Re: [PATCH] powerpc/32: Remove PAGE_KERNEL_TEXT to fix startup failure

Christophe Leroy
2025-09-05T05:20:35Z

Le 05/09/2025 à 05:55, Ritesh Harjani a écrit :
> Christophe Leroy <christophe.leroy@csgroup.eu> writes:
>
>> PAGE_KERNEL_TEXT is an old macro that is used to tell kernel whether
>> kernel text has to be mapped read-only or read-write based on build
>> time options.
>>
>> But nowadays, with functionnalities like jump_labels, static links,
>> etc ... more only less all kernels need to be read-write at some
>> point, and some combinations of configs failed to work due to
>> innacurate setting of PAGE_KERNEL_TEXT. On the other hand, today
>> we have CONFIG_STRICT_KERNEL_RWX which implements a more controlled
>> access to kernel modifications.
>>
>> Instead of trying to keep PAGE_KERNEL_TEXT accurate with all
>> possible options that may imply kernel text modification, always
>> set kernel text read-write at startup and rely on
>> CONFIG_STRICT_KERNEL_RWX to provide accurate protection.
>>
>> Reported-by: Erhard Furtner <erhard_f@mailbox.org>
>> Closes: https://lore.kernel.org/all/342b4120-911c-4723-82ec-d8c9b03a8aef@mailbox.org/
>> Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
>> ---
>> arch/powerpc/include/asm/pgtable.h | 12 ------------
>> arch/powerpc/mm/book3s32/mmu.c | 4 ++--
>> arch/powerpc/mm/pgtable_32.c | 2 +-
>> 3 files changed, 3 insertions(+), 15 deletions(-)
>>
>
> AFAIU - mmu_mark_initmem_nx gets called during kernel_init() which is
> way after static call initialization correct? i.e.
>
> start_kernel
> ...
> jump_label_init()
> static_call_init()
> ...
> ...
> rest_init() /* Do the rest non-__init'ed, we're now alive */
> kernel_init()
> free_initmem() -> mark_initmem_nx() -> __mark_initmem_nx -> mmu_mark_initmem_nx()
> mark_readonly()
> if (IS_ENABLED(CONFIG_STRICT_KERNEL_RWX) && rodata_enabled) {
> jump_label_init_ro()
> mark_rodata_ro() -> ....
> ...
> ...
>
> Then I guess we mainly only need __mapin_ram_chunk() to be PAGE_KERNEL_X (RWX)
> instead of PAGE_KERNEL_TEXT (ROX), isn't it?
>
> Let me quickly validate it...
> ...Ok, so I was able to get just this diff to be working.
>
> Thoughts?

setibat() doesn't take into account whether it is RO or RW. Only X or NX
is taken into account, so it doesn't matter whether it is X or ROX.

Then allthough you are right in principle, once the PAGE_KERNEL_TEXT is
removed from __mapin_ram_chunk() it becomes completely useless, so
better get rid of PAGE_KERNEL_TEXT completely.

>
> diff --git a/arch/powerpc/mm/pgtable_32.c b/arch/powerpc/mm/pgtable_32.c
> index 15276068f657..0c9ef705803e 100644
> --- a/arch/powerpc/mm/pgtable_32.c
> +++ b/arch/powerpc/mm/pgtable_32.c
> @@ -104,7 +104,7 @@ static void __init __mapin_ram_chunk(unsigned long offset, unsigned long top)
> p = memstart_addr + s;
> for (; s < top; s += PAGE_SIZE) {
> ktext = core_kernel_text(v);
> - map_kernel_page(v, p, ktext ? PAGE_KERNEL_TEXT : PAGE_KERNEL);
> + map_kernel_page(v, p, ktext ? PAGE_KERNEL_X : PAGE_KERNEL);
> v += PAGE_SIZE;
> p += PAGE_SIZE;
> }
>
> -ritesh
>
>
>
>> diff --git a/arch/powerpc/include/asm/pgtable.h b/arch/powerpc/include/asm/pgtable.h
>> index 93d77ad5a92f..d8f944a5a037 100644
>> --- a/arch/powerpc/include/asm/pgtable.h
>> +++ b/arch/powerpc/include/asm/pgtable.h
>> @@ -20,18 +20,6 @@ struct mm_struct;
>> #include <asm/nohash/pgtable.h>
>> #endif /* !CONFIG_PPC_BOOK3S */
>>
>> -/*
>> - * Protection used for kernel text. We want the debuggers to be able to
>> - * set breakpoints anywhere, so don't write protect the kernel text
>> - * on platforms where such control is possible.
>> - */
>> -#if defined(CONFIG_KGDB) || defined(CONFIG_XMON) || defined(CONFIG_BDI_SWITCH) || \
>> - defined(CONFIG_KPROBES) || defined(CONFIG_DYNAMIC_FTRACE)
>> -#define PAGE_KERNEL_TEXT PAGE_KERNEL_X
>> -#else
>> -#define PAGE_KERNEL_TEXT PAGE_KERNEL_ROX
>> -#endif
>> -
>> /* Make modules code happy. We don't set RO yet */
>> #define PAGE_KERNEL_EXEC PAGE_KERNEL_X
>>
>> diff --git a/arch/powerpc/mm/book3s32/mmu.c b/arch/powerpc/mm/book3s32/mmu.c
>> index be9c4106e22f..c42ecdf94e48 100644
>> --- a/arch/powerpc/mm/book3s32/mmu.c
>> +++ b/arch/powerpc/mm/book3s32/mmu.c
>> @@ -204,7 +204,7 @@ int mmu_mark_initmem_nx(void)
>>
>> for (i = 0; i < nb - 1 && base < top;) {
>> size = bat_block_size(base, top);
>> - setibat(i++, PAGE_OFFSET + base, base, size, PAGE_KERNEL_TEXT);
>> + setibat(i++, PAGE_OFFSET + base, base, size, PAGE_KERNEL_X);
>> base += size;
>> }
>> if (base < top) {
>> @@ -215,7 +215,7 @@ int mmu_mark_initmem_nx(void)
>> pr_warn("Some RW data is getting mapped X. "
>> "Adjust CONFIG_DATA_SHIFT to avoid that.\n");
>> }
>> - setibat(i++, PAGE_OFFSET + base, base, size, PAGE_KERNEL_TEXT);
>> + setibat(i++, PAGE_OFFSET + base, base, size, PAGE_KERNEL_X);
>> base += size;
>> }
>> for (; i < nb; i++)
>> diff --git a/arch/powerpc/mm/pgtable_32.c b/arch/powerpc/mm/pgtable_32.c
>> index 15276068f657..0c9ef705803e 100644
>> --- a/arch/powerpc/mm/pgtable_32.c
>> +++ b/arch/powerpc/mm/pgtable_32.c
>> @@ -104,7 +104,7 @@ static void __init __mapin_ram_chunk(unsigned long offset, unsigned long top)
>> p = memstart_addr + s;
>> for (; s < top; s += PAGE_SIZE) {
>> ktext = core_kernel_text(v);
>> - map_kernel_page(v, p, ktext ? PAGE_KERNEL_TEXT : PAGE_KERNEL);
>> + map_kernel_page(v, p, ktext ? PAGE_KERNEL_X : PAGE_KERNEL);
>> v += PAGE_SIZE;
>> p += PAGE_SIZE;
>> }
>> --
>> 2.49.0