[PATCH bpf-next] libbpf: make remark about zero-initializing bpf_*_info structs

[Matt Bobrowski <mattbobrowski@google.com>]

In some situations, if you fail to zero-initialize the bpf_*_info
buffer 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_*_info buffers for the respective
LIBBPF helpers to prevent exactly this situation.

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_*_info is 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 bpf_*_info types known to the kernel.

Signed-off-by: Matt Bobrowski <mattbobrowski@google.com>
---
 src/bpf.h | 20 ++++++++++++++++----
 1 file changed, 16 insertions(+), 4 deletions(-)

diff --git a/src/bpf.h b/src/bpf.h
index f866e98..568bcb3 100644
--- a/src/bpf.h
+++ b/src/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 before calling into this helper. 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 before calling into this helper. 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
@@ -534,7 +540,10 @@ LIBBPF_API int bpf_map_get_info_by_fd(int map_fd, struct bpf_map_info *info, __u
  * 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 before calling into this helper. 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 before calling into this helper. 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