Re: [PATCH v2 12/13] arm64: add support for relocatable kernel

Ard Biesheuvel
2016-01-08T12:38:56Z

On 8 January 2016 at 13:36, Mark Rutland <mark.rutland@arm.com> wrote:
> On Wed, Dec 30, 2015 at 04:26:11PM +0100, Ard Biesheuvel wrote:
>> This adds support for runtime relocation of the kernel Image, by
>> building it as a PIE (ET_DYN) executable and applying the dynamic
>> relocations in the early boot code.
>>
>> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
>> ---
>> Documentation/arm64/booting.txt | 3 +-
>> arch/arm64/Kconfig | 13 ++++
>> arch/arm64/Makefile | 6 +-
>> arch/arm64/include/asm/memory.h | 3 +
>> arch/arm64/kernel/head.S | 75 +++++++++++++++++++-
>> arch/arm64/kernel/setup.c | 22 +++---
>> arch/arm64/kernel/vmlinux.lds.S | 9 +++
>> scripts/sortextable.c | 4 +-
>> 8 files changed, 117 insertions(+), 18 deletions(-)
>
> [...]
>
>> +#ifdef CONFIG_ARM64_RELOCATABLE_KERNEL
>> +
>> +#define R_AARCH64_RELATIVE 0x403
>> +#define R_AARCH64_ABS64 0x101
>
> Let's not duplicate asm/elf.h.
>
> I have a patch to split the reloc types out into a separate header we
> can reuse from assembly -- I'll send that momentarily. We can add
> R_AARCH64_RELATIVE atop of that.
>

OK.

>> +
>> + /*
>> + * Iterate over each entry in the relocation table, and apply the
>> + * relocations in place.
>> + */
>> + adr_l x8, __dynsym_start // start of symbol table
>> + adr_l x9, __reloc_start // start of reloc table
>> + adr_l x10, __reloc_end // end of reloc table
>> +
>> +0: cmp x9, x10
>> + b.hs 2f
>> + ldp x11, x12, [x9], #24
>> + ldr x13, [x9, #-8]
>> + cmp w12, #R_AARCH64_RELATIVE
>> + b.ne 1f
>> + add x13, x13, x23 // relocate
>> + str x13, [x11, x23]
>> + b 0b
>> +
>> +1: cmp w12, #R_AARCH64_ABS64
>> + b.ne 0b
>> + add x12, x12, x12, lsl #1 // symtab offset: 24x top word
>> + add x12, x8, x12, lsr #(32 - 3) // ... shifted into bottom word
>> + ldrsh w14, [x12, #6] // Elf64_Sym::st_shndx
>> + ldr x15, [x12, #8] // Elf64_Sym::st_value
>> + cmp w14, #-0xf // SHN_ABS (0xfff1) ?
>> + add x14, x15, x23 // relocate
>> + csel x15, x14, x15, ne
>> + add x15, x13, x15
>> + str x15, [x11, x23]
>> + b 0b
>
> We need to clean each of these relocated instructions to the PoU to be
> visible for I-cache fetches.
>
> As this is normal-cacheable we can post the maintenance with a DC CVAU
> immediately after the store (no barriers necessary), and rely on the DSB
> at 2f to complete all of those.
>

Dynamic relocations never apply to instructions, so i don't think that
is necessary here.

>> +
>> +2: adr_l x8, kimage_vaddr // make relocated kimage_vaddr
>> + dc cvac, x8 // value visible to secondaries
>> + dsb sy // with MMU off
>
> Then we need:
>
> ic iallu
> dsb nsh
> isb
>
> To make sure the I-side is consistent with the PoU.
>
> As secondaries will do similarly in __enable_mmu we don't need to add
> any code for them.
>
>> +#endif
>> +
>> adr_l sp, initial_sp, x4
>> str_l x21, __fdt_pointer, x5 // Save FDT pointer
>>
>> - ldr x0, =KIMAGE_VADDR // Save the offset between
>> + ldr_l x0, kimage_vaddr // Save the offset between
>> sub x24, x0, x24 // the kernel virtual and
>> str_l x24, kimage_voffset, x0 // physical mappings
>>
>> @@ -462,6 +527,10 @@ ENDPROC(__mmap_switched)
>> * hotplug and needs to have the same protections as the text region
>> */
>> .section ".text","ax"
>> +
>> +ENTRY(kimage_vaddr)
>> + .quad _text - TEXT_OFFSET
>> +
>> /*
>> * If we're fortunate enough to boot at EL2, ensure that the world is
>> * sane before dropping to EL1.
>> @@ -622,7 +691,7 @@ ENTRY(secondary_startup)
>> adrp x26, swapper_pg_dir
>> bl __cpu_setup // initialise processor
>>
>> - ldr x8, =KIMAGE_VADDR
>> + ldr x8, kimage_vaddr
>> ldr w9, 0f
>> sub x27, x8, w9, sxtw // address to jump to after enabling the MMU
>> b __enable_mmu
>> diff --git a/arch/arm64/kernel/setup.c b/arch/arm64/kernel/setup.c
>> index 96177a7c0f05..2faee6042e99 100644
>> --- a/arch/arm64/kernel/setup.c
>> +++ b/arch/arm64/kernel/setup.c
>> @@ -292,16 +292,15 @@ 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 __initdata = {
>> - .addr = (void *)KIMAGE_VADDR,
>> - .size = 0,
>> - .flags = VM_IOREMAP,
>> - .caller = setup_arch,
>> - };
>> -
>> - vmlinux_vm.size = round_up((unsigned long)_end - KIMAGE_VADDR,
>> - 1 << SWAPPER_BLOCK_SHIFT);
>> - vmlinux_vm.phys_addr = __pa(KIMAGE_VADDR);
>> + static struct vm_struct vmlinux_vm __initdata;
>> +
>> + vmlinux_vm.addr = (void *)kimage_vaddr;
>> + vmlinux_vm.size = round_up((u64)_end - kimage_vaddr,
>> + SWAPPER_BLOCK_SIZE);
>> + vmlinux_vm.phys_addr = __pa(kimage_vaddr);
>> + vmlinux_vm.flags = VM_IOREMAP;
>> + vmlinux_vm.caller = setup_arch;
>> +
>> vm_area_add_early(&vmlinux_vm);
>>
>> pr_info("Boot CPU: AArch64 Processor [%08x]\n", read_cpuid_id());
>> @@ -367,7 +366,8 @@ void __init setup_arch(char **cmdline_p)
>> conswitchp = &dummy_con;
>> #endif
>> #endif
>> - if (boot_args[1] || boot_args[2] || boot_args[3]) {
>> + if ((!IS_ENABLED(CONFIG_ARM64_RELOCATABLE_KERNEL) && boot_args[1]) ||
>> + boot_args[2] || boot_args[3]) {
>> pr_err("WARNING: x1-x3 nonzero in violation of boot protocol:\n"
>> "\tx1: %016llx\n\tx2: %016llx\n\tx3: %016llx\n"
>> "This indicates a broken bootloader or old kernel\n",
>
> At this point it may make sense to split this out into a separate
> function. If the handshake is more involved we'll need more code to
> verify this, and it'd be nice to split that from setup_arch.
>

OK

>> diff --git a/arch/arm64/kernel/vmlinux.lds.S
>> b/arch/arm64/kernel/vmlinux.lds.S
>> index f935f082188d..cc1486039338 100644
>> --- a/arch/arm64/kernel/vmlinux.lds.S
>> +++ b/arch/arm64/kernel/vmlinux.lds.S
>> @@ -148,6 +148,15 @@ SECTIONS
>> .altinstr_replacement : {
>> *(.altinstr_replacement)
>> }
>> + .rela : ALIGN(8) {
>> + __reloc_start = .;
>> + *(.rela .rela*)
>> + __reloc_end = .;
>> + }
>> + .dynsym : ALIGN(8) {
>> + __dynsym_start = .;
>> + *(.dynsym)
>> + }
>>
>> . = ALIGN(PAGE_SIZE);
>> __init_end = .;
>> diff --git a/scripts/sortextable.c b/scripts/sortextable.c
>> index af247c70fb66..5ecbedefdb0f 100644
>> --- a/scripts/sortextable.c
>> +++ b/scripts/sortextable.c
>> @@ -266,9 +266,9 @@ do_file(char const *const fname)
>> break;
>> } /* end switch */
>> if (memcmp(ELFMAG, ehdr->e_ident, SELFMAG) != 0
>> - || r2(&ehdr->e_type) != ET_EXEC
>> + || (r2(&ehdr->e_type) != ET_EXEC && r2(&ehdr->e_type) != ET_DYN)
>> || ehdr->e_ident[EI_VERSION] != EV_CURRENT) {
>> - fprintf(stderr, "unrecognized ET_EXEC file %s\n", fname);
>> + fprintf(stderr, "unrecognized ET_EXEC/ET_DYN file %s\n", fname);
>> fail_file();
>> }
>
> This change should probably be a preparatory patch.
>
> Otherwise, looks good!
>

Cheers.