linux-kernel - Re: [PATCH v2] Revert "fuse: Apply flags2 only when userspace set the FUSE_INIT

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Message-ID: <1b03f355170333f20ee20e47c5f355dc73d3a91c.camel@linaro.org>
Date:   Wed, 18 Oct 2023 15:26:01 +0100
From:   André Draszik <andre.draszik@...aro.org>
To:     Bernd Schubert <bschubert@....com>
Cc:     Miklos Szeredi <mszeredi@...hat.com>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] Revert "fuse: Apply flags2 only when userspace set
 the FUSE_INIT_EXT"

On Wed, 2023-10-18 at 11:52 +0000, Bernd Schubert wrote:
> On 10/18/23 13:46, André Draszik wrote:
> > On Wed, 2023-10-18 at 11:39 +0000, Bernd Schubert wrote:
> > > On 10/18/23 13:15, André Draszik wrote:
> > > > From: André Draszik <andre.draszik@...aro.org>
> > > > 
> > > > This reverts commit 3066ff93476c35679cb07a97cce37d9bb07632ff.
> > > > 
> > > > This patch breaks all existing userspace by requiring updates
> > > > as
> > > > mentioned in the commit message, which is not allowed.
> > > > 
> > > > Revert to restore compatibility with existing userspace
> > > > implementations.
> > > 
> > > Which fuse file system does it exactly break? In fact there
> > > haven't
> > > been
> > > added too many flags after - what exactly is broken?
> > 
> > The original patch broke the existing kernel <-> user ABI by now
> > requiring user space applications to pass in an extra flag.
> > There are various side-effects of this, like unbootable systems,
> > just
> > because the kernel was updated.
> > Breaking the ABI is the one thing that is not allowed. This is not
> > specific to any particular fuse file system.
> 
> How exactly did it break it?

At least in Android, creating new files, or reading existing files
returns -EFAULT

>  These are feature flags - is there really a 
> file system that relies on these flag to the extend that it does not 
> work anymore?

I don't know enough about the implementation details, but even outside
Android user space had to be updated as a prerequisite for this kernel
patch:
https://lore.kernel.org/all/YmUKZQKNAGimupv7@redhat.com/
https://github.com/libfuse/libfuse/pull/662

Which means any non-Android user space predating those changes isn't
working anymore either.



Cheers,
Andre