pipe, sysctl: drop 'min' parameter from pipe-max-size converter

Patch series "pipe: buffer limits fixes and cleanups", v2.

This series simplifies the sysctl handler for pipe-max-size and fixes
another set of bugs related to the pipe buffer limits:

- The root user wasn't allowed to exceed the limits when creating new
  pipes.

- There was an off-by-one error when checking the limits, so a limit of
  N was actually treated as N - 1.

- F_SETPIPE_SZ accepted values over UINT_MAX.

- Reading the pipe buffer limits could be racy.

This patch (of 7):

Before validating the given value against pipe_min_size,
do_proc_dopipe_max_size_conv() calls round_pipe_size(), which rounds the
value up to pipe_min_size.  Therefore, the second check against
pipe_min_size is redundant.  Remove it.

Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Eric Biggers <[email protected]>
Acked-by: Kees Cook <[email protected]>
Acked-by: Joe Lawrence <[email protected]>
Cc: Alexander Viro <[email protected]>
Cc: "Luis R . Rodriguez" <[email protected]>
Cc: Michael Kerrisk <[email protected]>
Cc: Mikulas Patocka <[email protected]>
Cc: Willy Tarreau <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

fs/pipe.c

@@ -35,11 +35,6 @@
  */
 unsigned int pipe_max_size = 1048576;
 
-/*
- * Minimum pipe size, as required by POSIX
- */
-unsigned int pipe_min_size = PAGE_SIZE;
-
 /* Maximum allocatable pages per user. Hard limit is unset by default, soft
  * matches default values.
  */
@@ -1024,8 +1019,9 @@ unsigned int round_pipe_size(unsigned int size)
 {
 	unsigned long nr_pages;
 
-	if (size < pipe_min_size)
-		size = pipe_min_size;
+	/* Minimum pipe size, as required by POSIX */
+	if (size < PAGE_SIZE)
+		size = PAGE_SIZE;
 
 	nr_pages = (size + PAGE_SIZE - 1) >> PAGE_SHIFT;
 	if (nr_pages == 0)