linux-kernel - Re: [PATCH v2] kernel: add pid_max to pid

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Message-ID: <Zz9E0pGTioTcH32m@tycho.pizza>
Date: Thu, 21 Nov 2024 07:33:54 -0700
From: Tycho Andersen <tycho@...ho.pizza>
To: Christian Brauner <brauner@...nel.org>
Cc: Yun Zhou <yun.zhou@...driver.com>, stgraber@...raber.org,
	cyphar@...har.com,
	Alexander Mikhalitsyn <aleksandr.mikhalitsyn@...onical.com>,
	mcgrof@...nel.org, kees@...nel.org, joel.granados@...nel.org,
	mhiramat@...nel.org, mathieu.desnoyers@...icios.com,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	linux-trace-kernel@...r.kernel.org
Subject: Re: [PATCH v2] kernel: add pid_max to pid_namespace

On Wed, Nov 20, 2024 at 10:06:27AM +0100, Christian Brauner wrote:
> On Tue, Nov 05, 2024 at 11:10:24AM +0800, Yun Zhou wrote:
> > It is necessary to have a different pid_max in different containers.
> > For example, multiple containers are running on a host, one of which
> > is Android, and its 32 bit bionic libc only accepts pid <= 65535. So
> > it requires the global pid_max <= 65535. This will cause configuration
> > conflicts with other containers and also limit the maximum number of
> > tasks for the entire system.
> > 
> > Signed-off-by: Yun Zhou <yun.zhou@...driver.com>
> > ---
> 
> Fwiw, I've done a patch like this years ago and then Alex revived it in
> [1] including selftests! There's downsides to consider:
> 
> [1]: https://lore.kernel.org/lkml/20240222160915.315255-1-aleksandr.mikhalitsyn@canonical.com

Thanks, looks like this patch has the same oddity.

For me it's enough to just walk up the tree when changing pid_max. It
seems unlikely that applications will create a sub pidns and then lower
the max in their own pid_max. Famous last words and all that.

Tycho