linux-kernel - Re: [RFC][PATCH] procfs: Add /proc/<pid>/mapped

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Date:	Wed, 14 Jan 2015 14:45:31 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Calvin Owens <calvinowens@...com>
Cc:	Rasmus Villemoes <linux@...musvillemoes.dk>,
	Siddhesh Poyarekar <siddhesh.poyarekar@...il.com>,
	Alexey Dobriyan <adobriyan@...il.com>,
	Oleg Nesterov <oleg@...hat.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Al Viro <viro@...iv.linux.org.uk>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Peter Feiner <pfeiner@...gle.com>,
	Grant Likely <grant.likely@...retlab.ca>,
	linux-kernel <linux-kernel@...r.kernel.org>, <kernel-team@...com>
Subject: Re: [RFC][PATCH] procfs: Add /proc/<pid>/mapped_files

On Wed, 14 Jan 2015 13:03:26 -0800 Calvin Owens <calvinowens@...com> wrote:

> > Well, only when combined with checking vm_file for being NULL. One would
> > also need to ensure that vm_pgoff is 0 for any non-stack,
> > non-file-backed VMA. At which point it is somewhat ugly. 
> > 
> > > One problem with caching the value on clone like this though is that
> > > the stack could change due to a setcontext, but AFAICT we don't care
> > > about that for the process stack either.
> > 
> > If it is important, I guess one could update the info when a task calls
> > setcontext.
> 
> If I understand the current behavior, the "[stack]" marker will get put
> next to *any* mapping that encompasses the current value in the task's
> %sp, regardless of how the mapping was created or ucontext stuff. If
> you use flags on the VMA structs things could potentially be marked as
> stacks even though %sp points somewhere else.
> 
> It's probable that nobody cares (you'd obviously have to be doing crazy
> things to be pointing %sp at arbitrary places), but that's why I was
> hesitant to mess with it.

Fixing the N^2 search would of course be much better than adding a new
proc file to sidestep it.

Could we do something like refreshing the new vma.vm_flags:VM_IS_STACK
on each thread at the time when /proc/PID/maps is opened?  So do a walk
of the threads, use each thread's sp to hunt down the thread's stack's
vma, then set VM_IS_STACK and fill in the new vma.stack_tid field?

There are still several flags unused in vma.vm_flags btw.

I'm not sure that we can repurpose vm_pgoff (or vm_private_data) for
this: a badly behaved thread could make its sp point at a random vma
then trick the kernel into scribbling on that vma's vm_proff?  Adding a
new field to the vma wouldn't kill us, I guess.  That would remove the
need for a VM_IS_STACK.


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