Re: [PATCH] arm64/pageattr: Propagate return value from __change_memory_common

[Dev Jain <dev.jain@arm.com>]

On 04/11/25 12:15 am, Yang Shi wrote:
>
>
> On 11/3/25 7:16 AM, Will Deacon wrote:
>> On Mon, Nov 03, 2025 at 11:43:06AM +0530, Dev Jain wrote:
>>> Post a166563e7ec3 ("arm64: mm: support large block mapping when 
>>> rodata=full"),
>>> __change_memory_common has a real chance of failing due to split 
>>> failure.
>>> Before that commit, this line was introduced in c55191e96caa, still 
>>> having
>>> a chance of failing if it needs to allocate pagetable memory in
>>> apply_to_page_range, although that has never been observed to be true.
>>> In general, we should always propagate the return value to the caller.
>>>
>>> Cc: stable@vger.kernel.org
>>> Fixes: c55191e96caa ("arm64: mm: apply r/o permissions of VM areas 
>>> to its linear alias as well")
>>> Signed-off-by: Dev Jain <dev.jain@arm.com>
>>> ---
>>> Based on Linux 6.18-rc4.
>>>
>>>   arch/arm64/mm/pageattr.c | 5 ++++-
>>>   1 file changed, 4 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/arch/arm64/mm/pageattr.c b/arch/arm64/mm/pageattr.c
>>> index 5135f2d66958..b4ea86cd3a71 100644
>>> --- a/arch/arm64/mm/pageattr.c
>>> +++ b/arch/arm64/mm/pageattr.c
>>> @@ -148,6 +148,7 @@ static int change_memory_common(unsigned long 
>>> addr, int numpages,
>>>       unsigned long size = PAGE_SIZE * numpages;
>>>       unsigned long end = start + size;
>>>       struct vm_struct *area;
>>> +    int ret;
>>>       int i;
>>>         if (!PAGE_ALIGNED(addr)) {
>>> @@ -185,8 +186,10 @@ static int change_memory_common(unsigned long 
>>> addr, int numpages,
>>>       if (rodata_full && (pgprot_val(set_mask) == PTE_RDONLY ||
>>>                   pgprot_val(clear_mask) == PTE_RDONLY)) {
>>>           for (i = 0; i < area->nr_pages; i++) {
>>> - __change_memory_common((u64)page_address(area->pages[i]),
>>> +            ret = 
>>> __change_memory_common((u64)page_address(area->pages[i]),
>>>                              PAGE_SIZE, set_mask, clear_mask);
>>> +            if (ret)
>>> +                return ret;
>> Hmm, this means we can return failure half-way through the operation. Is
>> that something callers are expecting to handle? If so, how can they tell
>> how far we got?
>
> IIUC the callers don't have to know whether it is half-way or not 
> because the callers will change the permission back (e.g. to RW) for 
> the whole range when freeing memory.

Yes, it is the caller's responsibility to set VM_FLUSH_RESET_PERMS flag. 
Upon vfree(), it will change the direct map permissions back to RW.

>
> Thanks,
> Yang
>
>>
>> Will
>