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

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Message-ID: <YnERsPIsiOCa8cty@zx2c4.com>
Date:   Tue, 3 May 2022 13:27:44 +0200
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     Lennart Poettering <mzxreary@...inter.de>
Cc:     linux-kernel@...r.kernel.org, linux-crypto@...r.kernel.org
Subject: Re: [PATCH 1/2] sysctl: read() must consume poll events, not poll()

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. :(

Jason