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

Mark Rutland
2016-01-08T12:36:38Z

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.

> +
> + /*
> + * 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.

> +
> +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.

> 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!

Thanks,
Mark.