linux-kernel - Re: [RFC v10][PATCH 05/13] Dump memory address space

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Date:	Mon, 01 Dec 2008 15:57:09 -0500
From:	Oren Laadan <orenl@...columbia.edu>
To:	Dave Hansen <dave@...ux.vnet.ibm.com>
CC:	Al Viro <viro@...IV.linux.org.uk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...l.org>,
	containers@...ts.linux-foundation.org,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	linux-api@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
	Serge Hallyn <serue@...ibm.com>, Ingo Molnar <mingo@...e.hu>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [RFC v10][PATCH 05/13] Dump memory address space



Dave Hansen wrote:
> On Fri, 2008-11-28 at 10:53 +0000, Al Viro wrote:
>>> +static int cr_ctx_checkpoint(struct cr_ctx *ctx, pid_t pid)
>>> +{
>>> +     ctx->root_pid = pid;
>>> +
>>> +     /*
>>> +      * assume checkpointer is in container's root vfs
>>> +      * FIXME: this works for now, but will change with real containers
>>> +      */
>>> +     ctx->vfsroot = &current->fs->root;
>>> +     path_get(ctx->vfsroot);
>> This is going to break as soon as you get another thread doing e.g. chroot(2)
>> while you are in there.
> 
> Yeah, we do need at least a read_lock(&current->fs->lock) to keep people
> from chroot()'ing underneath us.

True.
(while adapting older and safer code I omitted these tests with no reason).

> 
>> And it's a really, _really_ bad idea to take a
>> pointer to shared object, increment refcount on the current *contents* of
>> said object and assume that dropping refcount on the later contents of the
>> same will balance out.
> 
> Absolutely.  I assume you mean get_fs_struct(current) instead of
> path_get().

True.

Should change the type of ctx->vfsroot to not be a pointer, and do:

>>> +     ctx->vfsroot = *current->fs->root;
>>> +     path_get(&ctx->vfsroot);

and adjust accordingly in where the refcount is dropped.

What we need here is a reference point (this will change later when we handle
multiple fs-namespaces), which is the path of the "container root". Assuming
locking is correct so that current->fs does not change under us, it's enough
to get that path and later release that path.

BW, the current->fs is assumed to not change during the checkpoint; if it does,
then it's a mis-use of the checkpoint interface, and the resulting behavior
is undefined - restart is guaranteed to restore the exact old state even if
checkpoint succeeds.

Thanks,

Oren.

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