seccomp_unotify.2: Document SECCOMP_ADDFD_FLAG_SEND

This flag was recently added to Linux 5.14 by a patch I wrote:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=0ae71c7720e3ae3aabd2e8a072d27f7bd173d25c

This patch adds documentation for the flag, the error code that the flag
added and explains in the caveat when it is useful.

Signed-off-by: Rodrigo Campos <rodrigo@kinvolk.io>
Signed-off-by: Alejandro Colomar <alx.manpages@gmail.com>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

diff --git a/man2/seccomp_unotify.2 b/man2/seccomp_unotify.2
index 2673d9bc756264485b48a40ab63fc23b6bf23e36..9bd27214f0496275c1c0229f69b3614848abe825 100644 (file)
--- a/man2/seccomp_unotify.2
+++ b/man2/seccomp_unotify.2
@@ -739,6 +739,17 @@ When allocating the file descriptor in the target,
 use the file descriptor number specified in the
 .I newfd
 field.
+.TP
+.BR SECCOMP_ADDFD_FLAG_SEND
+Available since Linux 5.14, combines the
+.B SECCOMP_IOCTL_NOTIF_ADDFD
+ioctl with
+.B SECCOMP_IOCTL_NOTIF_SEND
+into an atomic operation. On successful invocation, the target process's
+errno will be 0 and the return value will be the file descriptor number that was
+installed in the target. If allocating the file descriptor in the tatget fails,
+the target's syscall continues to be blocked until a successful response is
+sent.
 .RE
 .TP
 .I srcfd
@@ -801,6 +812,13 @@ Allocating the file descriptor in the target would cause the target's
 limit to be exceeded (see
 .BR getrlimit (2)).
 .TP
+.B EBUSY
+If the flag
+.B SECCOMP_IOCTL_NOTIF_SEND
+is used, this means the operation can't proceed until other
+.B SECCOMP_IOCTL_NOTIF_ADDFD
+requests are processed.
+.TP
 .B EINPROGRESS
 The user-space notification specified in the
 .I id
@@ -1131,6 +1149,14 @@ that would
 normally be restarted by the
 .BR SA_RESTART
 flag.
+.PP
+Furthermore, if the supervisor response is a file descriptor
+added with
+.B SECCOMP_IOCTL_NOTIF_ADDFD,
+then the flag
+.B SECCOMP_ADDFD_FLAG_SEND
+can be used to atomically add the file descriptor and return that value,
+making sure no file descriptors are inadvertently leaked into the target.
 .\" FIXME
 .\" About the above, Kees Cook commented:
 .\"