[PATCH v2 8/8] pipe: cap initial pipe capacity according to pipe-max-size limit

["Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com>]

This is a patch that provides behavior that is more consistent, and
probably less surprising to users. I consider the change optional, and
welcome opinions about whether it should be applied.

By default, pipes are created with a capacity of 64 kiB.  However,
/proc/sys/fs/pipe-max-size may be set smaller than this value.  In this
scenario, an unprivileged user could thus create a pipe whose initial
capacity exceeds the limit. Therefore, it seems logical to cap the
initial pipe capacity according to the value of pipe-max-size.

The test program shown earlier in this patch series can be used to
demonstrate the effect of the change brought about with this patch:

    # cat /proc/sys/fs/pipe-max-size
    1048576
    # sudo -u mtk ./test_F_SETPIPE_SZ 1
    Initial pipe capacity: 65536
    # echo 10000 > /proc/sys/fs/pipe-max-size
    # cat /proc/sys/fs/pipe-max-size
    16384
    # sudo -u mtk ./test_F_SETPIPE_SZ 1
    Initial pipe capacity: 16384
    # ./test_F_SETPIPE_SZ 1
    Initial pipe capacity: 65536

The last two executions of 'test_F_SETPIPE_SZ' show that pipe-max-size
caps the initial allocation for a new pipe for unprivileged users, but
not for privileged users.

Patch history:

v2
   * Switch order of tests in an 'if' statement to avoid a function
     call (to capability()) in normal path. [Thanks to Willy Tarreau]

Cc: Willy Tarreau <w@1wt.eu>
Cc: Vegard Nossum <vegard.nossum@oracle.com>
Cc: socketpair@gmail.com
Cc: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Cc: Jens Axboe <axboe@fb.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: linux-api@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
---
 fs/pipe.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/fs/pipe.c b/fs/pipe.c
index 741560b..10c5ad5 100644
--- a/fs/pipe.c
+++ b/fs/pipe.c
@@ -631,6 +631,9 @@ struct pipe_inode_info *alloc_pipe_info(void)
 	if (pipe == NULL)
 		goto out_free_uid;
 
+	if (pipe_bufs * PAGE_SIZE > pipe_max_size && !capable(CAP_SYS_RESOURCE))
+		pipe_bufs = pipe_max_size >> PAGE_SHIFT;
+
 	user_bufs = account_pipe_buffers(user, 0, pipe_bufs);
 
 	if (too_many_pipe_buffers_soft(user_bufs)) {
-- 
2.5.5