RE: [PATCH v8 09/10] x86, mpx: cleanup unused bound tables

[Ren, Qiaowei]

On 2014-09-16, Hansen, Dave wrote:
> On 09/11/2014 01:46 AM, Qiaowei Ren wrote:
>> +/*
>> + * Free the backing physical pages of bounds table 'bt_addr'.
>> + * Assume start...end is within that bounds table.
>> + */
>> +static int __must_check zap_bt_entries(struct mm_struct *mm,
>> +		unsigned long bt_addr,
>> +		unsigned long start, unsigned long end) {
>> +	struct vm_area_struct *vma;
>> +
>> +	/* Find the vma which overlaps this bounds table */
>> +	vma = find_vma(mm, bt_addr);
>> +	/*
>> +	 * The table entry comes from userspace and could be
>> +	 * pointing anywhere, so make sure it is at least
>> +	 * pointing to valid memory.
>> +	 */
>> +	if (!vma || !(vma->vm_flags & VM_MPX) ||
>> +			vma->vm_start > bt_addr ||
>> +			vma->vm_end < bt_addr+MPX_BT_SIZE_BYTES)
>> +		return -EINVAL;
> 
> If someone did *ANYTHING* to split the VMA, this check would fail.  I
> think that's a little draconian, considering that somebody could do a
> NUMA policy on part of a VM_MPX VMA and cause it to be split.
> 
> This check should look across the entire 'bt_addr ->
> bt_addr+MPX_BT_SIZE_BYTES' range, find all of the VM_MPX VMAs, and zap
> only those.
> 
> If we encounter a non-VM_MPX vma, it should be ignored.
>
Ok.

>> +	if (ret == -EFAULT)
>> +		return ret;
>> +
>> +	/*
>> +	 * unmap those bounds table which are entirely covered in this
>> +	 * virtual address region.
>> +	 */
> 
> Entirely covered *AND* not at the edges, right?
> 
Yes.

>> +	bde_start = mm->bd_addr + MPX_GET_BD_ENTRY_OFFSET(start);
>> +	bde_end = mm->bd_addr + MPX_GET_BD_ENTRY_OFFSET(end-1);
>> +	for (bd_entry = bde_start + 1; bd_entry < bde_end; bd_entry++) {
> 
> This needs a big fat comment that it is only freeing the bounds tables that are 1.
> fully covered 2. not at the edges of the mapping, even if full aligned
> 
> Does this get any nicer if we have unmap_side_bts() *ONLY* go after
> bounds tables that are partially owned by the region being unmapped?
> 
> It seems like we really should do this:
> 
> 	for (each bt fully owned)
> 		unmap_single_bt()
> 	if (start edge unaligned)
> 		free start edge
> 	if (end edge unaligned)
> 		free end edge
> 
> I bet the unmap_side_bts() code gets simpler if we do that, too.
> 
Maybe. I will try this.

Thanks,
Qiaowei
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