On Wed, Feb 14, 2024 at 09:14:23AM +0000, Matt Bobrowski wrote:
> In some situations, if you fail to zero-initialize the
> bpf_{prog,map,btf,link}_info structs supplied to the set of LIBBPF
> helpers bpf_{prog,map,btf,link}_get_info_by_fd(), you can expect the
> helper to return an error. This can possibly leave people in a
> situation where they're scratching their heads for an unnnecessary
> amount of time. Make an explicit remark about the requirement of
> zero-initializing the supplied bpf_{prog,map,btf,link}_info structs
> for the respective LIBBPF helpers.
>
> Internally, LIBBPF helpers bpf_{prog,map,btf,link}_get_info_by_fd()
> call into bpf_obj_get_info_by_fd() where the bpf(2)
> BPF_OBJ_GET_INFO_BY_FD command is used. This specific command is
> effectively backed by restrictions enforced by the
> bpf_check_uarg_tail_zero() helper. This function ensures that if the
> size of the supplied bpf_{prog,map,btf,link}_info structs are larger
> than what the kernel can handle, trailing bits are zeroed. This can be
> a problem when compiling against UAPI headers that don't necessarily
> match the sizes of the same underlying types known to the kernel.
>
> Signed-off-by: Matt Bobrowski <mattbobrowski@xxxxxxxxxx>
> ---
>
> v2 to v3:
>
> * Modified the comment wording a little to make it less
> misleading. Specifically, noting that the supplied
> bpf_{prog,map,btf,link}_info structs should be zero-initialized or
> initialized as expected. In some cases, subsequent invocations to
> bpf_{prog,map,btf,link}_get_info_by_fd() helpers don't necessarily
> require the bpf_{prog,map,btf,link}_info struct to be
> zero-initialized, but rather that it just has been properly
> initialized at some point.
Acked-by: Jiri Olsa <jolsa@xxxxxxxxxx>
thanks,
jirka
>
> tools/lib/bpf/bpf.h | 22 +++++++++++++++++-----
> 1 file changed, 17 insertions(+), 5 deletions(-)
>
> diff --git a/tools/lib/bpf/bpf.h b/tools/lib/bpf/bpf.h
> index f866e98b2436..ab2570d28aec 100644
> --- a/tools/lib/bpf/bpf.h
> +++ b/tools/lib/bpf/bpf.h
> @@ -500,7 +500,10 @@ LIBBPF_API int bpf_obj_get_info_by_fd(int bpf_fd, void *info, __u32 *info_len);
> * program corresponding to *prog_fd*.
> *
> * Populates up to *info_len* bytes of *info* and updates *info_len* with the
> - * actual number of bytes written to *info*.
> + * actual number of bytes written to *info*. Note that *info* should be
> + * zero-initialized or initialized as expected by the requested *info*
> + * type. Failing to (zero-)initialize *info* under certain circumstances can
> + * result in this helper returning an error.
> *
> * @param prog_fd BPF program file descriptor
> * @param info pointer to **struct bpf_prog_info** that will be populated with
> @@ -517,7 +520,10 @@ LIBBPF_API int bpf_prog_get_info_by_fd(int prog_fd, struct bpf_prog_info *info,
> * map corresponding to *map_fd*.
> *
> * Populates up to *info_len* bytes of *info* and updates *info_len* with the
> - * actual number of bytes written to *info*.
> + * actual number of bytes written to *info*. Note that *info* should be
> + * zero-initialized or initialized as expected by the requested *info*
> + * type. Failing to (zero-)initialize *info* under certain circumstances can
> + * result in this helper returning an error.
> *
> * @param map_fd BPF map file descriptor
> * @param info pointer to **struct bpf_map_info** that will be populated with
> @@ -530,11 +536,14 @@ LIBBPF_API int bpf_prog_get_info_by_fd(int prog_fd, struct bpf_prog_info *info,
> LIBBPF_API int bpf_map_get_info_by_fd(int map_fd, struct bpf_map_info *info, __u32 *info_len);
>
> /**
> - * @brief **bpf_btf_get_info_by_fd()** obtains information about the
> + * @brief **bpf_btf_get_info_by_fd()** obtains information about the
> * BTF object corresponding to *btf_fd*.
> *
> * Populates up to *info_len* bytes of *info* and updates *info_len* with the
> - * actual number of bytes written to *info*.
> + * actual number of bytes written to *info*. Note that *info* should be
> + * zero-initialized or initialized as expected by the requested *info*
> + * type. Failing to (zero-)initialize *info* under certain circumstances can
> + * result in this helper returning an error.
> *
> * @param btf_fd BTF object file descriptor
> * @param info pointer to **struct bpf_btf_info** that will be populated with
> @@ -551,7 +560,10 @@ LIBBPF_API int bpf_btf_get_info_by_fd(int btf_fd, struct bpf_btf_info *info, __u
> * link corresponding to *link_fd*.
> *
> * Populates up to *info_len* bytes of *info* and updates *info_len* with the
> - * actual number of bytes written to *info*.
> + * actual number of bytes written to *info*. Note that *info* should be
> + * zero-initialized or initialized as expected by the requested *info*
> + * type. Failing to (zero-)initialize *info* under certain circumstances can
> + * result in this helper returning an error.
> *
> * @param link_fd BPF link file descriptor
> * @param info pointer to **struct bpf_link_info** that will be populated with
> --
> 2.43.0.687.g38aa6559b0-goog
>
> /M
