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

From: Oren Laadan
Date: Mon Dec 01 2008 - 15:57:54 EST

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.

(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().


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.



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