From 1159d27852207e8efb8d6ef2dae5aaa87ec4e225 Mon Sep 17 00:00:00 2001 From: Matt Bobrowski Date: Wed, 14 Feb 2024 09:14:23 +0000 Subject: libbpf: Make remark about zero-initializing bpf_*_info structs 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 Signed-off-by: Andrii Nakryiko Acked-by: Jiri Olsa Link: https://lore.kernel.org/bpf/ZcyEb8x4VbhieWsL@google.com --- 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 -- cgit v1.2.3