il.com> wrote:
> > 
> >> On Mon, Apr 22, 2024 at 7:46 AM Xin Liu <liuxin350@...wei.com> wrote:
> >>>
> >>> In btf__add_int, the size of the new btf_kind_int type is limited.
> >>> When the size is greater than 16, btf__add_int fails to be added
> >>> and -EINVAL is returned. This is usually effective.
> >>>
> >>> However, when the built-in type __builtin_aarch64_simd_xi in the
> >>> NEON instruction is used in the code in the arm64 system, the value
> >>> of DW_AT_byte_size is 64. This causes btf__add_int to fail to
> >>> properly add btf information to it.
> >>>
> >>> like this:
> >>>   ...
> >>>    <1><cf>: Abbrev Number: 2 (DW_TAG_base_type)
> >>>     <d0>   DW_AT_byte_size   : 64              // over max size 16
> >>>     <d1>   DW_AT_encoding    : 5        (signed)
> >>>     <d2>   DW_AT_name        : (indirect string, offset: 0x53): __builtin_aarch64_simd_xi
> >>>    <1><d6>: Abbrev Number: 0
> >>>   ...
> >>>
> >>> An easier way to solve this problem is to treat it as a base type
> >>> and set byte_size to 64. This patch is modified along these lines.
> >>>
> >>> Fixes: 4a3b33f8579a ("libbpf: Add BTF writing APIs")
> >>> Signed-off-by: Xin Liu <liuxin350@...wei.com>
> >>> ---
> >>>  tools/lib/bpf/btf.c | 2 +-
> >>>  1 file changed, 1 insertion(+), 1 deletion(-)
> >>>
> >>> diff --git a/tools/lib/bpf/btf.c b/tools/lib/bpf/btf.c
> >>> index 2d0840ef599a..0af121293b65 100644
> >>> --- a/tools/lib/bpf/btf.c
> >>> +++ b/tools/lib/bpf/btf.c
> >>> @@ -1934,7 +1934,7 @@ int btf__add_int(struct btf *btf, const char *name, size_t byte_sz, int encoding
> >>>         if (!name || !name[0])
> >>>                 return libbpf_err(-EINVAL);
> >>>         /* byte_sz must be power of 2 */
> >>> -       if (!byte_sz || (byte_sz & (byte_sz - 1)) || byte_sz > 16)
> >>> +       if (!byte_sz || (byte_sz & (byte_sz - 1)) || byte_sz > 64)
> >>
> >>
> >> maybe we should just remove byte_sz upper limit? We can probably
> >> imagine 256-byte integers at some point, so why bother artificially
> >> restricting it?
> >>
> >> pw-bot: cr
> > 
> > In the current definition of btf_kind_int, bits has only 8 bits, followed
> > by 8 bits of unused interval. When we expand, we should only use 16 bits
> > at most, so the maximum value should be 8192(1 << 16 / 8), directly removing
> > the limit of byte_sz. It may not fit the current design. For INT type btfs
> > greater than 255, how to dump is still a challenge.
> > 
> > Does the current version support a maximum of 8192 bytes?
> > 
> 
> Presuming we expanded BTF_INT_BITS() as per
> 
> -#define BTF_INT_BITS(VAL)       ((VAL)  & 0x000000ff)
> +#define BTF_INT_BITS(VAL)       ((VAL)  & 0x0000ffff)
> 
> ...as you say we'd be able to represent a 65535-bit value. So if we
> preserve the power-of-two restriction on byte sizes, we'd have to choose
> between either having ints which
> 
> - have a byte_sz maximum of <= 4096 bytes, with all 32768 bits usable; or
> - have a byte_sz maximum of <= 8192 bytes, with 65535 out of 65536 bits
> usable
> 
> The first option seems more intuitive to me.
> 
> In terms of dumping, we could probably just dump a hex representation of
> the relevant bytes.
> 

Currently, there is actually no scenario to use built-in structs in btf. 
As Song and Andrii said, can we remove this restriction first?

> >>
> >>>                 return libbpf_err(-EINVAL);
> >>>         if (encoding & ~(BTF_INT_SIGNED | BTF_INT_CHAR | BTF_INT_BOOL))
> >>>                 return libbpf_err(-EINVAL);
> >>> --
> >>> 2.33.0
> >>>
> >>
> >