Re: [PATCH v1 0/3] overlayfs: Optimize override/revert creds

[Vinicius Costa Gomes]

Christian Brauner <brauner@xxxxxxxxxx> writes:

> On Wed, Apr 24, 2024 at 12:15:25PM -0700, Vinicius Costa Gomes wrote:
>> Christian Brauner <brauner@xxxxxxxxxx> 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@xxxxxxxxx/
>> >> 
>> >> 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