Re: [PATCH] fs/file: fix the check in find_next_fd()

[Al Viro <viro@zeniv.linux.org.uk>]

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).

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...