Messages in this thread | | | From | Ard Biesheuvel <> | Date | Mon, 17 Apr 2023 16:02:23 +0200 | Subject | Re: [PATCH v3 25/60] arm64: head: Clear BSS and the kernel page tables in one go |
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On Mon, 17 Apr 2023 at 16:00, Ryan Roberts <ryan.roberts@arm.com> 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@kernel.org> > > --- > > 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.
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