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Message-ID: <87y191wem5.fsf@intel.com>
Date: Thu, 25 Apr 2024 10:12:34 -0700
From: Vinicius Costa Gomes <vinicius.gomes@...el.com>
To: Christian Brauner <brauner@...nel.org>
Cc: amir73il@...il.com, hu1.chen@...el.com, miklos@...redi.hu,
malini.bhandaru@...el.com, tim.c.chen@...el.com, mikko.ylinen@...el.com,
lizhen.you@...el.com, linux-unionfs@...r.kernel.org,
linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1 0/3] overlayfs: Optimize override/revert creds
Christian Brauner <brauner@...nel.org> writes:
> On Wed, Apr 24, 2024 at 12:15:25PM -0700, Vinicius Costa Gomes wrote:
>> Christian Brauner <brauner@...nel.org> writes:
>>
>> > On Tue, Apr 02, 2024 at 07:18:05PM -0700, Vinicius Costa Gomes wrote:
>> >> Hi,
>> >>
>> >> Changes from RFC v3:
>> >> - Removed the warning "fixes" patches, as they could hide potencial
>> >> bugs (Christian Brauner);
>> >> - Added "cred-specific" macros (Christian Brauner), from my side,
>> >> added a few '_' to the guards to signify that the newly introduced
>> >> helper macros are preferred.
>> >> - Changed a few guard() to scoped_guard() to fix the clang (17.0.6)
>> >> compilation error about 'goto' bypassing variable initialization;
>> >>
>> >> Link to RFC v3:
>> >>
>> >> https://lore.kernel.org/r/20240216051640.197378-1-vinicius.gomes@intel.com/
>> >>
>> >> Changes from RFC v2:
>> >> - Added separate patches for the warnings for the discarded const
>> >> when using the cleanup macros: one for DEFINE_GUARD() and one for
>> >> DEFINE_LOCK_GUARD_1() (I am uncertain if it's better to squash them
>> >> together);
>> >> - Reordered the series so the backing file patch is the first user of
>> >> the introduced helpers (Amir Goldstein);
>> >> - Change the definition of the cleanup "class" from a GUARD to a
>> >> LOCK_GUARD_1, which defines an implicit container, that allows us
>> >> to remove some variable declarations to store the overriden
>> >> credentials (Amir Goldstein);
>> >> - Replaced most of the uses of scoped_guard() with guard(), to reduce
>> >> the code churn, the remaining ones I wasn't sure if I was changing
>> >> the behavior: either they were nested (overrides "inside"
>> >> overrides) or something calls current_cred() (Amir Goldstein).
>> >>
>> >> New questions:
>> >> - The backing file callbacks are now called with the "light"
>> >> overriden credentials, so they are kind of restricted in what they
>> >> can do with their credentials, is this acceptable in general?
>> >
>> > Until we grow additional users, I think yes. Just needs to be
>> > documented.
>> >
>>
>> Will add some documentation for it, then.
>>
>> >> - in ovl_rename() I had to manually call the "light" the overrides,
>> >> both using the guard() macro or using the non-light version causes
>> >> the workload to crash the kernel. I still have to investigate why
>> >> this is happening. Hints are appreciated.
>> >
>> > Do you have a reproducer? Do you have a splat from dmesg?
>>
>> Just to be sure, with this version of the series the crash doesn't
>> happen. It was only happening when I was using the guard() macro
>> everywhere.
>>
>> I just looked at my crash collection and couldn't find the splats, from
>> what I remember I lost connection to the machine, and wasn't able to
>> retrieve the splat.
>>
>> I believe the crash and clang 17 compilation error point to the same
>> problem, that in ovl_rename() some 'goto' skips the declaration of the
>> (implicit) variable that the guard() macro generates. And it ends up
>> doing a revert_creds_light() on garbage memory when ovl_rename()
>> returns.
>
> If this is a compiler bug this warrants at least a comment in the commit
> message because right now people will be wondering why that place
> doesn't use a guard. Ideally we can just use guards everywhere though
> and report this as a bug against clang, I think.
>
I am seeing this like a bug/mising feature in gcc (at least in the
version I was using), as clang (correctly) refuses to compile the buggy
code (I agree with the error).
But I will add a comment to the code explaining why guard() cannot be
used in that case.
Cheers,
--
Vinicius
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