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[PATCH 1/4] arm64: Move fixmap page tables to end of kernel image

Ard Biesheuvel posted 4 patches 2 weeks, 5 days ago
There is a newer version of this series
[PATCH 1/4] arm64: Move fixmap page tables to end of kernel image
Posted by Ard Biesheuvel 2 weeks, 5 days ago 
From: Ard Biesheuvel <ardb@kernel.org>

Move the fixmap page tables out of the BSS section, and place them at
the end of the image, right before the init_pg_dir section where some of
the other statically allocated page tables live.

These page tables are currently the only data objects in vmlinux that
are meant to be accessed via the kernel image's linear alias, and so
placing them together allows the remainder of the data/bss section to be
remapped read-only or unmapped entirely.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
---
 arch/arm64/kernel/vmlinux.lds.S | 5 +++++
 arch/arm64/mm/fixmap.c          | 7 ++++---
 2 files changed, 9 insertions(+), 3 deletions(-)

diff --git a/arch/arm64/kernel/vmlinux.lds.S b/arch/arm64/kernel/vmlinux.lds.S
index ad6133b89e7a..df530e6f3e53 100644
--- a/arch/arm64/kernel/vmlinux.lds.S
+++ b/arch/arm64/kernel/vmlinux.lds.S
@@ -334,6 +334,11 @@ SECTIONS
 	__pi___bss_start = __bss_start;
 
 	. = ALIGN(PAGE_SIZE);
+	.pgdir : {
+		__pgdir_start = .;
+		*(.fixmap_bss)
+	}
+
 	__pi_init_pg_dir = .;
 	. += INIT_DIR_SIZE;
 	__pi_init_pg_end = .;
diff --git a/arch/arm64/mm/fixmap.c b/arch/arm64/mm/fixmap.c
index c5c5425791da..b649ea1a46e4 100644
--- a/arch/arm64/mm/fixmap.c
+++ b/arch/arm64/mm/fixmap.c
@@ -31,9 +31,10 @@ static_assert(NR_BM_PMD_TABLES == 1);
 
 #define BM_PTE_TABLE_IDX(addr)	__BM_TABLE_IDX(addr, PMD_SHIFT)
 
-static pte_t bm_pte[NR_BM_PTE_TABLES][PTRS_PER_PTE] __page_aligned_bss;
-static pmd_t bm_pmd[PTRS_PER_PMD] __page_aligned_bss __maybe_unused;
-static pud_t bm_pud[PTRS_PER_PUD] __page_aligned_bss __maybe_unused;
+#define __fixmap_bss	__section(".fixmap_bss") __aligned(PAGE_SIZE)
+static pte_t bm_pte[NR_BM_PTE_TABLES][PTRS_PER_PTE] __fixmap_bss;
+static pmd_t bm_pmd[PTRS_PER_PMD] __fixmap_bss __maybe_unused;
+static pud_t bm_pud[PTRS_PER_PUD] __fixmap_bss __maybe_unused;
 
 static inline pte_t *fixmap_pte(unsigned long addr)
 {
-- 
2.52.0.457.g6b5491de43-goog
Re: [PATCH 1/4] arm64: Move fixmap page tables to end of kernel image
Posted by Anshuman Khandual 2 weeks, 1 day ago 

On 19/01/26 10:17 PM, Ard Biesheuvel wrote:
> From: Ard Biesheuvel <ardb@kernel.org>
> 
> Move the fixmap page tables out of the BSS section, and place them at
> the end of the image, right before the init_pg_dir section where some of
> the other statically allocated page tables live.
> 
> These page tables are currently the only data objects in vmlinux that
> are meant to be accessed via the kernel image's linear alias, and so
> placing them together allows the remainder of the data/bss section to be
> remapped read-only or unmapped entirely.
> 
> Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
> ---
>  arch/arm64/kernel/vmlinux.lds.S | 5 +++++
>  arch/arm64/mm/fixmap.c          | 7 ++++---
>  2 files changed, 9 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/arm64/kernel/vmlinux.lds.S b/arch/arm64/kernel/vmlinux.lds.S
> index ad6133b89e7a..df530e6f3e53 100644
> --- a/arch/arm64/kernel/vmlinux.lds.S
> +++ b/arch/arm64/kernel/vmlinux.lds.S
> @@ -334,6 +334,11 @@ SECTIONS
>  	__pi___bss_start = __bss_start;
>  
>  	. = ALIGN(PAGE_SIZE);
> +	.pgdir : {
> +		__pgdir_start = .;

Does __fixmap_pgdir_start with an end marker __fixmap_pgdir_end sound better ?

> +		*(.fixmap_bss)
> +	}
> +
>  	__pi_init_pg_dir = .;
>  	. += INIT_DIR_SIZE;
>  	__pi_init_pg_end = .;
> diff --git a/arch/arm64/mm/fixmap.c b/arch/arm64/mm/fixmap.c
> index c5c5425791da..b649ea1a46e4 100644
> --- a/arch/arm64/mm/fixmap.c
> +++ b/arch/arm64/mm/fixmap.c
> @@ -31,9 +31,10 @@ static_assert(NR_BM_PMD_TABLES == 1);
>  
>  #define BM_PTE_TABLE_IDX(addr)	__BM_TABLE_IDX(addr, PMD_SHIFT)
>  
> -static pte_t bm_pte[NR_BM_PTE_TABLES][PTRS_PER_PTE] __page_aligned_bss;
> -static pmd_t bm_pmd[PTRS_PER_PMD] __page_aligned_bss __maybe_unused;
> -static pud_t bm_pud[PTRS_PER_PUD] __page_aligned_bss __maybe_unused;
> +#define __fixmap_bss	__section(".fixmap_bss") __aligned(PAGE_SIZE)
> +static pte_t bm_pte[NR_BM_PTE_TABLES][PTRS_PER_PTE] __fixmap_bss;
> +static pmd_t bm_pmd[PTRS_PER_PMD] __fixmap_bss __maybe_unused;
> +static pud_t bm_pud[PTRS_PER_PUD] __fixmap_bss __maybe_unused;
>  
>  static inline pte_t *fixmap_pte(unsigned long addr)
>  {
Re: [PATCH 1/4] arm64: Move fixmap page tables to end of kernel image
Posted by Ard Biesheuvel 2 weeks, 1 day ago 
Hello Anshuman,

On Fri, 23 Jan 2026 at 07:12, Anshuman Khandual
<anshuman.khandual@arm.com> wrote:
>
>
>
> On 19/01/26 10:17 PM, Ard Biesheuvel wrote:
> > From: Ard Biesheuvel <ardb@kernel.org>
> >
> > Move the fixmap page tables out of the BSS section, and place them at
> > the end of the image, right before the init_pg_dir section where some of
> > the other statically allocated page tables live.
> >
> > These page tables are currently the only data objects in vmlinux that
> > are meant to be accessed via the kernel image's linear alias, and so
> > placing them together allows the remainder of the data/bss section to be
> > remapped read-only or unmapped entirely.
> >
> > Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
> > ---
> >  arch/arm64/kernel/vmlinux.lds.S | 5 +++++
> >  arch/arm64/mm/fixmap.c          | 7 ++++---
> >  2 files changed, 9 insertions(+), 3 deletions(-)
> >
> > diff --git a/arch/arm64/kernel/vmlinux.lds.S b/arch/arm64/kernel/vmlinux.lds.S
> > index ad6133b89e7a..df530e6f3e53 100644
> > --- a/arch/arm64/kernel/vmlinux.lds.S
> > +++ b/arch/arm64/kernel/vmlinux.lds.S
> > @@ -334,6 +334,11 @@ SECTIONS
> >       __pi___bss_start = __bss_start;
> >
> >       . = ALIGN(PAGE_SIZE);
> > +     .pgdir : {
> > +             __pgdir_start = .;
>
> Does __fixmap_pgdir_start with an end marker __fixmap_pgdir_end sound better ?
>

No. __pgdir_start covers the init_pg_dir as well. And I don't think we
should be adding end markers that we do not intend to use.

> > +             *(.fixmap_bss)
> > +     }
> > +
> >       __pi_init_pg_dir = .;
> >       . += INIT_DIR_SIZE;
> >       __pi_init_pg_end = .;
> > diff --git a/arch/arm64/mm/fixmap.c b/arch/arm64/mm/fixmap.c
> > index c5c5425791da..b649ea1a46e4 100644
> > --- a/arch/arm64/mm/fixmap.c
> > +++ b/arch/arm64/mm/fixmap.c
> > @@ -31,9 +31,10 @@ static_assert(NR_BM_PMD_TABLES == 1);
> >
> >  #define BM_PTE_TABLE_IDX(addr)       __BM_TABLE_IDX(addr, PMD_SHIFT)
> >
> > -static pte_t bm_pte[NR_BM_PTE_TABLES][PTRS_PER_PTE] __page_aligned_bss;
> > -static pmd_t bm_pmd[PTRS_PER_PMD] __page_aligned_bss __maybe_unused;
> > -static pud_t bm_pud[PTRS_PER_PUD] __page_aligned_bss __maybe_unused;
> > +#define __fixmap_bss __section(".fixmap_bss") __aligned(PAGE_SIZE)
> > +static pte_t bm_pte[NR_BM_PTE_TABLES][PTRS_PER_PTE] __fixmap_bss;
> > +static pmd_t bm_pmd[PTRS_PER_PMD] __fixmap_bss __maybe_unused;
> > +static pud_t bm_pud[PTRS_PER_PUD] __fixmap_bss __maybe_unused;
> >
> >  static inline pte_t *fixmap_pte(unsigned long addr)
> >  {
>

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