linux-kernel - Re: [PATCH v2 1/1] mm: shift VM_GROWS* check from mmap_region() to do_mmap

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Date:	Mon, 22 Jul 2013 16:30:51 -0700 (PDT)
From:	Hugh Dickins <hughd@...gle.com>
To:	Oleg Nesterov <oleg@...hat.com>
cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	Colin Cross <ccross@...roid.com>,
	David Rientjes <rientjes@...gle.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/1] mm: shift VM_GROWS* check from mmap_region() to
 do_mmap_pgoff()

On Sat, 20 Jul 2013, Oleg Nesterov wrote:

> mmap() doesn't allow the non-anonymous mappings with VM_GROWS* bit set.
> In particular this means that mmap_region()->vma_merge(file, vm_flags)
> must always fail if "vm_flags & VM_GROWS" is set incorrectly.
> 
> So it does not make sense to check VM_GROWS* after we already allocated
> the new vma, the only caller, do_mmap_pgoff(), which can pass this flag
> can do the check itself.
> 
> And this looks a bit more correct, mmap_region() already unmapped the
> old mapping at this stage. But if mmap() is going to fail, it should
> avoid do_munmap() if possible.
> 
> Note: we check VM_GROWS at the end to ensure that do_mmap_pgoff() won't
> return EINVAL in the case when it currently returns another error code.
> 
> Many thanks to Hugh who nacked the buggy v1.

Aw shucks, no need for that!

> 
> Signed-off-by: Oleg Nesterov <oleg@...hat.com>

Looks fine to me now, thanks Oleg; as do your other two.

Acked-by: Hugh Dickins <hughd@...gle.com>

> ---
>  mm/mmap.c |   10 ++++------
>  1 files changed, 4 insertions(+), 6 deletions(-)
> 
> diff --git a/mm/mmap.c b/mm/mmap.c
> index fbad7b0..92d9f54 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -1302,6 +1302,8 @@ unsigned long do_mmap_pgoff(struct file *file, unsigned long addr,
>  
>  			if (!file->f_op || !file->f_op->mmap)
>  				return -ENODEV;
> +			if (vm_flags & (VM_GROWSDOWN|VM_GROWSUP))
> +				return -EINVAL;
>  			break;
>  
>  		default:
> @@ -1310,6 +1312,8 @@ unsigned long do_mmap_pgoff(struct file *file, unsigned long addr,
>  	} else {
>  		switch (flags & MAP_TYPE) {
>  		case MAP_SHARED:
> +			if (vm_flags & (VM_GROWSDOWN|VM_GROWSUP))
> +				return -EINVAL;
>  			/*
>  			 * Ignore pgoff.
>  			 */
> @@ -1544,11 +1548,7 @@ munmap_back:
>  	vma->vm_pgoff = pgoff;
>  	INIT_LIST_HEAD(&vma->anon_vma_chain);
>  
> -	error = -EINVAL;	/* when rejecting VM_GROWSDOWN|VM_GROWSUP */
> -
>  	if (file) {
> -		if (vm_flags & (VM_GROWSDOWN|VM_GROWSUP))
> -			goto free_vma;
>  		if (vm_flags & VM_DENYWRITE) {
>  			error = deny_write_access(file);
>  			if (error)
> @@ -1573,8 +1573,6 @@ munmap_back:
>  		pgoff = vma->vm_pgoff;
>  		vm_flags = vma->vm_flags;
>  	} else if (vm_flags & VM_SHARED) {
> -		if (unlikely(vm_flags & (VM_GROWSDOWN|VM_GROWSUP)))
> -			goto free_vma;
>  		error = shmem_zero_setup(vma);
>  		if (error)
>  			goto free_vma;
> -- 
> 1.5.5.1
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