RE: [PATCH] bpf: Allow small structs to be type of function argument

[John Fastabend <john.fastabend@gmail.com>]

Jiri Olsa wrote:
> This way we can have trampoline on function
> that has arguments with types like:
> 
>   kuid_t uid
>   kgid_t gid
> 
> which unwind into small structs like:
> 
>   typedef struct {
>         uid_t val;
>   } kuid_t;
> 
>   typedef struct {
>         gid_t val;
>   } kgid_t;
> 
> And we can use them in bpftrace like:
> (assuming d_path changes are in)
> 
>   # bpftrace -e 'lsm:path_chown { printf("uid %d, gid %d\n", args->uid, args->gid) }'
>   Attaching 1 probe...
>   uid 0, gid 0
>   uid 1000, gid 1000
>   ...
> 
> Signed-off-by: Jiri Olsa <jolsa@kernel.org>
> ---
>  kernel/bpf/btf.c | 12 +++++++++++-
>  1 file changed, 11 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
> index 58c9af1d4808..f8fee5833684 100644
> --- a/kernel/bpf/btf.c
> +++ b/kernel/bpf/btf.c
> @@ -362,6 +362,14 @@ static bool btf_type_is_struct(const struct btf_type *t)
>  	return kind == BTF_KIND_STRUCT || kind == BTF_KIND_UNION;
>  }
>  
> +/* type is struct and its size is within 8 bytes
> + * and it can be value of function argument
> + */
> +static bool btf_type_is_struct_arg(const struct btf_type *t)
> +{
> +	return btf_type_is_struct(t) && (t->size <= sizeof(u64));

Can you comment on why sizeof(u64) here? The int types can be larger
than 64 for example and don't have a similar check, maybe the should
as well?

Here is an example from some made up program I ran through clang and
bpftool.

[2] INT '__int128' size=16 bits_offset=0 nr_bits=128 encoding=SIGNED

We also have btf_type_int_is_regular to decide if the int is of some
"regular" size but I don't see it used in these paths.

> +}
> +
>  static bool __btf_type_is_struct(const struct btf_type *t)
>  {
>  	return BTF_INFO_KIND(t->info) == BTF_KIND_STRUCT;
> @@ -3768,7 +3776,7 @@ bool btf_ctx_access(int off, int size, enum bpf_access_type type,
>  	/* skip modifiers */
>  	while (btf_type_is_modifier(t))
>  		t = btf_type_by_id(btf, t->type);
> -	if (btf_type_is_int(t) || btf_type_is_enum(t))
> +	if (btf_type_is_int(t) || btf_type_is_enum(t) || btf_type_is_struct_arg(t))
>  		/* accessing a scalar */
>  		return true;
>  	if (!btf_type_is_ptr(t)) {
> @@ -4161,6 +4169,8 @@ static int __get_type_size(struct btf *btf, u32 btf_id,
>  		return sizeof(void *);
>  	if (btf_type_is_int(t) || btf_type_is_enum(t))
>  		return t->size;
> +	if (btf_type_is_struct_arg(t))
> +		return t->size;
>  	*bad_type = t;
>  	return -EINVAL;
>  }
> -- 
> 2.25.4
>