Re: [PATCH RFC bpf-next 1/7] bpf: Add bpf_btf_get_struct_member_info() helper

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential
issue to consider:
- [Medium] bpf: 32-bit return value truncation in
  bpf_btf_get_struct_member_info()
--

commit 053d54c272170ad0de7a256a743bef2e2fac76d1
Author: Siddharth Nayyar <sidnayyar@google.com>

bpf: Add bpf_btf_get_struct_member_info() helper

This commit adds the bpf_btf_get_struct_member_info() helper for
BPF_PROG_TYPE_SYSCALL programs to query struct member byte offsets and
indices or struct sizes from vmlinux or module BTF.

> diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
> index 228b93d4f8c94..8d9188dda34e0 100644
> --- a/kernel/bpf/btf.c
> +++ b/kernel/bpf/btf.c
> @@ -9682,6 +9682,79 @@ const struct bpf_func_proto bpf_btf_find_by_name_kind_proto = {
>  	.arg4_type	= ARG_ANYTHING,
>  };
>  
> +BPF_CALL_4(bpf_btf_get_struct_member_info, u32, btf_id, int, btf_obj_fd,
> +	   const char *, name, int, name_sz)
> +{
> +	const struct btf_member *member;
> +	const struct btf_type *t;
> +	struct btf *btf;
> +	long ret = -EINVAL;
> +	u32 i;

[ ... ]

> +	for_each_member(i, t, member) {
> +		const char *mname = btf_name_by_offset(btf, member->name_off);
> +
> +		if (!mname || !*mname)
> +			continue;
> +		if (!strcmp(mname, name)) {
> +			u32 moff;
> +
> +			if (__btf_member_bitfield_size(t, member)) {
> +				ret = -EOPNOTSUPP;
> +				goto out;
> +			}
> +			moff = __btf_member_bit_offset(t, member) / 8;
> +			ret = moff | (((u64)i) << 32);
> +			goto out;
> +		}
> +	}
> +
> +out:
> +	btf_put(btf);
> +	return ret;
> +}

[Severity: Medium]
Does storing the 64-bit return value in long ret truncate the upper 32
bits on 32-bit architectures?

Because long is 32 bits wide on 32-bit kernels, assigning
moff | (((u64)i) << 32) to ret discards the member index i in the upper
32 bits before returning, breaking callers such as struct_ops light
skeletons when querying any member at index i > 0.  Declaring ret as s64
would preserve both negative error codes and the full 64-bit value.

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20261106-lskel-struct-ops-v1-0-a4486ffc79cb@google.com?part=1