Re: [PATCH v2 1/2] mm/vmalloc: Add a safer version of find_vm_area() for debug

[Uladzislau Rezki]

On Fri, Sep 01, 2023 at 12:33:21AM +0000, Joel Fernandes wrote:
> On Fri, Sep 01, 2023 at 12:19:17AM +0000, Joel Fernandes wrote:
> > On Thu, Aug 31, 2023 at 09:47:52PM +0200, Uladzislau Rezki wrote:
> > > On Thu, Aug 31, 2023 at 05:18:25PM +0000, Joel Fernandes (Google) wrote:
> > > > It is unsafe to dump vmalloc area information when trying to do so from
> > > > some contexts. Add a safer trylock version of the same function to do a
> > > > best-effort VMA finding and use it from vmalloc_dump_obj().
> > > > 
> > > > [apply test robot feedback on unused function fix.]
> > > > 
> > > > Reported-by: Zhen Lei <thunder.leizhen@xxxxxxxxxxxxxxx>
> > > > Cc: Paul E. McKenney <paulmck@xxxxxxxxxx>
> > > > Cc: rcu@xxxxxxxxxxxxxxx
> > > > Cc: Zqiang <qiang.zhang1211@xxxxxxxxx>
> > > > Reviewed-by: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
> > > > Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
> > > > ---
> > > > v1->v2: Apply review tags and test robot feedback.
> > > > 
> > > >  mm/vmalloc.c | 39 ++++++++++++++++++++++++++++++++++++++-
> > > >  1 file changed, 38 insertions(+), 1 deletion(-)
> > > > 
> > > > diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> > > > index 93cf99aba335..f09e882ae3b8 100644
> > > > --- a/mm/vmalloc.c
> > > > +++ b/mm/vmalloc.c
> > > > @@ -1865,6 +1865,20 @@ struct vmap_area *find_vmap_area(unsigned long addr)
> > > >  	return va;
> > > >  }
> > > >  
> > > > +#ifdef CONFIG_PRINTK
> > > > +static struct vmap_area *find_vmap_area_trylock(unsigned long addr)
> > > > +{
> > > > +	struct vmap_area *va;
> > > > +
> > > > +	if (!spin_trylock(&vmap_area_lock))
> > > > +		return NULL;
> > > > +	va = __find_vmap_area(addr, &vmap_area_root);
> > > > +	spin_unlock(&vmap_area_lock);
> > > > +
> > > > +	return va;
> > > > +}
> > > > +#endif
> > > > +
> > > >  static struct vmap_area *find_unlink_vmap_area(unsigned long addr)
> > > >  {
> > > >  	struct vmap_area *va;
> > > > @@ -2671,6 +2685,29 @@ struct vm_struct *find_vm_area(const void *addr)
> > > >  	return va->vm;
> > > >  }
> > > >  
> > > > +/**
> > > > + * try_to_find_vm_area - find a continuous kernel virtual area
> > > > + * @addr:	  base address
> > > > + *
> > > > + * This function is the same as find_vm_area() except that it is
> > > > + * safe to call if vmap_area_lock is already held and returns NULL
> > > > + * if it is. See comments in find_vmap_area() for other details.
> > > > + *
> > > > + * Return: the area descriptor on success or %NULL on failure.
> > > > + */
> > > > +#ifdef CONFIG_PRINTK
> > > > +static struct vm_struct *try_to_find_vm_area(const void *addr)
> > > > +{
> > > > +	struct vmap_area *va;
> > > > +
> > > > +	va = find_vmap_area_trylock((unsigned long)addr);
> > > > +	if (!va)
> > > > +		return NULL;
> > > > +
> > > > +	return va->vm;
> > > > +}
> > > > +#endif
> > > > +
> > > >  /**
> > > >   * remove_vm_area - find and remove a continuous kernel virtual area
> > > >   * @addr:	    base address
> > > > @@ -4277,7 +4314,7 @@ bool vmalloc_dump_obj(void *object)
> > > >  	struct vm_struct *vm;
> > > >  	void *objp = (void *)PAGE_ALIGN((unsigned long)object);
> > > >  
> > > > -	vm = find_vm_area(objp);
> > > > +	vm = try_to_find_vm_area(objp);
> > > >  	if (!vm)
> > > >  		return false;
> > > >  	pr_cont(" %u-page vmalloc region starting at %#lx allocated at %pS\n",
> > 
> > Hi Vlad,
> > Thanks for taking a look.
> > 
> > > I am not sure if this patch makes a lot of sense. I agree, this is a
> > > problem and it mitigates it. But it is broken in terms of once you drop
> > > the lock, the VA should not be accessed.
> > 
> > Just to note the lockless-access issue you are referring to is not introduced
> > by this patch but is rather in the existing code. Also just to note this is
> > debug code.
> > 
> > > Is that a real issue or it gets triggered due to some syntetic test case?
> > 
> > It is a real issue. See 2/2.
> > 
> > > If i were you, i would go with open-coded version of trylock. Because
> > > there is only one user so far.
> > 
> > Taking your open coding and locking suggestions, I came up with the below
> > which actually results in a smaller patch. Does it look good to you?
> > 
> > diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> > index 93cf99aba335..aaf6bad997a7 100644
> 
> And with some trivial compiler errors fixed (sorry should have build tested
> but wanted to just share the idea earlier):
> 
> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> index 93cf99aba335..2c6a0e2ff404 100644
> --- a/mm/vmalloc.c
> +++ b/mm/vmalloc.c
> @@ -4274,14 +4274,32 @@ void pcpu_free_vm_areas(struct vm_struct **vms, int nr_vms)
>  #ifdef CONFIG_PRINTK
>  bool vmalloc_dump_obj(void *object)
>  {
> -	struct vm_struct *vm;
>  	void *objp = (void *)PAGE_ALIGN((unsigned long)object);
> +	const void *caller;
> +	struct vm_struct *vm;
> +	struct vmap_area *va;
> +	unsigned long addr;
> +	unsigned int nr_pages;
>  
> -	vm = find_vm_area(objp);
> -	if (!vm)
> +	if (!spin_trylock(&vmap_area_lock))
> +		return false;
> +	va = __find_vmap_area((unsigned long)objp, &vmap_area_root);
> +	if (!va) {
> +		spin_unlock(&vmap_area_lock);
>  		return false;
> +	}
> +
> +	vm = va->vm;
> +	if (!vm) {
> +		spin_unlock(&vmap_area_lock);
> +		return false;
> +	}
> +	addr = (unsigned long)vm->addr;
> +	caller = vm->caller;
> +	nr_pages = vm->nr_pages;
> +	spin_unlock(&vmap_area_lock);
>  	pr_cont(" %u-page vmalloc region starting at %#lx allocated at %pS\n",
> -		vm->nr_pages, (unsigned long)vm->addr, vm->caller);
> +		nr_pages, addr, caller);
>  	return true;
>  }
>  #endif
>
Looks good to me and thank you for fixing a locking issue :)
I think you will re-spin and resend it one more time?

--
Uladzislau Rezki