linux-kernel - Re: [PATCH RFC v3 02/10] sched_getattr: port to copy_struct_to

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Message-ID: <87jzaqdpfe.fsf@oldenburg.str.redhat.com>
Date: Mon, 20 Jan 2025 06:28:37 +0100
From: Florian Weimer <fweimer@...hat.com>
To: Xi Ruoyao <xry111@...111.site>
Cc: Christian Brauner <brauner@...nel.org>,  Aleksa Sarai
 <cyphar@...har.com>,  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>,  Arnd Bergmann
 <arnd@...db.de>,  Shuah Khan <shuah@...nel.org>,  Kees Cook
 <kees@...nel.org>,  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,  libc-alpha@...rceware.org
Subject: Re: [PATCH RFC v3 02/10] sched_getattr: port to copy_struct_to_user

* Xi Ruoyao:

> On Wed, 2024-12-11 at 11:23 +0100, Christian Brauner wrote:
>> On Tue, Dec 10, 2024 at 07:14:07PM +0100, Florian Weimer wrote:
>> > * Aleksa Sarai:
>> > 
>> > > sched_getattr(2) doesn't care about trailing non-zero bytes in the
>> > > (ksize > usize) case, so just use copy_struct_to_user() without checking
>> > > ignored_trailing.
>> > 
>> > I think this is what causes glibc's misc/tst-sched_setattr test to fail
>> > on recent kernels.  The previous non-modifying behavior was documented
>> > in the manual page:
>> > 
>> >        If the caller-provided attr buffer is larger than the kernel's
>> >        sched_attr structure, the additional bytes in the user-space
>> >        structure are not touched.
>> > 
>> > I can just drop this part of the test if the kernel deems both behaviors
>> > valid.
>
>> I think in general both behaviors are valid but I would consider zeroing
>> the unknown parts of the provided buffer to be the safer option. And all
>> newer extensible struct system calls do that.
>
> Florian,
>
> So should we drop the test before Glibc-2.41 release?  I'm seeing the
> failure during my machine test.

I was waiting for a verdict from the kernel developers.  I didn't expect
such a change to happen given the alleged UAPI policy.

Thanks,
Florian