[PATCH 5.15 681/913] fs: fix fd table size alignment properly

From: Greg Kroah-Hartman
Date: Tue Apr 05 2022 - 18:10:41 EST

From: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>

[ Upstream commit d888c83fcec75194a8a48ccd283953bdba7b2550 ]

Jason Donenfeld reports that my commit 1c24a186398f ("fs: fd tables have
to be multiples of BITS_PER_LONG") doesn't work, and the reason is an
embarrassing brown-paper-bag bug.

Yes, we want to align the number of fds to BITS_PER_LONG, and yes, the
reason they might not be aligned is because the incoming 'max_fd'
argument might not be aligned.

But aligining the argument - while simple - will cause a "infinitely
big" maxfd (eg NR_OPEN_MAX) to just overflow to zero. Which most
definitely isn't what we want either.

The obvious fix was always just to do the alignment last, but I had
moved it earlier just to make the patch smaller and the code look
simpler. Duh. It certainly made _me_ look simple.

Fixes: 1c24a186398f ("fs: fd tables have to be multiples of BITS_PER_LONG")
Reported-and-tested-by: Jason A. Donenfeld <Jason@xxxxxxxxx>
Cc: Fedor Pchelkin <aissur0002@xxxxxxxxx>
Cc: Alexey Khoroshilov <khoroshilov@xxxxxxxxx>
Cc: Christian Brauner <brauner@xxxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
fs/file.c | 3 +--
1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/fs/file.c b/fs/file.c
index c01c29417ae6..ee9317346702 100644
--- a/fs/file.c
+++ b/fs/file.c
@@ -303,10 +303,9 @@ static unsigned int sane_fdtable_size(struct fdtable *fdt, unsigned int max_fds)
unsigned int count;

count = count_open_files(fdt);
- max_fds = ALIGN(max_fds, BITS_PER_LONG);
if (max_fds < NR_OPEN_DEFAULT)
max_fds = NR_OPEN_DEFAULT;
- return min(count, max_fds);
+ return ALIGN(min(count, max_fds), BITS_PER_LONG);