Re: [PATCH bpf-next v5 06/11] bpf: Add verifier support for 16-byte returns in R0:R2

[Eduard Zingerman <eddyz87@gmail.com>]

On Thu, 2026-08-13 at 13:02 -0700, Yonghong Song wrote:
> LLVM 23 added support for returning a value in two registers for an
> __int128, or a struct/union whose size is greater than 8 but not more than
> 16 bytes. See LLVM patches [1] and [2].
> 
> Before LLVM 23 the BPF backend could not return these values at all. A
> by-value struct or union return (of any size) was rejected at compile time
> with:
> 
>   error: aggregate returns are not supported
> 
> and an __int128 return failed later in the backend with:
> 
>   fatal error: error in backend: unable to allocate function return #1
> 
> Both are resolved in LLVM 23, which lowers such returns into the R0:R2
> register pair.
> 
> This patch models that pair at calls to global and static BPF subprograms
> and at kfunc calls: R2 is marked alongside R0 at the call, propagated out
> of a callee at its exit, and held to the same scalar-only and no-stack-
> pointer rules that R0 already is. A struct returned by a kfunc must be
> composed of scalars, since its bytes reach the program as raw register
> contents and a pointer field would otherwise be laundered into a scalar.
> 
> An extension program is the one caller of the convention that cannot take
> part in it: its own return value is the program exit code, read out of R0
> alone, so it has no way to hand back an upper half. Replacing a function
> whose return value is larger than 8 bytes is therefore rejected with
> -EOPNOTSUPP rather than supported.
> 
>   [1] https://github.com/llvm/llvm-project/pull/190894
>   [2] https://github.com/llvm/llvm-project/pull/206876
> 
> Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
> ---

Acked-by: Eduard Zingerman <eddyz87@gmail.com>

> @@ -19404,6 +19458,13 @@ int bpf_check_attach_target(struct bpf_verifier_log *log,
>  			return -EOPNOTSUPP;
>  		}
>  
> +		if (prog_extension && tgt_info->fmodel.ret_size > 8) {
> +			bpf_log(log,
> +				"Cannot replace function %s with a >8 byte return value\n",
> +				tname);
> +			return -EOPNOTSUPP;
> +		}
> +

As commented in v4 [1], this check belongs to btf_check_type_match().

[1] https://lore.kernel.org/bpf/e1bf30cac04d54aa328fee5282de348a89494048.camel@gmail.com/

>  		/*
>  		 * *.multi programs don't need an address during program
>  		 * verification, we just take the module ref if needed.