linux-kernel - Re: (subset) [PATCH RFC v3 00/10] extensible syscalls: CHECK_FIELDS to allow for easier feature detection

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Message-ID: <20241021.213312-daft.handset.rotten.piers-MlL0KIs8tUv7@cyphar.com>
Date: Tue, 22 Oct 2024 08:38:17 +1100
From: Aleksa Sarai <cyphar@...har.com>
To: Christian Brauner <brauner@...nel.org>
Cc: Kees Cook <kees@...nel.org>, Florian Weimer <fweimer@...hat.com>, 
	Arnd Bergmann <arnd@...db.de>, Mark Rutland <mark.rutland@....com>, 
	linux-kernel@...r.kernel.org, linux-api@...r.kernel.org, linux-fsdevel@...r.kernel.org, 
	linux-arch@...r.kernel.org, linux-kselftest@...r.kernel.org, stable@...r.kernel.org, 
	Ingo Molnar <mingo@...hat.com>, Peter Zijlstra <peterz@...radead.org>, 
	Juri Lelli <juri.lelli@...hat.com>, Vincent Guittot <vincent.guittot@...aro.org>, 
	Dietmar Eggemann <dietmar.eggemann@....com>, Steven Rostedt <rostedt@...dmis.org>, 
	Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>, 
	Valentin Schneider <vschneid@...hat.com>, Alexander Viro <viro@...iv.linux.org.uk>, 
	Jan Kara <jack@...e.cz>, Shuah Khan <shuah@...nel.org>
Subject: Re: (subset) [PATCH RFC v3 00/10] extensible syscalls: CHECK_FIELDS
 to allow for easier feature detection

On 2024-10-21, Christian Brauner <brauner@...nel.org> wrote:
> On Thu, 10 Oct 2024 07:40:33 +1100, Aleksa Sarai wrote:
> > This is something that I've been thinking about for a while. We had a
> > discussion at LPC 2020 about this[1] but the proposals suggested there
> > never materialised.
> > 
> > In short, it is quite difficult for userspace to detect the feature
> > capability of syscalls at runtime. This is something a lot of programs
> > want to do, but they are forced to create elaborate scenarios to try to
> > figure out if a feature is supported without causing damage to the
> > system. For the vast majority of cases, each individual feature also
> > needs to be tested individually (because syscall results are
> > all-or-nothing), so testing even a single syscall's feature set can
> > easily inflate the startup time of programs.
> > 
> > [...]
> 
> I think the copy_struct_to_user() is useful especially now that we'll gain
> another user with pidfd_info.

Once we start extending pidfd_info, it might be necessary to add some
more helpers to make it easier to figure out what bits to set in the
returned request mask.

> ---
> 
> Applied to the vfs.usercopy branch of the vfs/vfs.git tree.
> Patches in the vfs.usercopy branch should appear in linux-next soon.
> 
> Please report any outstanding bugs that were missed during review in a
> new review to the original patch series allowing us to drop it.
> 
> It's encouraged to provide Acked-bys and Reviewed-bys even though the
> patch has now been applied. If possible patch trailers will be updated.
> 
> Note that commit hashes shown below are subject to change due to rebase,
> trailer updates or similar. If in doubt, please check the listed branch.
> 
> tree:   https://git.kernel.org/pub/scm/linux/kernel/git/vfs/vfs.git
> branch: vfs.usercopy
> 
> [01/10] uaccess: add copy_struct_to_user helper
>         https://git.kernel.org/vfs/vfs/c/424a55a4a908
> [02/10] sched_getattr: port to copy_struct_to_user
>         https://git.kernel.org/vfs/vfs/c/112cca098a70

-- 
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
<https://www.cyphar.com/>

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