linux-kernel - Re: [PATCH 1/2] sysctl: read() must consume poll events, not poll()

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Message-ID: <CAHmME9o5n6TLfB3GsXz4KOt9iwxbT0e4ut65AFMsJsAm7ayNRg@mail.gmail.com>
Date:   Thu, 12 May 2022 20:32:51 +0200
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     Luis Chamberlain <mcgrof@...nel.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Lucas De Marchi <lucas.demarchi@...el.com>,
        Lennart Poettering <mzxreary@...inter.de>,
        linux-kernel@...r.kernel.org, linux-crypto@...r.kernel.org
Subject: Re: [PATCH 1/2] sysctl: read() must consume poll events, not poll()

Hi Eric,

On 5/12/22, Eric W. Biederman <ebiederm@...ssion.com> wrote:
> Luis Chamberlain <mcgrof@...nel.org> writes:
>
>> On Tue, May 03, 2022 at 01:27:44PM +0200, Jason A. Donenfeld wrote:
>>> On Mon, May 02, 2022 at 05:43:21PM +0200, Lennart Poettering wrote:
>>> > On Mo, 02.05.22 17:30, Jason A. Donenfeld (Jason@...c4.com) wrote:
>>> >
>>> > > Just wanted to double check with you that this change wouldn't break
>>> > > how
>>> > > you're using it in systemd for /proc/sys/kernel/hostname:
>>> > >
>>> > > https://github.com/systemd/systemd/blob/39cd62c30c2e6bb5ec13ebc1ecf0d37ed015b1b8/src/journal/journald-server.c#L1832
>>> > > https://github.com/systemd/systemd/blob/39cd62c30c2e6bb5ec13ebc1ecf0d37ed015b1b8/src/resolve/resolved-manager.c#L465
>>> > >
>>> > > I couldn't find anybody else actually polling on it. Interestingly,
>>> > > it
>>> > > looks like sd_event_add_io uses epoll() inside, but you're not
>>> > > hitting
>>> > > the bug that Jann pointed out (because I suppose you're not poll()ing
>>> > > on
>>> > > an epoll fd).
>>> >
>>> > Well, if you made sure this still works, I am fine either way ;-)
>>>
>>> Actually... ugh. It doesn't work. systemd uses uname() to read the host
>>> name, and doesn't actually read() the file descriptor after receiving
>>> the poll event on it. So I guess I'll forget this, and maybe we'll have
>>> to live with sysctl's poll() being broken. :(
>
> We should be able to modify calling uname() to act the same as reading
> the file descriptor.

How? That sounds like madness. read() takes a fd. uname() doesn't. Are
you proposing we walk through the fds of the process calling uname()
til we find a matching one and then twiddle it's private context
state? I mean I guess that'd work, but...

Jason