linux-kernel - Re: [PATCH] vfs: vfs_tmpfile: ensure O

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Date:   Thu, 3 Nov 2022 12:33:14 -0700
From:   William McVicker <willmcvicker@...gle.com>
To:     Peter Griffin <peter.griffin@...aro.org>
Cc:     Peter Griffin <peter.griffin@...aro.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Miklos Szeredi <mszeredi@...hat.com>, stable@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        Peter Griffin <gpeter@...gle.com>
Subject: Re: [PATCH] vfs: vfs_tmpfile: ensure O_EXCL flag is enforced

On 11/03/2022, Miklos Szeredi wrote:
> On Thu, 3 Nov 2022 at 18:04, Peter Griffin <peter.griffin@...aro.org> wrote:
> >
> > If O_EXCL is *not* specified, then linkat() can be
> > used to link the temporary file into the filesystem.
> > If O_EXCL is specified then linkat() should fail (-1).
> >
> > After commit 863f144f12ad ("vfs: open inside ->tmpfile()")
> > the O_EXCL flag is no longer honored by the vfs layer for
> > tmpfile, which means the file can be linked even if O_EXCL
> > flag is specified, which is a change in behaviour for
> > userspace!
> >
> > The open flags was previously passed as a parameter, so it
> > was uneffected by the changes to file->f_flags caused by
> > finish_open(). This patch fixes the issue by storing
> > file->f_flags in a local variable so the O_EXCL test
> > logic is restored.
> >
> > This regression was detected by Android CTS Bionic fcntl()
> > tests running on android-mainline [1].
> >
> > [1] https://android.googlesource.com/platform/bionic/+/
> >     refs/heads/master/tests/fcntl_test.cpp#352
> 
> Looks good.
> 
> Acked-by: Miklos Szeredi <mszeredi@...hat.com>
> 
> Thanks,
> Miklos
> >

Thanks Peter for tracking this down! I tested this on the android-mainline
version of 6.1-rc3 on a Pixel 6 device.

Tested-by: Will McVicker <willmcvicker@...gle.com>

Regards,
Will