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

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Date:	Mon, 5 Mar 2012 13:14:36 -0800
From:	Matt Helsley <matthltc@...ibm.com>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Matt Helsley <matthltc@...ibm.com>,
	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 Mon, Mar 05, 2012 at 04:28:26PM +0100, Oleg Nesterov wrote:
> I think the patch is simple and self-explanatory, it simply
> does s/mm->exe_file/mm->exe_path/.
> 
> Why do we need mm->exe_file? IIUC, there are 2 reasons:
> 
> 	1. we do not want O(n) proc/pid/exe looking for the 1st
> 	   VM_EXECUTABLE vma.

(Frankly this always seemed like a bit of a hack to me. If someone got
creative with text layout then it could break this hack...)

> 
> 	2. we do not want to rely on vma->vm_file->f_path,
> 	   bprm->file->f_op->mmap can change ->vm_file.
> 
> 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.

> 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. 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.

> PR_SET_MM_EXE_FILE anyway, see http://marc.info/?t=133052865500016
> So perhaps we can add PR_SET_MM_EXE_PATH which accepts NULL as well
> and kill this counter?
> 
> Oleg.

Cheers,
	-Matt Helsley

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