Re: [RFC PATCH 1/2] lib, stackdepot: check stackdepot handle before accessing slabs.

[Vlastimil Babka <vbabka@suse.cz>]

+CC linux-mm, Alex

On 9/1/21 07:19, Imran Khan wrote:
> stack_depot_save allocates slabs that will be used for storing
> objects in future.If this slab allocation fails we may get to
> a situation where space allocation for a new stack_record fails,
> causing stack_depot_save to return 0 as handle.
> If user of this handle ends up invoking stack_depot_fetch with
> this handle value, current implementation of stack_depot_fetch
> will end up using slab from wrong index.
> To avoid this check handle value at the beginning.
> Also issue a warning for nil handle values and when slab allocation
> for stackdepot fails for the first time.
> 
> Signed-off-by: Imran Khan <imran.f.khan@oracle.com>
> Suggested-by: Vlastimil Babka <vbabka@suse.cz>

Agree but without the warnings please, especially the "stack depot handle is
absent" one. It's just something that can happen e.g. in GFP_NOWAIT contexts
and no need to spam dmesg.

> ---
>  lib/stackdepot.c | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/lib/stackdepot.c b/lib/stackdepot.c
> index 0a2e417f83cb..1d42ef9ef766 100644
> --- a/lib/stackdepot.c
> +++ b/lib/stackdepot.c
> @@ -232,6 +232,10 @@ unsigned int stack_depot_fetch(depot_stack_handle_t handle,
>  	struct stack_record *stack;
>  
>  	*entries = NULL;
> +	if (!handle) {
> +		WARN(1, "stack depot handle is absent.\n");
> +		return 0;
> +	}
>  	if (parts.slabindex > depot_index) {
>  		WARN(1, "slab index %d out of bounds (%d) for stack id %08x\n",
>  			parts.slabindex, depot_index, handle);
> @@ -303,6 +307,8 @@ depot_stack_handle_t stack_depot_save(unsigned long *entries,
>  		page = alloc_pages(alloc_flags, STACK_ALLOC_ORDER);
>  		if (page)
>  			prealloc = page_address(page);
> +		else
> +			WARN_ONCE(1, "slab allocation for stack depot failed.\n");
>  	}
>  
>  	raw_spin_lock_irqsave(&depot_lock, flags);
>