[PATCH] fs: pipe, fix undefined behaviour

[Jiri Slaby <jslaby@suse.cz>]

echo | ./program
where ./program contains fcntl(0, F_SETPIPE_SZ, 0) this is triggered:
UBSAN: Undefined behaviour in ../include/linux/log2.h:63:13
shift exponent 64 is too large for 64-bit type 'long unsigned int'
CPU: 3 PID: 4978 Comm: syz-executor Not tainted 4.8.1-0-syzkaller #1
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.9.1-0-gb3ef39f-prebuilt.qemu-project.org 04/01/2014
...
Call Trace:
...
 [<ffffffffa18ced53>] ? pipe_fcntl+0xa3/0x7a0
 [<ffffffffa18f1940>] ? SyS_fcntl+0x930/0xf30
 [<ffffffffa2d1fb00>] ? entry_SYSCALL_64_fastpath+0x23/0xc1

Documentaion of roundup_pow_of_two explicitly mentions that passing
size 0 is undefined. So return 0 from round_pipe_size prematurely to
fix this.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: linux-fsdevel@vger.kernel.org
---
 fs/pipe.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/fs/pipe.c b/fs/pipe.c
index 8e0d9f26dfad..23c0c06ffc33 100644
--- a/fs/pipe.c
+++ b/fs/pipe.c
@@ -1024,6 +1024,9 @@ static inline unsigned int round_pipe_size(unsigned int size)
 {
 	unsigned long nr_pages;
 
+	if (!size)
+		return 0;
+
 	nr_pages = (size + PAGE_SIZE - 1) >> PAGE_SHIFT;
 	return roundup_pow_of_two(nr_pages) << PAGE_SHIFT;
 }
-- 
2.10.1