Re: [PATCH 1/8] execmem: drop unused execmem_update_copy()

[Christophe Leroy <christophe.leroy@csgroup.eu>]

Le 07/07/2025 à 13:49, Mike Rapoport a écrit :
> On Mon, Jul 07, 2025 at 12:10:43PM +0200, Christophe Leroy wrote:
>>
>> Le 04/07/2025 à 15:49, Mike Rapoport a écrit :
>>> From: "Mike Rapoport (Microsoft)" <rppt@kernel.org>
>>>
>>> The execmem_update_copy() that used text poking was required when memory
>>> allocated from ROX cache was always read-only. Since now its permissions
>>> can be switched to read-write there is no need in a function that updates
>>> memory with text poking.
>>
>> Erm. Looks like I missed the patch that introduced this change.
>>
>> On some variant of powerpc, namely book3s/32, this is not feasible.
> 
> The only user of EXECMEM_ROX_CACHE for now is x86-64, we can always revisit
> when powerpc book3s/32 would want to opt in to cache usage.
> 
> And it seems that [MODULES_VADDR, MODULES_END] is already mapped with
> "large pages", isn't it?

I don't think so. It uses execmem_alloc() which sets VM_ALLOW_HUGE_VMAP 
only when using EXECMEM_ROX_CACHE. And book3s/32 doesn't have large pages.

Only 8xx has large pages but they are not PMD aligned (PMD_SIZE is 4M 
while large pages are 512k and 8M) so it wouldn't work well with 
existing execmem_vmalloc().

> 
>> The granularity for setting the NX (non exec) bit is 256 Mbytes sections.
>> So the area dedicated to execmem [MODULES_VADDR; MODULES_END[ always have
>> the NX bit unset.
>>
>> You can change any page within this area from ROX to RWX but you can't make
>> it RW without X. If you want RW without X you must map it in the VMALLOC
>> area, as VMALLOC area have NX bit always set.
> 
> So what will happen when one callse
> 
> 	set_memory_nx()
> 	set_memory_rw()
> 
> in such areas?

Nothing will happen. It will successfully unset the X bit on the PTE but 
that will be ignored by the HW which only relies on the segment's NX bit 
which is set for the entire VMALLOC area and unset for the entire MODULE 
area.

That's one of the reasons why it has 
CONFIG_ARCH_WANTS_MODULES_DATA_IN_VMALLOC , to make sure text is 
allocated in exec area and data in no-exec area.

Christophe