fs: fd tables have to be multiples of BITS_PER_LONG

fs: fd tables have to be multiples of BITS_PER_LONG

This has always been the rule: fdtables have several bitmaps in them,
and as a result they have to be sized properly for bitmaps.  We walk
those bitmaps in chunks of 'unsigned long' in serveral cases, but even
when we don't, we use the regular kernel bitops that are defined to work
on arrays of 'unsigned long', not on some byte array.

Now, the distinction between arrays of bytes and 'unsigned long'
normally only really ends up being noticeable on big-endian systems, but
Fedor Pchelkin and Alexey Khoroshilov reported that copy_fd_bitmaps()
could be called with an argument that wasn't even a multiple of
BITS_PER_BYTE.  And then it fails to do the proper copy even on
little-endian machines.

The bug wasn't in copy_fd_bitmap(), but in sane_fdtable_size(), which
didn't actually sanitize the fdtable size sufficiently, and never made
sure it had the proper BITS_PER_LONG alignment.

That's partly because the alignment historically came not from having to
explicitly align things, but simply from previous fdtable sizes, and
from count_open_files(), which counts the file descriptors by walking
them one 'unsigned long' word at a time and thus naturally ends up doing
sizing in the proper 'chunks of unsigned long'.

But with the introduction of close_range(), we now have an external
source of "this is how many files we want to have", and so
sane_fdtable_size() needs to do a better job.

This also adds that explicit alignment to alloc_fdtable(), although
there it is mainly just for documentation at a source code level.  The
arithmetic we do there to pick a reasonable fdtable size already aligns
the result sufficiently.

In fact,clang notices that the added ALIGN() in that function doesn't
actually do anything, and does not generate any extra code for it.

It turns out that gcc ends up confusing itself by combining a previous
constant-sized shift operation with the variable-sized shift operations
in roundup_pow_of_two().  And probably due to that doesn't notice that
the ALIGN() is a no-op.  But that's a (tiny) gcc misfeature that doesn't
matter.  Having the explicit alignment makes sense, and would actually
matter on a 128-bit architecture if we ever go there.

This also adds big comments above both functions about how fdtable sizes
have to have that BITS_PER_LONG alignment.

Fixes: 60997c3d45d9 ("close_range: add CLOSE_RANGE_UNSHARE")
Reported-by: Fedor Pchelkin <aissur0002@gmail.com>
Reported-by: Alexey Khoroshilov <khoroshilov@ispras.ru>
Link: https://lore.kernel.org/all/20220326114009.1690-1-aissur0002@gmail.com/
Tested-and-acked-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>