Re: [PATCH v3 25/60] arm64: head: Clear BSS and the kernel page tables in one go

[Ard Biesheuvel]

On Mon, 17 Apr 2023 at 16:00, Ryan Roberts <ryan.roberts@xxxxxxx> wrote:
>
> On 07/03/2023 14:04, Ard Biesheuvel wrote:
> > We will move the CPU feature overrides into BSS in a subsequent patch,
> > and this requires that BSS is zeroed before the feature override
> > detection code runs. So let's map BSS read-write in the ID map, and zero
> > it via this mapping.
> >
> > Since the kernel page tables are right next to it, and also zeroed via
> > the ID map, let's drop the separate clear_page_tables() function, and
> > just zero everything in one go.
> >
> > Signed-off-by: Ard Biesheuvel <ardb@xxxxxxxxxx>
> > ---
> >  arch/arm64/kernel/head.S | 33 +++++++-------------
> >  1 file changed, 11 insertions(+), 22 deletions(-)
> >
> > diff --git a/arch/arm64/kernel/head.S b/arch/arm64/kernel/head.S
> > index 0fa44b3188c1e204..ade0cb99c8a83a3d 100644
> > --- a/arch/arm64/kernel/head.S
> > +++ b/arch/arm64/kernel/head.S
> > @@ -177,17 +177,6 @@ SYM_CODE_START_LOCAL(preserve_boot_args)
> >       ret
> >  SYM_CODE_END(preserve_boot_args)
> >
> > -SYM_FUNC_START_LOCAL(clear_page_tables)
> > -     /*
> > -      * Clear the init page tables.
> > -      */
> > -     adrp    x0, init_pg_dir
> > -     adrp    x1, init_pg_end
> > -     sub     x2, x1, x0
> > -     mov     x1, xzr
> > -     b       __pi_memset                     // tail call
> > -SYM_FUNC_END(clear_page_tables)
> > -
> >  /*
> >   * Macro to populate page table entries, these entries can be pointers to the next level
> >   * or last level entries pointing to physical memory.
> > @@ -386,9 +375,9 @@ SYM_FUNC_START_LOCAL(create_idmap)
> >
> >       map_memory x0, x1, x3, x6, x7, x3, IDMAP_PGD_ORDER, x10, x11, x12, x13, x14, EXTRA_SHIFT
> >
> > -     /* Remap the kernel page tables r/w in the ID map */
> > +     /* Remap BSS and the kernel page tables r/w in the ID map */
> >       adrp    x1, _text
> > -     adrp    x2, init_pg_dir
> > +     adrp    x2, __bss_start
> >       adrp    x3, _end
> >       bic     x4, x2, #SWAPPER_BLOCK_SIZE - 1
> >       mov     x5, SWAPPER_RW_MMUFLAGS
> > @@ -489,14 +478,6 @@ SYM_FUNC_START_LOCAL(__primary_switched)
> >       mov     x0, x20
> >       bl      set_cpu_boot_mode_flag
> >
> > -     // Clear BSS
> > -     adr_l   x0, __bss_start
> > -     mov     x1, xzr
> > -     adr_l   x2, __bss_stop
> > -     sub     x2, x2, x0
> > -     bl      __pi_memset
> > -     dsb     ishst                           // Make zero page visible to PTW
> > -
> >  #if VA_BITS > 48
> >       adr_l   x8, vabits_actual               // Set this early so KASAN early init
> >       str     x25, [x8]                       // ... observes the correct value
> > @@ -780,6 +761,15 @@ SYM_FUNC_START_LOCAL(__primary_switch)
> >       adrp    x1, reserved_pg_dir
> >       adrp    x2, init_idmap_pg_dir
> >       bl      __enable_mmu
> > +
> > +     // Clear BSS
> > +     adrp    x0, __bss_start
> > +     mov     x1, xzr
> > +     adrp    x2, init_pg_end
> > +     sub     x2, x2, x0
> > +     bl      __pi_memset
> > +     dsb     ishst                           // Make zero page visible to PTW
>
> Is it possible to add an assert somewhere (or at the very least a comment in
> vmlinux.lds.S) to ensure that nothing gets inserted between the BSS and the page
> tables? It feels a bit fragile otherwise.
>

I'm not sure that matters. The contents are not covered by the loaded
image so they are undefined otherwise in any case.

> I also wonder what's the point in calling __pi_memset() from here? Why not just
> do it all in C?
>

That happens in one of the subsequent patches.