fs: fix fd table size alignment properly

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@zx2c4.com>
Cc: Fedor Pchelkin <aissur0002@gmail.com>
Cc: Alexey Khoroshilov <khoroshilov@ispras.ru>
Cc: Christian Brauner <brauner@kernel.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/fs/file.c b/fs/file.c
index c01c29417ae6ce303d5dccd730e9ade12c42596d..ee931734670257393aa9ee1383596e233c69d05b 100644 (file)
--- 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);
 }
 
 /*