linux-kernel - Re: [PATCH] powerpc/32: Remove PAGE_KERNEL

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Message-ID: <68ba6bee.170a0220.3b821b.ca9d@mx.google.com>
Date: Fri, 05 Sep 2025 09:25:15 +0530
From: Ritesh Harjani (IBM) <ritesh.list@...il.com>
To: Christophe Leroy <christophe.leroy@...roup.eu>, Andrew Donnellan <ajd@...ux.ibm.com>, Michael Ellerman <mpe@...erman.id.au>, Nicholas Piggin <npiggin@...il.com>, Madhavan Srinivasan <maddy@...ux.ibm.com>
Cc: Christophe Leroy <christophe.leroy@...roup.eu>, linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org, Erhard Furtner <erhard_f@...lbox.org>
Subject: Re: [PATCH] powerpc/32: Remove PAGE_KERNEL_TEXT to fix startup failure

Christophe Leroy <christophe.leroy@...roup.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@...lbox.org>
> Closes: https://lore.kernel.org/all/342b4120-911c-4723-82ec-d8c9b03a8aef@mailbox.org/
> Signed-off-by: Christophe Leroy <christophe.leroy@...roup.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?

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