linux-kernel - Re: [PATCH v3 3/3] exec: do unshare_files after de

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Message-ID: <20180915163704.GA31693@redhat.com>
Date:   Sat, 15 Sep 2018 18:37:04 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     Jeff Layton <jlayton@...nel.org>
Cc:     viro@...iv.linux.org.uk, ebiederm@...ssion.com,
        berrange@...hat.com, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH v3 3/3] exec: do unshare_files after de_thread

On 09/14, Jeff Layton wrote:
>
> POSIX mandates that open fds and their associated file locks should be
> preserved across an execve. This works, unless the process is
> multithreaded at the time that execve is called.
>
> In that case, we'll end up unsharing the files_struct but the locks will
> still have their fl_owner set to the address of the old one. Eventually,
> when the other threads die and the last reference to the old
> files_struct is put, any POSIX locks get torn down since it looks like
> a close occurred on them.
>
> The result is that all of your open files will be intact with none of
> the locks you held before execve. The simple answer to this is "use OFD
> locks", but this is a nasty surprise and it violates the spec.
>
> Fix this by doing unshare_files later during exec,

See my reply to 1/3... if we can forget about the races with get_files_struct()
we can probably make a much simpler patch, plus we do not need 2/2, afaics.

What I really can't understand is why we need to _change_ current->files
early in do_execve().

IOW. Lets ignore do_close_on_exec(), lets ignore the fact that unshare_fd()
can fail and thus it makes sense to call it before point-of-no-return.

Any other reason why we can't simply call unshare_files() at the end of
__do_execve_file() on success?

Oleg.