linux-kernel - Re: [PATCH] fs/file: fix the check in find_next

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Date: Thu, 30 May 2024 09:50:03 +0800
From: Yuntao Wang <yuntao.wang@...ux.dev>
To: viro@...iv.linux.org.uk
Cc: brauner@...nel.org,
	jack@...e.cz,
	linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	yuntao.wang@...ux.dev
Subject: Re: [PATCH] fs/file: fix the check in find_next_fd()

On Wed, 29 May 2024 20:03:28 +0100, Al Viro <viro@...iv.linux.org.uk> wrote:

> On Thu, May 30, 2024 at 12:06:56AM +0800, Yuntao Wang wrote:
> > The maximum possible return value of find_next_zero_bit(fdt->full_fds_bits,
> > maxbit, bitbit) is maxbit. This return value, multiplied by BITS_PER_LONG,
> > gives the value of bitbit, which can never be greater than maxfd, it can
> > only be equal to maxfd at most, so the following check 'if (bitbit > maxfd)'
> > will never be true.
> > 
> > Moreover, when bitbit equals maxfd, it indicates that there are no unused
> > fds, and the function can directly return.
> > 
> > Fix this check.
> 
> Hmm...  The patch is correct, AFAICS.  I _think_ what happened is that
> Linus decided to play it safe around the last word.  In the reality
> ->max_fds is always a multiple of BITS_PER_LONG, so there's no boundary
> effects - a word can not cross the ->max_fds boundary, so "no zero
> bits in full_fds_bits under max_fds/BITS_PER_LONG" does mean there's
> no point checking in range starting at round_down(max_fds, BITS_PER_LONG).

Yes.

> Perhaps a comment along the lines of
> 
>         unsigned int maxfd = fdt->max_fds; // always a multiple of BITS_PER_LONG
> 
> would be useful in there...

Actually, we can simplify this issue. When 'bitbit >= maxfd', it indicates that
there are no unused fds in 'fdt->open_fds', and we can directly return maxfd,
regardless of whether maxfd is a multiple of BITS_PER_LONG or not. Therefore, I
think this comment may not be very necessary.

Of course, I don't oppose adding this comment.