[PATCH v3 07/21] arm64: move kernel image to base of vmalloc area

Ard Biesheuvel
2016-01-13T08:39:41Z

On 12 January 2016 at 19:14, Mark Rutland <mark.rutland@arm.com> wrote:
> On Mon, Jan 11, 2016 at 02:19:00PM +0100, Ard Biesheuvel wrote:
>> This moves the module area to right before the vmalloc area, and
>> moves the kernel image to the base of the vmalloc area. This is
>> an intermediate step towards implementing kASLR, where the kernel
>> image can be located anywhere in the vmalloc area.
>>
>> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
>> ---
>> arch/arm64/include/asm/kasan.h | 20 ++++---
>> arch/arm64/include/asm/kernel-pgtable.h | 5 +-
>> arch/arm64/include/asm/memory.h | 18 ++++--
>> arch/arm64/include/asm/pgtable.h | 7 ---
>> arch/arm64/kernel/setup.c | 12 ++++
>> arch/arm64/mm/dump.c | 12 ++--
>> arch/arm64/mm/init.c | 20 +++----
>> arch/arm64/mm/kasan_init.c | 21 +++++--
>> arch/arm64/mm/mmu.c | 62 ++++++++++++++------
>> 9 files changed, 118 insertions(+), 59 deletions(-)
>>
>> diff --git a/arch/arm64/include/asm/kasan.h b/arch/arm64/include/asm/kasan.h
>> index de0d21211c34..2c583dbf4746 100644
>> --- a/arch/arm64/include/asm/kasan.h
>> +++ b/arch/arm64/include/asm/kasan.h
>> @@ -1,20 +1,16 @@
>> #ifndef __ASM_KASAN_H
>> #define __ASM_KASAN_H
>>
>> -#ifndef __ASSEMBLY__
>> -
>> #ifdef CONFIG_KASAN
>>
>> #include <linux/linkage.h>
>> -#include <asm/memory.h>
>> -#include <asm/pgtable-types.h>
>>
>> /*
>> * KASAN_SHADOW_START: beginning of the kernel virtual addresses.
>> * KASAN_SHADOW_END: KASAN_SHADOW_START + 1/8 of kernel virtual addresses.
>> */
>> -#define KASAN_SHADOW_START (VA_START)
>> -#define KASAN_SHADOW_END (KASAN_SHADOW_START + (1UL << (VA_BITS - 3)))
>> +#define KASAN_SHADOW_START (VA_START)
>> +#define KASAN_SHADOW_END (KASAN_SHADOW_START + (_AC(1, UL) << (VA_BITS - 3)))
>>
>> /*
>> * This value is used to map an address to the corresponding shadow
>> @@ -26,16 +22,22 @@
>> * should satisfy the following equation:
>> * KASAN_SHADOW_OFFSET = KASAN_SHADOW_END - (1ULL << 61)
>> */
>> -#define KASAN_SHADOW_OFFSET (KASAN_SHADOW_END - (1ULL << (64 - 3)))
>> +#define KASAN_SHADOW_OFFSET (KASAN_SHADOW_END - (_AC(1, ULL) << (64 - 3)))
>> +
>
> I couldn't immediately spot where KASAN_SHADOW_* were used in assembly.
> I guess there's some other definition built atop of them that I've
> missed.
>
> Where should I be looking?
>

Well, the problem is that KIMAGE_VADDR will be defined in terms of
KASAN_SHADOW_END if KASAN is enabled. But since KASAN always uses the
first 1/8 of that VA space, I am going to rework this so that the
non-KASAN constants never depend on the actual values but only on
CONFIG_KASAN

>> +#ifndef __ASSEMBLY__
>> +#include <asm/pgtable-types.h>
>>
>> void kasan_init(void);
>> void kasan_copy_shadow(pgd_t *pgdir);
>> asmlinkage void kasan_early_init(void);
>> +#endif
>>
>> #else
>> +
>> +#ifndef __ASSEMBLY__
>> static inline void kasan_init(void) { }
>> static inline void kasan_copy_shadow(pgd_t *pgdir) { }
>> #endif
>>
>> -#endif
>> -#endif
>> +#endif /* CONFIG_KASAN */
>> +#endif /* __ASM_KASAN_H */
>> diff --git a/arch/arm64/include/asm/kernel-pgtable.h b/arch/arm64/include/asm/kernel-pgtable.h
>> index a459714ee29e..daa8a7b9917a 100644
>> --- a/arch/arm64/include/asm/kernel-pgtable.h
>> +++ b/arch/arm64/include/asm/kernel-pgtable.h
>> @@ -70,8 +70,9 @@
>> /*
>> * Initial memory map attributes.
>> */
>> -#define SWAPPER_PTE_FLAGS (PTE_TYPE_PAGE | PTE_AF | PTE_SHARED)
>> -#define SWAPPER_PMD_FLAGS (PMD_TYPE_SECT | PMD_SECT_AF | PMD_SECT_S)
>> +#define SWAPPER_PTE_FLAGS (PTE_TYPE_PAGE | PTE_AF | PTE_SHARED | PTE_UXN)
>> +#define SWAPPER_PMD_FLAGS (PMD_TYPE_SECT | PMD_SECT_AF | PMD_SECT_S | \
>> + PMD_SECT_UXN)
>
> This will only affect the tables created in head.S. Before we start
> userspace we'll have switched over to a new set of tables using
> PAGE_KERNEL (including UXN).
>
> Given that, this doesn't look necessary for the vmalloc area changes. Am
> I missing something?
>

No, this was carried over from an older version of the series, when
the kernel mapping, after having been moved below PAGE_OFFSET, would
not be overridden by the memblock based linear mapping routines, and
so would missing the UXN bit. But with your changes, this can indeed
be dropped.

>> #if ARM64_SWAPPER_USES_SECTION_MAPS
>> #define SWAPPER_MM_MMUFLAGS (PMD_ATTRINDX(MT_NORMAL) | SWAPPER_PMD_FLAGS)
>> diff --git a/arch/arm64/include/asm/memory.h b/arch/arm64/include/asm/memory.h
>> index bea9631b34a8..e45d3141ad98 100644
>> --- a/arch/arm64/include/asm/memory.h
>> +++ b/arch/arm64/include/asm/memory.h
>> @@ -51,14 +51,24 @@
>> #define VA_BITS (CONFIG_ARM64_VA_BITS)
>> #define VA_START (UL(0xffffffffffffffff) << VA_BITS)
>> #define PAGE_OFFSET (UL(0xffffffffffffffff) << (VA_BITS - 1))
>> -#define KIMAGE_VADDR (PAGE_OFFSET)
>> -#define MODULES_END (KIMAGE_VADDR)
>> -#define MODULES_VADDR (MODULES_END - SZ_64M)
>> -#define PCI_IO_END (MODULES_VADDR - SZ_2M)
>> +#define PCI_IO_END (PAGE_OFFSET - SZ_2M)
>> #define PCI_IO_START (PCI_IO_END - PCI_IO_SIZE)
>> #define FIXADDR_TOP (PCI_IO_START - SZ_2M)
>> #define TASK_SIZE_64 (UL(1) << VA_BITS)
>>
>> +#ifndef CONFIG_KASAN
>> +#define MODULES_VADDR (VA_START)
>> +#else
>> +#include <asm/kasan.h>
>> +#define MODULES_VADDR (KASAN_SHADOW_END)
>> +#endif
>> +
>> +#define MODULES_VSIZE (SZ_64M)
>> +#define MODULES_END (MODULES_VADDR + MODULES_VSIZE)
>> +
>> +#define KIMAGE_VADDR (MODULES_END)
>> +#define VMALLOC_START (MODULES_END)
>> +
>> #ifdef CONFIG_COMPAT
>> #define TASK_SIZE_32 UL(0x100000000)
>> #define TASK_SIZE (test_thread_flag(TIF_32BIT) ? \
>> diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
>> index 7b4e16068c9f..a910a44d7ab3 100644
>> --- a/arch/arm64/include/asm/pgtable.h
>> +++ b/arch/arm64/include/asm/pgtable.h
>> @@ -42,13 +42,6 @@
>> */
>> #define VMEMMAP_SIZE ALIGN((1UL << (VA_BITS - PAGE_SHIFT)) * sizeof(struct page), PUD_SIZE)
>>
>> -#ifndef CONFIG_KASAN
>> -#define VMALLOC_START (VA_START)
>> -#else
>> -#include <asm/kasan.h>
>> -#define VMALLOC_START (KASAN_SHADOW_END + SZ_64K)
>> -#endif
>> -
>> #define VMALLOC_END (PAGE_OFFSET - PUD_SIZE - VMEMMAP_SIZE - SZ_64K)
>
> It's a shame VMALLOC_START and VMALLOC_END are now in different headers.
> It would be nice if we could keep them together.
>
> As VMEMMAP_SIZE depends on sizeof(struct page), it's not just a simple
> move. We could either place that in the !__ASSEMBLY__ portion of
> memory.h, or we could add S_PAGE to asm-offsets.
>
> If that's too painful now, we can leave that for subsequent cleanup;
> there's other stuff in that area I'd like to unify at some point (e.g.
> the mem_init and dump.c section boundary descriptions).
>

No, I think I can probably do a bit better than this. I will address it in v4

>>
>> #define vmemmap ((struct page *)(VMALLOC_END + SZ_64K))
>> diff --git a/arch/arm64/kernel/setup.c b/arch/arm64/kernel/setup.c
>> index cfed56f0ad26..c67ba4453ec6 100644
>> --- a/arch/arm64/kernel/setup.c
>> +++ b/arch/arm64/kernel/setup.c
>> @@ -53,6 +53,7 @@
>> #include <asm/cpufeature.h>
>> #include <asm/cpu_ops.h>
>> #include <asm/kasan.h>
>> +#include <asm/kernel-pgtable.h>
>> #include <asm/sections.h>
>> #include <asm/setup.h>
>> #include <asm/smp_plat.h>
>> @@ -291,6 +292,17 @@ u64 __cpu_logical_map[NR_CPUS] = { [0 ... NR_CPUS-1] = INVALID_HWID };
>>
>> void __init setup_arch(char **cmdline_p)
>> {
>> + static struct vm_struct vmlinux_vm;
>> +
>> + vmlinux_vm.addr = (void *)KIMAGE_VADDR;
>> + vmlinux_vm.size = round_up((u64)_end - KIMAGE_VADDR,
>> + SWAPPER_BLOCK_SIZE);
>
> With the fine grained tables we should only need to round up to
> PAGE_SIZE (though _end is implicitly page-aligned anyway). Given that,
> is the SWAPPER_BLOCK_SIZE rounding necessary?
>

No, probably not.

>> + vmlinux_vm.phys_addr = __pa(KIMAGE_VADDR);
>> + vmlinux_vm.flags = VM_MAP;
>
> I was going to say we should set VM_KASAN also per its description in
> include/vmalloc.h, though per its uses its not clear if it will ever
> matter.
>

No, we shouldn't. Even if we are never going to unmap this vma,
setting the flag will result in the shadow area being freed using
vfree(), while it was not allocated via vmalloc() so that is likely to
cause trouble.

>> + vmlinux_vm.caller = setup_arch;
>> +
>> + vm_area_add_early(&vmlinux_vm);
>
> Do we need to register the kernel VA range quite this early, or could we
> do this around paging_init/map_kernel time?
>

No. Locally, I moved it into map_kernel_chunk, so that we have
separate areas for _text, _init and _data, and we can unmap the _init
entirely rather than only stripping the exec bit. I haven't quite
figured out how to get rid of the vma area, but perhaps it make sense
to keep it reserved, so that modules don't end up there later (which
is possible with the module region randomization I have implemented
for v4) since I don't know how well things like kallsyms etc cope with
that.

>> +
>> pr_info("Boot CPU: AArch64 Processor [%08x]\n", read_cpuid_id());
>>
>> sprintf(init_utsname()->machine, ELF_PLATFORM);
>> diff --git a/arch/arm64/mm/dump.c b/arch/arm64/mm/dump.c
>> index 5a22a119a74c..e83ffb00560c 100644
>> --- a/arch/arm64/mm/dump.c
>> +++ b/arch/arm64/mm/dump.c
>> @@ -35,7 +35,9 @@ struct addr_marker {
>> };
>>
>> enum address_markers_idx {
>> - VMALLOC_START_NR = 0,
>> + MODULES_START_NR = 0,
>> + MODULES_END_NR,
>> + VMALLOC_START_NR,
>> VMALLOC_END_NR,
>> #ifdef CONFIG_SPARSEMEM_VMEMMAP
>> VMEMMAP_START_NR,
>> @@ -45,12 +47,12 @@ enum address_markers_idx {
>> FIXADDR_END_NR,
>> PCI_START_NR,
>> PCI_END_NR,
>> - MODULES_START_NR,
>> - MODUELS_END_NR,
>> KERNEL_SPACE_NR,
>> };
>>
>> static struct addr_marker address_markers[] = {
>> + { MODULES_VADDR, "Modules start" },
>> + { MODULES_END, "Modules end" },
>> { VMALLOC_START, "vmalloc() Area" },
>> { VMALLOC_END, "vmalloc() End" },
>> #ifdef CONFIG_SPARSEMEM_VMEMMAP
>> @@ -61,9 +63,7 @@ static struct addr_marker address_markers[] = {
>> { FIXADDR_TOP, "Fixmap end" },
>> { PCI_IO_START, "PCI I/O start" },
>> { PCI_IO_END, "PCI I/O end" },
>> - { MODULES_VADDR, "Modules start" },
>> - { MODULES_END, "Modules end" },
>> - { PAGE_OFFSET, "Kernel Mapping" },
>> + { PAGE_OFFSET, "Linear Mapping" },
>> { -1, NULL },
>> };
>>
>> diff --git a/arch/arm64/mm/init.c b/arch/arm64/mm/init.c
>> index f3b061e67bfe..baa923bda651 100644
>> --- a/arch/arm64/mm/init.c
>> +++ b/arch/arm64/mm/init.c
>> @@ -302,22 +302,26 @@ void __init mem_init(void)
>> #ifdef CONFIG_KASAN
>> " kasan : 0x%16lx - 0x%16lx (%6ld GB)\n"
>> #endif
>> + " modules : 0x%16lx - 0x%16lx (%6ld MB)\n"
>> " vmalloc : 0x%16lx - 0x%16lx (%6ld GB)\n"
>> + " .init : 0x%p" " - 0x%p" " (%6ld KB)\n"
>> + " .text : 0x%p" " - 0x%p" " (%6ld KB)\n"
>> + " .data : 0x%p" " - 0x%p" " (%6ld KB)\n"
>> #ifdef CONFIG_SPARSEMEM_VMEMMAP
>> " vmemmap : 0x%16lx - 0x%16lx (%6ld GB maximum)\n"
>> " 0x%16lx - 0x%16lx (%6ld MB actual)\n"
>> #endif
>> " fixed : 0x%16lx - 0x%16lx (%6ld KB)\n"
>> " PCI I/O : 0x%16lx - 0x%16lx (%6ld MB)\n"
>> - " modules : 0x%16lx - 0x%16lx (%6ld MB)\n"
>> - " memory : 0x%16lx - 0x%16lx (%6ld MB)\n"
>> - " .init : 0x%p" " - 0x%p" " (%6ld KB)\n"
>> - " .text : 0x%p" " - 0x%p" " (%6ld KB)\n"
>> - " .data : 0x%p" " - 0x%p" " (%6ld KB)\n",
>> + " memory : 0x%16lx - 0x%16lx (%6ld MB)\n",
>> #ifdef CONFIG_KASAN
>> MLG(KASAN_SHADOW_START, KASAN_SHADOW_END),
>> #endif
>> + MLM(MODULES_VADDR, MODULES_END),
>> MLG(VMALLOC_START, VMALLOC_END),
>> + MLK_ROUNDUP(__init_begin, __init_end),
>> + MLK_ROUNDUP(_text, _etext),
>> + MLK_ROUNDUP(_sdata, _edata),
>> #ifdef CONFIG_SPARSEMEM_VMEMMAP
>> MLG((unsigned long)vmemmap,
>> (unsigned long)vmemmap + VMEMMAP_SIZE),
>> @@ -326,11 +330,7 @@ void __init mem_init(void)
>> #endif
>> MLK(FIXADDR_START, FIXADDR_TOP),
>> MLM(PCI_IO_START, PCI_IO_END),
>> - MLM(MODULES_VADDR, MODULES_END),
>> - MLM(PAGE_OFFSET, (unsigned long)high_memory),
>> - MLK_ROUNDUP(__init_begin, __init_end),
>> - MLK_ROUNDUP(_text, _etext),
>> - MLK_ROUNDUP(_sdata, _edata));
>> + MLM(PAGE_OFFSET, (unsigned long)high_memory));
>>
>> #undef MLK
>> #undef MLM
>> diff --git a/arch/arm64/mm/kasan_init.c b/arch/arm64/mm/kasan_init.c
>> index 0ca411fc5ea3..acdd1ac166ec 100644
>> --- a/arch/arm64/mm/kasan_init.c
>> +++ b/arch/arm64/mm/kasan_init.c
>> @@ -17,9 +17,11 @@
>> #include <linux/start_kernel.h>
>>
>> #include <asm/mmu_context.h>
>> +#include <asm/kernel-pgtable.h>
>> #include <asm/page.h>
>> #include <asm/pgalloc.h>
>> #include <asm/pgtable.h>
>> +#include <asm/sections.h>
>> #include <asm/tlbflush.h>
>>
>> static pgd_t tmp_pg_dir[PTRS_PER_PGD] __initdata __aligned(PGD_SIZE);
>> @@ -33,7 +35,7 @@ static void __init kasan_early_pte_populate(pmd_t *pmd, unsigned long addr,
>> if (pmd_none(*pmd))
>> pmd_populate_kernel(&init_mm, pmd, kasan_zero_pte);
>>
>> - pte = pte_offset_kernel(pmd, addr);
>> + pte = pte_offset_kimg(pmd, addr);
>> do {
>> next = addr + PAGE_SIZE;
>> set_pte(pte, pfn_pte(virt_to_pfn(kasan_zero_page),
>> @@ -51,7 +53,7 @@ static void __init kasan_early_pmd_populate(pud_t *pud,
>> if (pud_none(*pud))
>> pud_populate(&init_mm, pud, kasan_zero_pmd);
>>
>> - pmd = pmd_offset(pud, addr);
>> + pmd = pmd_offset_kimg(pud, addr);
>> do {
>> next = pmd_addr_end(addr, end);
>> kasan_early_pte_populate(pmd, addr, next);
>> @@ -68,7 +70,7 @@ static void __init kasan_early_pud_populate(pgd_t *pgd,
>> if (pgd_none(*pgd))
>> pgd_populate(&init_mm, pgd, kasan_zero_pud);
>>
>> - pud = pud_offset(pgd, addr);
>> + pud = pud_offset_kimg(pgd, addr);
>> do {
>> next = pud_addr_end(addr, end);
>> kasan_early_pmd_populate(pud, addr, next);
>> @@ -126,8 +128,14 @@ static void __init clear_pgds(unsigned long start,
>>
>> void __init kasan_init(void)
>> {
>> + u64 kimg_shadow_start, kimg_shadow_end;
>> struct memblock_region *reg;
>>
>> + kimg_shadow_start = round_down((u64)kasan_mem_to_shadow(_text),
>> + SWAPPER_BLOCK_SIZE);
>> + kimg_shadow_end = round_up((u64)kasan_mem_to_shadow(_end),
>> + SWAPPER_BLOCK_SIZE);
>
> This rounding looks suspect to me, given it's applied to the shadow
> addresses rather than the kimage addresses. That's roughly equivalent to
> kasan_mem_to_shadow(round_up(_end, 8 * SWAPPER_BLOCK_SIZE).
>
> I don't think we need any rounding for the kimage addresses. The image
> end is page-granular (and the fine-grained mapping will reflect that).
> Any accesses between _end and roud_up(_end, SWAPPER_BLOCK_SIZE) would be
> bugs (and would most likely fault) regardless of KASAN.
>
> Or am I just being thick here?
>

Well, the problem here is that vmemmap_populate() is used as a
surrogate vmalloc() since that is not available yet, and
vmemmap_populate() allocates in SWAPPER_BLOCK_SIZE granularity.
If I remove the rounding, I get false positive kasan errors which I
have not quite diagnosed yet, but are probably due to the fact that
the rounding performed by vmemmap_populate() goes in the wrong
direction.

I do wonder what that means for memblocks that are not multiples of 16
MB, though (below)

>> +
>> /*
>> * We are going to perform proper setup of shadow memory.
>> * At first we should unmap early shadow (clear_pgds() call bellow).
>> @@ -141,8 +149,13 @@ void __init kasan_init(void)
>>
>> clear_pgds(KASAN_SHADOW_START, KASAN_SHADOW_END);
>>
>> + vmemmap_populate(kimg_shadow_start, kimg_shadow_end,
>> + pfn_to_nid(virt_to_pfn(kimg_shadow_start)));
>
> That virt_to_pfn doesn't look right -- kimg_shadow_start is neither a
> linear address nor an image address. As pfn_to_nid is hard-coded to 0
> for !NUMA this happens to be ok for us for the moment.
>
> I think we should follow the x86 KASAN code and use NUMA_NO_NODE for
> this for now.
>

Ack

>> +
>> kasan_populate_zero_shadow((void *)KASAN_SHADOW_START,
>> - kasan_mem_to_shadow((void *)MODULES_VADDR));
>> + (void *)kimg_shadow_start);
>> + kasan_populate_zero_shadow((void *)kimg_shadow_end,
>> + kasan_mem_to_shadow((void *)PAGE_OFFSET));
>>
>> for_each_memblock(memory, reg) {
>> void *start = (void *)__phys_to_virt(reg->base);
>> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
>> index 75b5f0dc3bdc..0b28f1469f9b 100644
>> --- a/arch/arm64/mm/mmu.c
>> +++ b/arch/arm64/mm/mmu.c
>> @@ -53,6 +53,10 @@ u64 idmap_t0sz = TCR_T0SZ(VA_BITS);
>> unsigned long empty_zero_page[PAGE_SIZE / sizeof(unsigned long)] __page_aligned_bss;
>> EXPORT_SYMBOL(empty_zero_page);
>>
>> +static pte_t bm_pte[PTRS_PER_PTE] __page_aligned_bss;
>> +static pmd_t bm_pmd[PTRS_PER_PMD] __page_aligned_bss;
>> +static pud_t bm_pud[PTRS_PER_PUD] __page_aligned_bss;
>> +
>> pgprot_t phys_mem_access_prot(struct file *file, unsigned long pfn,
>> unsigned long size, pgprot_t vma_prot)
>> {
>> @@ -349,14 +353,14 @@ static void __init __map_memblock(pgd_t *pgd, phys_addr_t start, phys_addr_t end
>> {
>>
>> unsigned long kernel_start = __pa(_stext);
>> - unsigned long kernel_end = __pa(_end);
>> + unsigned long kernel_end = __pa(_etext);
>>
>> /*
>> - * The kernel itself is mapped at page granularity. Map all other
>> - * memory, making sure we don't overwrite the existing kernel mappings.
>> + * Take care not to create a writable alias for the
>> + * read-only text and rodata sections of the kernel image.
>> */
>>
>> - /* No overlap with the kernel. */
>> + /* No overlap with the kernel text */
>> if (end < kernel_start || start >= kernel_end) {
>> __create_pgd_mapping(pgd, start, __phys_to_virt(start),
>> end - start, PAGE_KERNEL,
>> @@ -365,7 +369,7 @@ static void __init __map_memblock(pgd_t *pgd, phys_addr_t start, phys_addr_t end
>> }
>>
>> /*
>> - * This block overlaps the kernel mapping. Map the portion(s) which
>> + * This block overlaps the kernel text mapping. Map the portion(s) which
>> * don't overlap.
>> */
>> if (start < kernel_start)
>> @@ -438,12 +442,29 @@ static void __init map_kernel(pgd_t *pgd)
>> map_kernel_chunk(pgd, __init_begin, __init_end, PAGE_KERNEL_EXEC);
>> map_kernel_chunk(pgd, _data, _end, PAGE_KERNEL);
>>
>> - /*
>> - * The fixmap falls in a separate pgd to the kernel, and doesn't live
>> - * in the carveout for the swapper_pg_dir. We can simply re-use the
>> - * existing dir for the fixmap.
>> - */
>> - set_pgd(pgd_offset_raw(pgd, FIXADDR_START), *pgd_offset_k(FIXADDR_START));
>> + if (pgd_index(FIXADDR_START) != pgd_index((u64)_end)) {
>
> To match the style of early_fixmap_init, and given we already mapped the
> kernel image, this could be:
>
> if (pgd_none(pgd_offset_raw(pgd, FIXADDR_START))) {
>
> Which also serves as a run-time check that the pgd entry really was
> clear.
>

Yes, that looks better. I will steal that :-)

> Other than that, this looks good to me!
>

Thanks!

>> + /*
>> + * The fixmap falls in a separate pgd to the kernel, and doesn't
>> + * live in the carveout for the swapper_pg_dir. We can simply
>> + * re-use the existing dir for the fixmap.
>> + */
>> + set_pgd(pgd_offset_raw(pgd, FIXADDR_START),
>> + *pgd_offset_k(FIXADDR_START));
>> + } else if (CONFIG_PGTABLE_LEVELS > 3) {
>> + /*
>> + * The fixmap shares its top level pgd entry with the kernel
>> + * mapping. This can really only occur when we are running
>> + * with 16k/4 levels, so we can simply reuse the pud level
>> + * entry instead.
>> + */
>> + BUG_ON(!IS_ENABLED(CONFIG_ARM64_16K_PAGES));
>> +
>> + set_pud(pud_set_fixmap_offset(pgd, FIXADDR_START),
>> + __pud(__pa(bm_pmd) | PUD_TYPE_TABLE));
>> + pud_clear_fixmap();
>> + } else {
>> + BUG();
>> + }
>>
>> kasan_copy_shadow(pgd);
>> }
>> @@ -569,10 +590,6 @@ void vmemmap_free(unsigned long start, unsigned long end)
>> }
>> #endif /* CONFIG_SPARSEMEM_VMEMMAP */
>>
>> -static pte_t bm_pte[PTRS_PER_PTE] __page_aligned_bss;
>> -static pmd_t bm_pmd[PTRS_PER_PMD] __page_aligned_bss;
>> -static pud_t bm_pud[PTRS_PER_PUD] __page_aligned_bss;
>> -
>> static inline pud_t * fixmap_pud(unsigned long addr)
>> {
>> return (CONFIG_PGTABLE_LEVELS > 3) ? &bm_pud[pud_index(addr)]
>> @@ -598,8 +615,19 @@ void __init early_fixmap_init(void)
>> unsigned long addr = FIXADDR_START;
>>
>> pgd = pgd_offset_k(addr);
>> - pgd_populate(&init_mm, pgd, bm_pud);
>> - pud = fixmap_pud(addr);
>> + if (CONFIG_PGTABLE_LEVELS > 3 && !pgd_none(*pgd)) {
>> + /*
>> + * We only end up here if the kernel mapping and the fixmap
>> + * share the top level pgd entry, which should only happen on
>> + * 16k/4 levels configurations.
>> + */
>> + BUG_ON(!IS_ENABLED(CONFIG_ARM64_16K_PAGES));
>> + pud = pud_offset_kimg(pgd, addr);
>> + memblock_free(__pa(bm_pud), sizeof(bm_pud));
>> + } else {
>> + pgd_populate(&init_mm, pgd, bm_pud);
>> + pud = fixmap_pud(addr);
>> + }
>> pud_populate(&init_mm, pud, bm_pmd);
>> pmd = fixmap_pmd(addr);
>> pmd_populate_kernel(&init_mm, pmd, bm_pte);
>> --
>> 2.5.0
>>