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

[Yonghong Song]

On 4/23/24 6:15 AM, Xin Liu wrote:
> On Mon, 22 Apr 2024 10:43:38 -0700 Andrii Nakryiko <andrii.nakryiko@xxxxxxxxx> wrote:
>
> > On Mon, Apr 22, 2024 at 7:46 AM Xin Liu <liuxin350@xxxxxxxxxx> 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@xxxxxxxxxx>
> > > ---
> > >   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.

Looking at this patch. Now I remember that I have an old pahole patch
to address similar issues
  https://lore.kernel.org/bpf/20230426055030.3743074-1-yhs@xxxxxx/
which is not merged and I forgot that.

In that particular case, the int size is 1024 bytes.
Currently the int type more than 16 bytes cannot be dumped in libbpf.
Do you have a particular use case to use your__builtin_aarch64_simd_xi() type
in bpf program? I guess probably not as BPF does not support
builtin function your__builtin_aarch64_simd_xi().

> Does the current version support a maximum of 8192 bytes?
>
> > >                  return libbpf_err(-EINVAL);
> > >          if (encoding & ~(BTF_INT_SIGNED | BTF_INT_CHAR | BTF_INT_BOOL))
> > >                  return libbpf_err(-EINVAL);
> > > --
> > > 2.33.0