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From:   ebiederm@xmission.com (Eric W. Biederman)
To:     Christoph Hellwig <hch@lst.de>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Jeremy Kerr <jk@ozlabs.org>, Arnd Bergmann <arnd@arndb.de>,
        linuxppc-dev@lists.ozlabs.org, linux-fsdevel@vger.kernel.org,
        linux-kernel@vger.kernel.org
References: <20200414070142.288696-1-hch@lst.de>
        <87r1wl68gf.fsf@x220.int.ebiederm.org> <20200419081926.GA12539@lst.de>
Date:   Sun, 19 Apr 2020 06:50:32 -0500
In-Reply-To: <20200419081926.GA12539@lst.de> (Christoph Hellwig's message of
        "Sun, 19 Apr 2020 10:19:26 +0200")
Message-ID: <87o8rn3d9z.fsf@x220.int.ebiederm.org>
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Subject: Re: remove set_fs calls from the exec and coredump code v2
Sender: linux-kernel-owner@vger.kernel.org
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Christoph Hellwig <hch@lst.de> writes:

> On Fri, Apr 17, 2020 at 05:41:52PM -0500, Eric W. Biederman wrote:
>> > this series gets rid of playing with the address limit in the exec and
>> > coredump code.  Most of this was fairly trivial, the biggest changes are
>> > those to the spufs coredump code.
>> >
>> > Changes since v1:
>> >  - properly spell NUL
>> >  - properly handle the compat siginfo case in ELF coredumps
>> 
>> Quick question is exec from a kernel thread within the scope of what you
>> are looking at?
>> 
>> There is a set_fs(USER_DS) in flush_old_exec whose sole purpose appears
>> to be to allow exec from kernel threads.  Where the kernel threads
>> run with set_fs(KERNEL_DS) until they call exec.
>
> This series doesn't really look at that area.  But I don't think exec
> from a kernel thread makes any sense, and cleaning up how to set the
> initial USER_DS vs KERNEL_DS state is something I'll eventually get to,
> it seems like a major mess at the moment.

Fair enough.  I just wanted to make certain that it is on people's radar
that when the kernel exec's init the arguments are read from kernel
memory and the set_fs(USER_DS) in flush_old_exec() that makes that not
work later.

It is subtle and easy to miss.  So I figured I would mention it since
I have been staring at the exec code a lot lately.

Eric