Re: [PATCH stable 1/3] random: restore O_NONBLOCK support

[Greg KH <gregkh@linuxfoundation.org>]

On Thu, Oct 13, 2022 at 09:36:52AM -0600, Jason A. Donenfeld wrote:
> commit cd4f24ae9404fd31fc461066e57889be3b68641b upstream.
> 
> Prior to 5.6, when /dev/random was opened with O_NONBLOCK, it would
> return -EAGAIN if there was no entropy. When the pools were unified in
> 5.6, this was lost. The post 5.6 behavior of blocking until the pool is
> initialized, and ignoring O_NONBLOCK in the process, went unnoticed,
> with no reports about the regression received for two and a half years.
> However, eventually this indeed did break somebody's userspace.
> 
> So we restore the old behavior, by returning -EAGAIN if the pool is not
> initialized. Unlike the old /dev/random, this can only occur during
> early boot, after which it never blocks again.
> 
> In order to make this O_NONBLOCK behavior consistent with other
> expectations, also respect users reading with preadv2(RWF_NOWAIT) and
> similar.
> 
> Fixes: 30c08efec888 ("random: make /dev/random be almost like /dev/urandom")
> Reported-by: Guozihua <guozihua@huawei.com>
> Reported-by: Zhongguohua <zhongguohua1@huawei.com>
> Cc: Al Viro <viro@zeniv.linux.org.uk>
> Cc: Theodore Ts'o <tytso@mit.edu>
> Cc: Andrew Lutomirski <luto@kernel.org>
> Cc: stable@vger.kernel.org
> Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
> ---
>  drivers/char/mem.c    | 4 ++--
>  drivers/char/random.c | 5 +++++
>  2 files changed, 7 insertions(+), 2 deletions(-)

Still breaks on older kernels:

drivers/char/random.c: In function ‘random_read_iter’:
drivers/char/random.c:1299:33: error: ‘IOCB_NOWAIT’ undeclared (first use in this function); did you mean ‘IPC_NOWAIT’?
 1299 |             ((kiocb->ki_flags & IOCB_NOWAIT) ||
      |                                 ^~~~~~~~~~~
      |                                 IPC_NOWAIT
drivers/char/random.c:1299:33: note: each undeclared identifier is reported only once for each function it appears in
drivers/char/mem.c:872:48: error: ‘FMODE_NOWAIT’ undeclared here (not in a function); did you mean ‘FOLL_NOWAIT’?
  872 |          [8] = { "random", 0666, &random_fops, FMODE_NOWAIT },
      |                                                ^~~~~~~~~~~~
      |                                                FOLL_NOWAIT