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

[Uladzislau Rezki]

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",
>
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.

Is that a real issue or it gets triggered due to some syntetic test case?

If i were you, i would go with open-coded version of trylock. Because
there is only one user so far.

--
Uladzislau Rezki