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

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Message-ID: <CAMj1kXG8Q+VjXP08-hZhQ16Fe39+_JiSpPfh389gQ7twHr7eBQ@mail.gmail.com>
Date: Fri, 23 Jan 2026 07:43:04 +0100
From: Ard Biesheuvel <ardb@...nel.org>
To: Anshuman Khandual <anshuman.khandual@....com>
Cc: Ard Biesheuvel <ardb+git@...gle.com>, linux-kernel@...r.kernel.org, 
	linux-arm-kernel@...ts.infradead.org, will@...nel.org, 
	catalin.marinas@....com, mark.rutland@....com, 
	Ryan Roberts <ryan.roberts@....com>, Liz Prucka <lizprucka@...gle.com>, 
	Seth Jenkins <sethjenkins@...gle.com>, Kees Cook <kees@...nel.org>, 
	linux-hardening@...r.kernel.org
Subject: Re: [PATCH 1/4] arm64: Move fixmap page tables to end of kernel image

Hello Anshuman,

On Fri, 23 Jan 2026 at 07:12, Anshuman Khandual
<anshuman.khandual@....com> wrote:
>
>
>
> On 19/01/26 10:17 PM, Ard Biesheuvel wrote:
> > From: Ard Biesheuvel <ardb@...nel.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@...nel.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)
> >  {
>