Re: [PATCH RERESEND 00/11] splice(file<>pipe) I/O on file as-if O_NONBLOCK

[Jens Axboe <axboe@kernel.dk>]

On 12/14/23 11:44 AM, Ahelenia Ziemia?ska wrote:
> First:  https://lore.kernel.org/lkml/cover.1697486714.git.nabijaczleweli@nabijaczleweli.xyz/t/#u
> Resend: https://lore.kernel.org/lkml/1cover.1697486714.git.nabijaczleweli@nabijaczleweli.xyz/t/#u
> Resending again per https://lore.kernel.org/lkml/20231214093859.01f6e2cd@kernel.org/t/#u
> 
> Hi!
> 
> As it stands, splice(file -> pipe):
> 1. locks the pipe,
> 2. does a read from the file,
> 3. unlocks the pipe.
> 
> For reading from regular files and blcokdevs this makes no difference.
> But if the file is a tty or a socket, for example, this means that until
> data appears, which it may never do, every process trying to read from
> or open the pipe enters an uninterruptible sleep,
> and will only exit it if the splicing process is killed.
> 
> This trivially denies service to:
> * any hypothetical pipe-based log collexion system
> * all nullmailer installations
> * me, personally, when I'm pasting stuff into qemu -serial chardev:pipe
> 
> This follows:
> 1. https://lore.kernel.org/linux-fsdevel/qk6hjuam54khlaikf2ssom6custxf5is2ekkaequf4hvode3ls@zgf7j5j4ubvw/t/#u
> 2. a security@ thread rooted in
>    <irrrblivicfc7o3lfq7yjm2lrxq35iyya4gyozlohw24gdzyg7@azmluufpdfvu>
> 3. https://nabijaczleweli.xyz/content/blogn_t/011-linux-splice-exclusion.html
> 
> Patches were posted and then discarded on principle or funxionality,
> all in all terminating in Linus posting
>> But it is possible that we need to just bite the bullet and say
>> "copy_splice_read() needs to use a non-blocking kiocb for the IO".
> 
> This does that, effectively making splice(file -> pipe)
> request (and require) O_NONBLOCK on reads fron the file:
> this doesn't affect splicing from regular files and blockdevs,
> since they're always non-blocking
> (and requesting the stronger "no kernel sleep" IOCB_NOWAIT is non-sensical),

Not sure how you got the idea that regular files or block devices is
always non-blocking, this is certainly not true without IOCB_NOWAIT.
Without IOCB_NOWAIT, you can certainly be waiting for previous IO to
complete.

> but always returns -EINVAL for ttys.
> Sockets behave as expected from O_NONBLOCK reads:
> splice if there's data available else -EAGAIN.
> 
> This should all pretty much behave as-expected.

Should it? Seems like there's a very high risk of breaking existing use
cases here.

Have you at all looked into the approach of enabling splice to/from
_without_ holding the pipe lock? That, to me, would seem like a much
saner approach, with the caveat that I have not looked into that at all
so there may indeed be reasons why this is not feasible.

-- 
Jens Axboe