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

[Xin Liu]

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.

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
> >
>