linux-kernel - [PATCH 5.17 0832/1126] fs: fix fd table size alignment properly

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Date:   Tue,  5 Apr 2022 09:26:19 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Fedor Pchelkin <aissur0002@...il.com>,
        Alexey Khoroshilov <khoroshilov@...ras.ru>,
        Christian Brauner <brauner@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Sasha Levin <sashal@...nel.org>,
        "Jason A . Donenfeld" <Jason@...c4.com>
Subject: [PATCH 5.17 0832/1126] fs: fix fd table size alignment properly

From: Linus Torvalds <torvalds@...ux-foundation.org>

[ 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@...c4.com>
Cc: Fedor Pchelkin <aissur0002@...il.com>
Cc: Alexey Khoroshilov <khoroshilov@...ras.ru>
Cc: Christian Brauner <brauner@...nel.org>
Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
 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);
 }

 /*
-- 
2.34.1