[PATCH RFC bpf-next 1/4] bpf: add struct largest member size in func model

[Andrii Nakryiko]

On Fri, Apr 11, 2025 at 1:32 PM Alexis Lothoré (eBPF Foundation)
<alexis.lothore at bootlin.com> wrote:
>
> In order to properly JIT the trampolines needed to attach BPF programs
> to functions, some architectures like ARM64 need to know about the
> alignment needed for the function arguments. Such alignment can
> generally be deduced from the argument size, but that's not completely
> true for composite types. In the specific case of ARM64, the AAPCS64 ABI
> defines that a composite type which needs to be passed through stack
> must be aligned on the maximum between 8 and the largest alignment
> constraint of its first-level members. So the JIT compiler needs more
> information about the arguments to make sure to generate code that
> respects those alignment constraints.
>
> For struct arguments, add information about the size of the largest
> first-level member in the struct btf_func_model to allow the JIT
> compiler to guess the needed alignment. The information is quite

I might be missing something, but how can the *size* of the field be
used to calculate that argument's *alignment*? i.e., I don't
understand why arg_largest_member_size needs to be calculated instead
of arg_largest_member_alignment...

> specific, but it allows to keep arch-specific concerns (ie: guessing the
> final needed alignment for an argument) isolated in each JIT compiler.

couldn't all this information be calculated in the JIT compiler (if
JIT needs that) from BTF?

>
> Signed-off-by: Alexis Lothoré (eBPF Foundation) <alexis.lothore at bootlin.com>
> ---
>  include/linux/bpf.h |  1 +
>  kernel/bpf/btf.c    | 25 +++++++++++++++++++++++++
>  2 files changed, 26 insertions(+)
>
> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> index 3f0cc89c0622cb1a097999afb78c17102593b6bb..8b34dcf60a0ce09228ff761b962ab67b6a3e2263 100644
> --- a/include/linux/bpf.h
> +++ b/include/linux/bpf.h
> @@ -1106,6 +1106,7 @@ struct btf_func_model {
>         u8 nr_args;
>         u8 arg_size[MAX_BPF_FUNC_ARGS];
>         u8 arg_flags[MAX_BPF_FUNC_ARGS];
> +       u8 arg_largest_member_size[MAX_BPF_FUNC_ARGS];
>  };
>
>  /* Restore arguments before returning from trampoline to let original function
> diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
> index 16ba36f34dfab7531babf5753cab9f368cddefa3..5d40911ec90210086a6175d569abb6e52d75ad17 100644
> --- a/kernel/bpf/btf.c
> +++ b/kernel/bpf/btf.c
> @@ -7318,6 +7318,29 @@ static int __get_type_size(struct btf *btf, u32 btf_id,
>         return -EINVAL;
>  }
>
> +static u8 __get_largest_member_size(struct btf *btf, const struct btf_type *t)
> +{
> +       const struct btf_member *member;
> +       const struct btf_type *mtype;
> +       u8 largest_member_size = 0;
> +       int i;
> +
> +       if (!__btf_type_is_struct(t))
> +               return largest_member_size;
> +
> +       for_each_member(i, t, member) {
> +               mtype = btf_type_by_id(btf, member->type);
> +               while (mtype && btf_type_is_modifier(mtype))
> +                       mtype = btf_type_by_id(btf, mtype->type);
> +               if (!mtype)
> +                       return -EINVAL;
> +               if (mtype->size > largest_member_size)
> +                       largest_member_size = mtype->size;
> +       }
> +
> +       return largest_member_size;
> +}
> +
>  static u8 __get_type_fmodel_flags(const struct btf_type *t)
>  {
>         u8 flags = 0;
> @@ -7396,6 +7419,8 @@ int btf_distill_func_proto(struct bpf_verifier_log *log,
>                 }
>                 m->arg_size[i] = ret;
>                 m->arg_flags[i] = __get_type_fmodel_flags(t);
> +               m->arg_largest_member_size[i] =
> +                       __get_largest_member_size(btf, t);
>         }
>         m->nr_args = nargs;
>         return 0;
>
> --
> 2.49.0
>