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Message-ID: <20181017095155.GA354@infradead.org>
Date: Wed, 17 Oct 2018 02:51:55 -0700
From: Christoph Hellwig <hch@...radead.org>
To: "Joel Fernandes (Google)" <joel@...lfernandes.org>
Cc: linux-kernel@...r.kernel.org, kernel-team@...roid.com,
jreck@...gle.com, john.stultz@...aro.org, tkjos@...gle.com,
gregkh@...uxfoundation.org,
Andrew Morton <akpm@...ux-foundation.org>, dancol@...gle.com,
"J. Bruce Fields" <bfields@...ldses.org>,
Jeff Layton <jlayton@...nel.org>,
Khalid Aziz <khalid.aziz@...cle.com>,
linux-fsdevel@...r.kernel.org, linux-kselftest@...r.kernel.org,
linux-mm@...ck.org, Mike Kravetz <mike.kravetz@...cle.com>,
minchan@...gle.com, Shuah Khan <shuah@...nel.org>
Subject: Re: [PATCH v2 1/2] mm: Add an F_SEAL_FS_WRITE seal to memfd
On Tue, Oct 09, 2018 at 03:20:41PM -0700, Joel Fernandes (Google) wrote:
> One of the main usecases Android has is the ability to create a region
> and mmap it as writeable, then drop its protection for "future" writes
> while keeping the existing already mmap'ed writeable-region active.
s/drop/add/ ?
Otherwise this doesn't make much sense to me.
> This usecase cannot be implemented with the existing F_SEAL_WRITE seal.
> To support the usecase, this patch adds a new F_SEAL_FS_WRITE seal which
> prevents any future mmap and write syscalls from succeeding while
> keeping the existing mmap active. The following program shows the seal
> working in action:
Where does the FS come from? I'd rather expect this to be implemented
as a 'force' style flag that applies the seal even if the otherwise
required precondition is not met.
> Note: This seal will also prevent growing and shrinking of the memfd.
> This is not something we do in Android so it does not affect us, however
> I have mentioned this behavior of the seal in the manpage.
This seems odd, as that is otherwise split into the F_SEAL_SHRINK /
F_SEAL_GROW flags.
> static int memfd_add_seals(struct file *file, unsigned int seals)
> {
> @@ -219,6 +220,9 @@ static int memfd_add_seals(struct file *file, unsigned int seals)
> }
> }
>
> + if ((seals & F_SEAL_FS_WRITE) && !(*file_seals & F_SEAL_FS_WRITE))
> + file->f_mode &= ~(FMODE_WRITE | FMODE_PWRITE);
> +
This seems to lack any synchronization for f_mode.
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