linux-kernel - Re: [PATCH v4 4/4] ovl: Optimize override/revert creds

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Message-ID: <87h689sf6p.fsf@intel.com>
Date: Thu, 14 Nov 2024 13:01:18 -0800
From: Vinicius Costa Gomes <vinicius.gomes@...el.com>
To: Amir Goldstein <amir73il@...il.com>
Cc: brauner@...nel.org, miklos@...redi.hu, hu1.chen@...el.com,
 malini.bhandaru@...el.com, tim.c.chen@...el.com, mikko.ylinen@...el.com,
 linux-unionfs@...r.kernel.org, linux-fsdevel@...r.kernel.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 4/4] ovl: Optimize override/revert creds

Amir Goldstein <amir73il@...il.com> writes:

> On Thu, Nov 14, 2024 at 9:56 AM Amir Goldstein <amir73il@...il.com> wrote:
>>
>> On Wed, Nov 13, 2024 at 8:30 PM Vinicius Costa Gomes
>> <vinicius.gomes@...el.com> wrote:
>> >
>> > Amir Goldstein <amir73il@...il.com> writes:
>> >
>> > > On Thu, Nov 7, 2024 at 1:57 AM Vinicius Costa Gomes
>> > > <vinicius.gomes@...el.com> wrote:
>> >
>> > [...]
>> >
>> > >
>> > > Vinicius,
>> > >
>> > > While testing fanotify with LTP tests (some are using overlayfs),
>> > > kmemleak consistently reports the problems below.
>> > >
>> > > Can you see the bug, because I don't see it.
>> > > Maybe it is a false positive...
>> >
>> > Hm, if the leak wasn't there before and we didn't touch anything related to
>> > prepare_creds(), I think that points to the leak being real.
>> >
>> > But I see your point, still not seeing it.
>> >
>> > This code should be equivalent to the code we have now (just boot
>> > tested):
>> >
>> > ----
>> > diff --git a/fs/overlayfs/dir.c b/fs/overlayfs/dir.c
>> > index 136a2c7fb9e5..7ebc2fd3097a 100644
>> > --- a/fs/overlayfs/dir.c
>> > +++ b/fs/overlayfs/dir.c
>> > @@ -576,8 +576,7 @@ static int ovl_setup_cred_for_create(struct dentry *dentry, struct inode *inode,
>> >          * We must be called with creator creds already, otherwise we risk
>> >          * leaking creds.
>> >          */
>> > -       WARN_ON_ONCE(override_creds(override_cred) != ovl_creds(dentry->d_sb));
>> > -       put_cred(override_cred);
>> > +       WARN_ON_ONCE(override_creds_light(override_cred) != ovl_creds(dentry->d_sb));
>> >
>> >         return 0;
>> >  }
>> > ----
>> >
>> > Does it change anything? (I wouldn't think so, just to try something)
>>
>> No, but I think this does:
>>
>
> Vinicius,
>
> Sorry, your fix is correct. I did not apply it properly when I tested.
>
> I edited the comment as follows and applied on top of the patch
> that I just sent [1]:
>

I just noticed there's a typo in the first sentence of the commit
message, the function name that we are using revert_creds_light() is
ovl_revert_creds(). Could you fix that while you are at it?

Glad that the fix works:

Acked-by: Vinicius Costa Gomes <vinicius.gomes@...el.com>

>
> -       put_cred(override_creds(override_cred));
> +
> +       /*
> +        * Caller is going to match this with revert_creds_light() and drop
> +        * reference on the returned creds.
> +        * We must be called with creator creds already, otherwise we risk
> +        * leaking creds.
> +        */
> +       old_cred = override_creds_light(override_cred);
> +       WARN_ON_ONCE(old_cred != ovl_creds(dentry->d_sb));
>
>         return override_cred;
>
> Thanks,
> Amir.
>
> [1] https://lore.kernel.org/linux-unionfs/20241114100536.628162-1-amir73il@gmail.com/

-- 
Vinicius