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	Sun, 04 Aug 2024 03:47:39 +0000
Date: Sun, 4 Aug 2024 04:47:39 +0100
From: Al Viro <viro@zeniv.linux.org.uk>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Christian Brauner <brauner@kernel.org>, linux-fsdevel@vger.kernel.org
Subject: Re: [PATCH] fix bitmap corruption on close_range() with
 CLOSE_RANGE_UNSHARE
Message-ID: <20240804034739.GB5334@ZenIV>
References: <20240803225054.GY5334@ZenIV>
 <CAHk-=wgDgy++ydcF6U2GOLAAxTB526ctk1SS7s1y=1HaexwcvA@mail.gmail.com>
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Sender: Al Viro <viro@ftp.linux.org.uk>

On Sun, Aug 04, 2024 at 01:34:05AM +0100, Al Viro wrote:

> static void copy_fd_bitmaps(struct fdtable *nfdt, struct fdtable *ofdt,
>                             unsigned int count, unsigned int size)
> {
> 	bitmap_copy_and_extend(nfdt->open_fds, ofdt->open_fds,
> 				count, size);
> 	bitmap_copy_and_extend(nfdt->close_on_exec, ofdt->close_on_exec,
> 				count, size);
> 	bitmap_copy_and_extend(nfdt->full_fds_bits, ofdt->full_fds_bits,
> 				count / BITS_PER_LONG, size / BITS_PER_LONG);
> }

One variant would be

#define fdt_words(fdt)	((fdt)->max_fds / BITS_PER_LONG) /* longs in fdt->open_fds[] */

static inline void copy_fd_bitmaps(struct fdtable *nfdt, struct fdtable *ofdt,
                            unsigned int copy_words)
{
	unsigned int nwords = fdt_words(nfdt);

	bitmap_copy_and_extend(nfdt->open_fds, ofdt->open_fds,
			copy_words * BITS_PER_LONG, nwords * BITS_PER_LONG);
	bitmap_copy_and_extend(nfdt->close_on_exec, ofdt->close_on_exec,
			copy_words * BITS_PER_LONG, nwords * BITS_PER_LONG);
	bitmap_copy_and_extend(nfdt->full_fds_bits, ofdt->full_fds_bits,
			copy_words, nwords);
}
with callers being
        copy_fd_bitmaps(nfdt, ofdt, fdt_words(ofdt));
and
        copy_fd_bitmaps(new_fdt, old_fdt, open_files / BITS_PER_LONG);

resp.  That would compile into pure memcpy()+memset() for the main bitmaps
and at least as good as bitmap_copy()+bitmap_clear() for full_fds_bits...