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Date:   Wed, 04 Aug 2021 15:48:36 -0400
From:   "Alex Xu (Hello71)" <alex_y_xu@yahoo.ca>
Subject: Re: [REGRESSION?] Simultaneous writes to a reader-less, non-full pipe
 can hang
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     acrichton@mozilla.com, Christian Brauner <christian@brauner.io>,
        David Howells <dhowells@redhat.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        keyrings@vger.kernel.org, Linux API <linux-api@vger.kernel.org>,
        linux-block <linux-block@vger.kernel.org>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Rasmus Villemoes <linux@rasmusvillemoes.dk>,
        LSM List <linux-security-module@vger.kernel.org>,
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        Nicolas Dichtel <nicolas.dichtel@6wind.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Ian Kent <raven@themaw.net>
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Excerpts from Linus Torvalds's message of August 4, 2021 12:31 pm:
> Your program is buggy.
>=20
> On Wed, Aug 4, 2021 at 8:37 AM Alex Xu (Hello71) <alex_y_xu@yahoo.ca> wro=
te:
>>
>>     pipe(pipefd);
>>     printf("init buffer: %d\n", fcntl(pipefd[1], F_GETPIPE_SZ));
>>     printf("new buffer:  %d\n", fcntl(pipefd[1], F_SETPIPE_SZ, 0));
>=20
> Yeah, what did you expect this to do? You said you want a minimal
> buffer, you get a really small buffer.
>=20
> Then you try to write multiple messages to the pipe that you just said
> should have a minimum size.
>=20
> Don't do that then.
>=20
>> /proc/x/stack shows that the remaining thread is hanging at pipe.c:560.
>> It looks like not only there needs to be space in the pipe, but also
>> slots.
>=20
> Correct. The fullness of a pipe is not about whether it has the
> possibility of merging more bytes into an existing not-full slot, but
> about whether it has empty slots left.
>=20
> Part of that is simply the POSIX pipe guarantees - a write of size
> PIPE_BUF or less is guaranteed to be atomic, so it mustn't be split
> among buffers.
>=20
> So a pipe must not be "writable" unless it has space for at least that
> much (think select/poll, which don't know the size of the write).
>=20
> The fact that we might be able to reuse a partially filled buffer for
> smaller writes is simply not relevant to that issue.
>=20
> And yes, we could have different measures of "could write" for
> different writes, but we just don't have or want that complexity.
>=20
> Please don't mess with F_SETPIPE_SZ unless you have a really good
> reason to do so, and actually understand what you are doing.
>=20
> Doing a F_SETPIPE_SZ, 0 basically means "I want the mimimum pipe size
> possible". And that one accepts exactly one write at a time.
>=20
> Of course, the exact semantics are much more complicated than that
> "exactly one write". The pipe write code will optimistically merge
> writes into a previous buffer, which means that depending on the
> pattern of your writes, the exact number of bytes you can write will
> be very different.
>=20
> But that "merge writes into a previous buffer" only appends to the
> buffer - not _reuse_ it - so when each buffer is one page in size,
> what happens is that you can merge up to 4096 bytes worth of writes,
> but then after that the pipe write will want a new buffer - even if
> the old buffer is now empty because of old reads.
>=20
> That's why your test program won't block immediately: both writers
> will actually start out happily doing writes into that one buffer that
> is allocated, but at some point that buffer ends, and it wants to
> allocate a new buffer.
>=20
> But you told it not to allocate more buffers, and the old buffer is
> never completely empty because your readers never read _everythign_,
> so it will hang, waiting for you to empty the one minimal buffer it
> allocated. And that will never happen.
>=20
> There's a very real reason why we do *not* by default say "pipes can
> only ever use only one buffer".
>=20
> I don't think this is a regression, but if you have an actual
> application - not a test program - that does crazy things like this
> and used to work (I'm not sure it has ever worked, though), we can
> look into making it work again.
>=20
> That said, I suspect the way to make it work is to just say "the
> minimum pipe size is two slots" rather than change the "we want at
> least one empty slot". Exactly because of that whole "look, we must
> not consider a pipe that doesn't have a slot writable".
>=20
> Because clearly people don't understand how subtle F_SETPIPE_SZ is.
> It's not really a "byte count", even though that is how it's
> expressed.
>=20
>                    Linus
>=20

I agree that if this only affects programs which intentionally adjust=20
the pipe buffer size, then it is not a huge issue. The problem,=20
admittedly buried very close to the bottom of my email, is that the=20
kernel will silently provide one-page pipe buffers if the user has=20
exceeded 16384 (by default) pipe buffer pages allocated. Try this=20
slightly more complicated program:

#define _GNU_SOURCE
#include <fcntl.h>
#include <pthread.h>
#include <signal.h>
#include <stdio.h>
#include <unistd.h>

static int pipefd[2];

void *thread_start(void *arg) {
    char buf[1];
    int i;
    for (i =3D 0; i < 1000000; i++) {
        read(pipefd[0], buf, sizeof(buf));
        if (write(pipefd[1], buf, sizeof(buf)) =3D=3D -1)
            break;
    }
    printf("done %d\n", i);
    return NULL;
}

int main() {
    signal(SIGPIPE, SIG_IGN);
    // pretend there are a very large number of make processes running
    for (int i =3D 0; i < 1025; i++)
    {
        pipe(pipefd);
        write(pipefd[1], "aa", 2);
    }
    printf("%d %d\n", pipefd[0], pipefd[1]);
    printf("init buffer: %d\n", fcntl(pipefd[1], F_GETPIPE_SZ));
    //printf("new buffer:  %d\n", fcntl(pipefd[1], F_SETPIPE_SZ, 0));
    pthread_t thread1, thread2;
    pthread_create(&thread1, NULL, thread_start, NULL);
    pthread_create(&thread2, NULL, thread_start, NULL);
    sleep(3);
    close(pipefd[0]);
    close(pipefd[1]);
    pthread_join(thread1, NULL);
    pthread_join(thread2, NULL);
}

You may need to raise your RLIMIT_NOFILE before running the program.

With default settings (fs.pipe-user-pages-soft =3D 16384) the init buffer=20
will be 4096, no mangling required. I think this could be probably be=20
solved if the kernel instead reduced over-quota pipes to two pages=20
instead of one page. If someone wants to set a one-page pipe buffer,=20
then they can suffer the consequences, but the kernel shouldn't silently=20
hand people that footgun.

Regards,
Alex.