Re: [PATCH resend] powerpc/code-patching: Avoid r/w mapping of the zero page

["Ard Biesheuvel" <ardb@kernel.org>]

On Wed, 20 May 2026, at 11:36, Christophe Leroy (CS GROUP) wrote:
> Le 20/05/2026 à 10:54, Ard Biesheuvel a écrit :
>> The only remaining use of map_patch_area() is mapping the zero page, and
>> immediately unmapping it again so that the intermediate page table
>> levels are all guaranteed to be populated.
>> 
>> The use of the zero page here is completely arbitrary, and not harmful
>> per se, but currently, it creates a writable mapping, and does so in a
>> manner that requires that the empty_zero_page[] symbol is not
>> const-qualified.
>> 
>> Given that this is about to change, and that map_patch_area() now never
>> maps anything other than the zero page, let's simplify the code and
>> - take the PA of empty_zero_page directly
>> - create a read-only temporary mapping.
>> 
>> This allows empty_zero_page[] to be repainted as const u8[] in a
>> subsequent patch, without making substantial changes to this code
>> patching logic.
>> 
>> Cc: Madhavan Srinivasan <maddy@linux.ibm.com>
>> Cc: Michael Ellerman <mpe@ellerman.id.au>
>> Cc: Nicholas Piggin <npiggin@gmail.com>
>> Cc: "Christophe Leroy (CS GROUP)" <chleroy@kernel.org>
>> Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
>> ---
>> Build tested only (Clang)
>> 
>> Resending from my non-Google email. Apologies if you are receiving this
>> twice.
>> 
>>   arch/powerpc/lib/code-patching.c | 11 +++++------
>>   1 file changed, 5 insertions(+), 6 deletions(-)
>> 
>> diff --git a/arch/powerpc/lib/code-patching.c b/arch/powerpc/lib/code-patching.c
>> index f84e0337cc02..13a8acf851f1 100644
>> --- a/arch/powerpc/lib/code-patching.c
>> +++ b/arch/powerpc/lib/code-patching.c
>> @@ -60,7 +60,7 @@ struct patch_context {
>>   
>>   static DEFINE_PER_CPU(struct patch_context, cpu_patching_context);
>>   
>> -static int map_patch_area(void *addr, unsigned long text_poke_addr);
>> +static int map_patch_area(unsigned long text_poke_addr);
>>   static void unmap_patch_area(unsigned long addr);
>>   
>>   static bool mm_patch_enabled(void)
>> @@ -117,7 +117,7 @@ static int text_area_cpu_up(unsigned int cpu)
>>   
>>   	// Map/unmap the area to ensure all page tables are pre-allocated
>>   	addr = (unsigned long)area->addr;
>> -	err = map_patch_area(empty_zero_page, addr);
>> +	err = map_patch_area(addr);
>
> I would get rid of map_patch_area() completely and just do:
>
> 	err = map_kernel_page(addr, __pa_symbol(empty_zero_page), PAGE_KERNEL_RO);
>

I think retaining the symmetry of map_patch_area() and unmap_patch_area()
makes sense too.

>
> By the way I don't know how vital it is to map in read-only but in case 
> it is see commit da3a3b0a0e38 ("powerpc/32s: map kasan zero shadow with 
> PAGE_READONLY instead of PAGE_KERNEL_RO")
>

In the end, it doesn't make any difference, given that the mapping is removed
again immediately. It is just neater to use _RO to map a const object, rather
than explicitly creating a read-write mapping of something that should really
never be written to.