linux-kernel - Re: [PATCH 3.2 056/185] mm: ensure get_unmapped_area() returns higher address than mmap_min

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Date:	Fri, 03 Jan 2014 04:26:43 +0000
From:	Ben Hutchings <ben@...adent.org.uk>
To:	linux-kernel@...r.kernel.org
Cc:	stable@...r.kernel.org, akpm@...ux-foundation.org,
	Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
	Kiyoshi Owada <owada.kiyoshi@...panasonic.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Akira Takeuchi <takeuchi.akr@...panasonic.com>
Subject: Re: [PATCH 3.2 056/185] mm: ensure get_unmapped_area() returns
 higher address than mmap_min_addr

On Sun, 2013-12-29 at 03:08 +0100, Ben Hutchings wrote:
> 3.2.54-rc1 review patch.  If anyone has any objections, please let me know.
> 
> ------------------
> 
> From: Akira Takeuchi <takeuchi.akr@...panasonic.com>
> 
> commit 2afc745f3e3079ab16c826be4860da2529054dd2 upstream.
[...]
> [bwh: Backported to 3.2:
>  As we do not have vm_unmapped_area(), make arch_get_unmapped_area_topdown()
>  calculate the lower limit for the new area's end address and then compare
>  addresses with this instead of with len.  In the process, fix an off-by-one
>  error which could result in returning 0 if mm->mmap_base == len.]

I'm dropping this as I have no good way to test the backport (it's not
used on x86) and I didn't get any confirmation that it's right.

Ben.

> Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
> ---
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -1368,7 +1368,7 @@ arch_get_unmapped_area(struct file *filp
>  	struct vm_area_struct *vma;
>  	unsigned long start_addr;
>  
> -	if (len > TASK_SIZE)
> +	if (len > TASK_SIZE - mmap_min_addr)
>  		return -ENOMEM;
>  
>  	if (flags & MAP_FIXED)
> @@ -1377,7 +1377,7 @@ arch_get_unmapped_area(struct file *filp
>  	if (addr) {
>  		addr = PAGE_ALIGN(addr);
>  		vma = find_vma(mm, addr);
> -		if (TASK_SIZE - len >= addr &&
> +		if (TASK_SIZE - len >= addr && addr >= mmap_min_addr &&
>  		    (!vma || addr + len <= vma->vm_start))
>  			return addr;
>  	}
> @@ -1442,9 +1442,10 @@ arch_get_unmapped_area_topdown(struct fi
>  	struct vm_area_struct *vma;
>  	struct mm_struct *mm = current->mm;
>  	unsigned long addr = addr0;
> +	unsigned long low_limit = max(PAGE_SIZE, mmap_min_addr);
>  
>  	/* requested length too big for entire address space */
> -	if (len > TASK_SIZE)
> +	if (len > TASK_SIZE - mmap_min_addr)
>  		return -ENOMEM;
>  
>  	if (flags & MAP_FIXED)
> @@ -1454,7 +1455,7 @@ arch_get_unmapped_area_topdown(struct fi
>  	if (addr) {
>  		addr = PAGE_ALIGN(addr);
>  		vma = find_vma(mm, addr);
> -		if (TASK_SIZE - len >= addr &&
> +		if (TASK_SIZE - len >= addr && addr >= mmap_min_addr &&
>  				(!vma || addr + len <= vma->vm_start))
>  			return addr;
>  	}
> @@ -1469,14 +1470,14 @@ arch_get_unmapped_area_topdown(struct fi
>  	addr = mm->free_area_cache;
>  
>  	/* make sure it can fit in the remaining address space */
> -	if (addr > len) {
> +	if (addr >= low_limit + len) {
>  		vma = find_vma(mm, addr-len);
>  		if (!vma || addr <= vma->vm_start)
>  			/* remember the address as a hint for next time */
>  			return (mm->free_area_cache = addr-len);
>  	}
>  
> -	if (mm->mmap_base < len)
> +	if (mm->mmap_base < low_limit + len)
>  		goto bottomup;
>  
>  	addr = mm->mmap_base-len;
> @@ -1498,7 +1499,7 @@ arch_get_unmapped_area_topdown(struct fi
>  
>  		/* try just below the current vma->vm_start */
>  		addr = vma->vm_start-len;
> -	} while (len < vma->vm_start);
> +	} while (vma->vm_start >= low_limit + len);
>  
>  bottomup:
>  	/*

-- 
Ben Hutchings
The program is absolutely right; therefore, the computer must be wrong.

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