On Wed, 29 May 2024 20:03:28 +0100, Al Viro <viro@xxxxxxxxxxxxxxxxxx> 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.
