linux-kernel - [PATCH 6/7] pipe: simplify round_pipe

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Date:   Sun,  7 Jan 2018 21:35:41 -0800
From:   Eric Biggers <ebiggers3@...il.com>
To:     linux-fsdevel@...r.kernel.org
Cc:     Alexander Viro <viro@...iv.linux.org.uk>,
        Joe Lawrence <joe.lawrence@...hat.com>,
        Michael Kerrisk <mtk.manpages@...il.com>,
        Willy Tarreau <w@....eu>,
        Mikulas Patocka <mpatocka@...hat.com>,
        "Luis R . Rodriguez" <mcgrof@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        linux-kernel@...r.kernel.org, Eric Biggers <ebiggers@...gle.com>
Subject: [PATCH 6/7] pipe: simplify round_pipe_size()

From: Eric Biggers <ebiggers@...gle.com>

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 that bit 31 isn't set, 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, also remove the check for '!nr_pages' in pipe_set_size(), since
round_pipe_size() always returns either 0 or a multiple of PAGE_SIZE.

Signed-off-by: Eric Biggers <ebiggers@...gle.com>
---
 fs/pipe.c | 15 +++------------
 1 file changed, 3 insertions(+), 12 deletions(-)

diff --git a/fs/pipe.c b/fs/pipe.c
index f1ee1e599495..774cafd947dc 100644
--- a/fs/pipe.c
+++ b/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);
 }
 
 /*
@@ -1054,9 +1048,6 @@ static long pipe_set_size(struct pipe_inode_info *pipe, unsigned long arg)
 		return -EINVAL;
 	nr_pages = size >> PAGE_SHIFT;
 
-	if (!nr_pages)
-		return -EINVAL;
-
 	/*
 	 * If trying to increase the pipe capacity, check that an
 	 * unprivileged user is not trying to exceed various limits
-- 
2.15.1