linux-kernel - Re: [PATCH 0/1] turn mm->exe_file into mm->exe

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Message-ID: <20120306162814.GA12836@redhat.com>
Date:	Tue, 6 Mar 2012 17:28:14 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Matt Helsley <matthltc@...ibm.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Alexey Dobriyan <adobriyan@...il.com>,
	Cyrill Gorcunov <gorcunov@...nvz.org>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Kees Cook <keescook@...omium.org>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Pavel Emelyanov <xemul@...allels.com>,
	Tejun Heo <tj@...nel.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/1] turn mm->exe_file into mm->exe_path

On 03/05, Matt Helsley wrote:
>
> On Mon, Mar 05, 2012 at 04:28:26PM +0100, Oleg Nesterov wrote:
> >
> > Unless there was another subtle reason, "struct path *exe_path"
> > can equally work but it looks more clear.
>
> PATCH 1/1 looks fine. I think Alexey Dobriyan was working on a similar
> patch years ago.

Good, can I take the above as your Acked-by/Reviewed-by ?


> > And can't we also remove added_exe_file_vma/removed_exe_file_vma?
> > Why do we need mm->num_exe_file_vmas? Afaics it is only needed to
> > "free" mm->exe_file if the application unmaps all these vmas. Say,
> > to allow to unmount fs.
>
> Yup. I know it's not pretty to have to track the exe file refs this way
> but I couldn't see any other way to keep a reference to the file (or
> path) and avoid pinning the mounted filesystem the exectuable is on.
>
> > Can't we simply add PR_CLEAR_MM_EXE_PATH instead? Of course it is
> > not enough if ->vm_file still has a reference. But c/r people want
>
> Relying solely on this prctl would break existing programs.

so we can't :/ I expected this answer.

> I believe Al
> Viro's example was a program that copies its text to a new executable
> area, unmaps the original, performs a pivot_root(), and finally umounts
> the old root. Removing the counter would cause the mount to be pinned
> for these programs and the umount would fail.

Yes, sure, the application should do PR_SET_MM_EXE_PATH(NULL) after unmap.
I am wondering if there is any apllication which actually does this.

But OK, it is hardly possible to argue with "break existing program".

Thanks,

Oleg.

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