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

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Date: Wed, 24 Apr 2024 19:01:32 +0200
From: Christian Brauner <brauner@...nel.org>
To: Vinicius Costa Gomes <vinicius.gomes@...el.com>
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

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.

>  - 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?