pipe: simplify round_pipe_size()

round_pipe_size() calculates the number of pages the requested size
corresponds to, then rounds the page count up to the next power of 2.

However, it also rounds everything < PAGE_SIZE up to PAGE_SIZE.
Therefore, there's no need to actually translate the size into a page
count; we just need to round the size up to the next power of 2.

We do need to verify the size isn't greater than (1 << 31), since on
32-bit systems roundup_pow_of_two() would be undefined in that case.  But
that can just be combined with the UINT_MAX check which we need anyway
now.

Finally, update pipe_set_size() to not redundantly check the return value
of round_pipe_size() for the "invalid size" case twice.

Link: http://lkml.kernel.org/r/20180111052902.14409-7-ebiggers3@gmail.com
Signed-off-by: Eric Biggers <ebiggers@google.com>
Acked-by: Kees Cook <keescook@chromium.org>
Acked-by: Joe Lawrence <joe.lawrence@redhat.com>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: "Luis R . Rodriguez" <mcgrof@kernel.org>
Cc: Michael Kerrisk <mtk.manpages@gmail.com>
Cc: Mikulas Patocka <mpatocka@redhat.com>
Cc: Willy Tarreau <w@1wt.eu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/pipe.c

@@ -1022,20 +1022,14 @@ const struct file_operations pipefifo_fops = {
  */
 unsigned int round_pipe_size(unsigned long size)
 {
-	unsigned long nr_pages;
-
-	if (size > UINT_MAX)
+	if (size > (1U << 31))
 		return 0;
 
 	/* 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)
-		return 0;
+		return PAGE_SIZE;
 
-	return roundup_pow_of_two(nr_pages) << PAGE_SHIFT;
+	return roundup_pow_of_two(size);
 }
 
 /*
@@ -1050,8 +1044,6 @@ static long pipe_set_size(struct pipe_inode_info *pipe, unsigned long arg)
 	long ret = 0;
 
 	size = round_pipe_size(arg);
-	if (size == 0)
-		return -EINVAL;
 	nr_pages = size >> PAGE_SHIFT;
 
 	if (!nr_pages)