bpf: Document BPF_PROG_PIN syscall command

Commit b2197755 ("bpf: add support for persistent maps/progs")
contains the original implementation and git logs, used as reference for
this documentation.

Also pull in the filename restriction as documented in commit 6d8cb045
("bpf: comment why dots in filenames under BPF virtual FS are not allowed")
Signed-off-by: Joe Stringer <joe@cilium.io>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Reviewed-by: Quentin Monnet <quentin@isovalent.com>
Acked-by: Toke Høiland-Jørgensen <toke@redhat.com>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20210302171947.2268128-5-joe@cilium.io

include/uapi/linux/bpf.h

@@ -219,6 +219,22 @@ union bpf_iter_link_info {
  *		Pin an eBPF program or map referred by the specified *bpf_fd*
  *		to the provided *pathname* on the filesystem.
  *
+ *		The *pathname* argument must not contain a dot (".").
+ *
+ *		On success, *pathname* retains a reference to the eBPF object,
+ *		preventing deallocation of the object when the original
+ *		*bpf_fd* is closed. This allow the eBPF object to live beyond
+ *		**close**\ (\ *bpf_fd*\ ), and hence the lifetime of the parent
+ *		process.
+ *
+ *		Applying **unlink**\ (2) or similar calls to the *pathname*
+ *		unpins the object from the filesystem, removing the reference.
+ *		If no other file descriptors or filesystem nodes refer to the
+ *		same object, it will be deallocated (see NOTES).
+ *
+ *		The filesystem type for the parent directory of *pathname* must
+ *		be **BPF_FS_MAGIC**.
+ *
  *	Return
  *		Returns zero on success. On error, -1 is returned and *errno*
  *		is set appropriately.
@@ -584,13 +600,19 @@ union bpf_iter_link_info {
  *
  * NOTES
  *	eBPF objects (maps and programs) can be shared between processes.
- *	For example, after **fork**\ (2), the child inherits file descriptors
- *	referring to the same eBPF objects. In addition, file descriptors
- *	referring to eBPF objects can be transferred over UNIX domain sockets.
- *	File descriptors referring to eBPF objects can be duplicated in the
- *	usual way, using **dup**\ (2) and similar calls. An eBPF object is
- *	deallocated only after all file descriptors referring to the object
- *	have been closed.
+ *
+ *	* After **fork**\ (2), the child inherits file descriptors
+ *	  referring to the same eBPF objects.
+ *	* File descriptors referring to eBPF objects can be transferred over
+ *	  **unix**\ (7) domain sockets.
+ *	* File descriptors referring to eBPF objects can be duplicated in the
+ *	  usual way, using **dup**\ (2) and similar calls.
+ *	* File descriptors referring to eBPF objects can be pinned to the
+ *	  filesystem using the **BPF_OBJ_PIN** command of **bpf**\ (2).
+ *
+ *	An eBPF object is deallocated only after all file descriptors referring
+ *	to the object have been closed and no references remain pinned to the
+ *	filesystem or attached (for example, bound to a program or device).
  */
 enum bpf_cmd {
 	BPF_MAP_CREATE,