Re: [PATCH] mm/mmap: Fix error return in do_vmi_align_munmap()

["Liam R. Howlett" <Liam.Howlett@Oracle.com>]

* David Woodhouse <dwmw2@infradead.org> [230628 06:43]:
> From: David Woodhouse <dwmw@amazon.co.uk>
> 
> If mas_store_gfp() in the gather loop failed, the 'error' variable that
> ultimately gets returned was not being set. In many cases, its original
> value of -ENOMEM was still in place, and that was fine. But if VMAs had
> been split at the start or end of the range, then 'error' could be zero.
> 
> Change to the 'error = foo(); if (error) goto …' idiom to fix the bug.
> 
> Also clean up a later case which avoided the same bug by *explicitly*
> setting error = -ENOMEM right before calling the function that might
> return -ENOMEM.
> 
> In a final cosmetic change, move the 'Point of no return' comment to
> *after* the goto. That's been in the wrong place since the preallocation
> was removed, and this new error path was added.
> 
> Fixes: 606c812eb1d5 ("mm/mmap: Fix error path in do_vmi_align_munmap()")
> Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>

Reviewed-by: Liam R. Howlett <Liam.Howlett@oracle.com>

> ---
>  mm/mmap.c | 9 +++++----
>  1 file changed, 5 insertions(+), 4 deletions(-)
> 
> diff --git a/mm/mmap.c b/mm/mmap.c
> index d600404580b2..13128e908470 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -2387,7 +2387,8 @@ do_vmi_align_munmap(struct vma_iterator *vmi, struct vm_area_struct *vma,
>  		}
>  		vma_start_write(next);
>  		mas_set_range(&mas_detach, next->vm_start, next->vm_end - 1);
> -		if (mas_store_gfp(&mas_detach, next, GFP_KERNEL))
> +		error = mas_store_gfp(&mas_detach, next, GFP_KERNEL);
> +		if (error)
>  			goto munmap_gather_failed;
>  		vma_mark_detached(next, true);
>  		if (next->vm_flags & VM_LOCKED)
> @@ -2436,12 +2437,12 @@ do_vmi_align_munmap(struct vma_iterator *vmi, struct vm_area_struct *vma,
>  		BUG_ON(count != test_count);
>  	}
>  #endif
> -	/* Point of no return */
> -	error = -ENOMEM;
>  	vma_iter_set(vmi, start);
> -	if (vma_iter_clear_gfp(vmi, start, end, GFP_KERNEL))
> +	error = vma_iter_clear_gfp(vmi, start, end, GFP_KERNEL);
> +	if (error)
>  		goto clear_tree_failed;
>  
> +	/* Point of no return */
>  	mm->locked_vm -= locked_vm;
>  	mm->map_count -= count;
>  	/*
> -- 
> 2.34.1
> 
>