Re: [PATCH] riscv: Introduce CONFIG_RELOCATABLE

From: Anup Patel
Date: Fri Jan 24 2020 - 23:58:31 EST

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On Fri, Jan 24, 2020 at 1:44 AM Alexandre Ghiti <alex@xxxxxxxx> wrote:
>
> This config allows to compile the kernel as PIE and to relocate it at any
> virtual address at runtime: this paves the way to KASLR and to 4-level
> page table folding at runtime. Runtime relocation is possible since
> relocation metadata are embedded into the kernel.
>
> Note that relocating at runtime introduces an overhead even if the kernel
> is loaded at the same address it was linked at and that the compiler
> options are those used in arm64 which uses the same RELA relocation format.
>
> Signed-off-by: Alexandre Ghiti <alex@xxxxxxxx>
> ---
> arch/riscv/Kconfig | 11 ++++
> arch/riscv/Makefile | 5 +-
> arch/riscv/boot/loader.lds.S | 2 +-
> arch/riscv/include/asm/page.h | 5 +-
> arch/riscv/kernel/head.S | 3 +-
> arch/riscv/kernel/vmlinux.lds.S | 10 ++--
> arch/riscv/mm/Makefile | 4 ++
> arch/riscv/mm/init.c | 92 ++++++++++++++++++++++++++++-----
> 8 files changed, 110 insertions(+), 22 deletions(-)
>
> diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
> index fa7dc03459e7..c652b4b850ce 100644
> --- a/arch/riscv/Kconfig
> +++ b/arch/riscv/Kconfig
> @@ -163,6 +163,17 @@ config PGTABLE_LEVELS
> default 3 if 64BIT
> default 2
>
> +config RELOCATABLE
> + bool
> + help
> + This builds a kernel as a Position Independent Executable (PIE),
> + which retains all relocation metadata required to relocate the
> + kernel binary at runtime to a different virtual address than the
> + address it was linked at.
> + Since RISCV uses the RELA relocation format, this requires a
> + relocation pass at runtime even if the kernel is loaded at the
> + same address it was linked at.
> +

I am sure this cannot be used for NOMMU support because of the
page table mappings.

I guess this should depend on MMU.

> source "arch/riscv/Kconfig.socs"
>
> menu "Platform type"
> diff --git a/arch/riscv/Makefile b/arch/riscv/Makefile
> index b9009a2fbaf5..5a115cf6a9c1 100644
> --- a/arch/riscv/Makefile
> +++ b/arch/riscv/Makefile
> @@ -9,7 +9,10 @@
> #
>
> OBJCOPYFLAGS := -O binary
> -LDFLAGS_vmlinux :=
> +ifeq ($(CONFIG_RELOCATABLE),y)
> +LDFLAGS_vmlinux := -shared -Bsymbolic -z notext -z norelro
> +KBUILD_CFLAGS += -fPIE
> +endif
> ifeq ($(CONFIG_DYNAMIC_FTRACE),y)
> LDFLAGS_vmlinux := --no-relax
> endif
> diff --git a/arch/riscv/boot/loader.lds.S b/arch/riscv/boot/loader.lds.S
> index 47a5003c2e28..a9ed218171aa 100644
> --- a/arch/riscv/boot/loader.lds.S
> +++ b/arch/riscv/boot/loader.lds.S
> @@ -7,7 +7,7 @@ ENTRY(_start)
>
> SECTIONS
> {
> - . = PAGE_OFFSET;
> + . = CONFIG_PAGE_OFFSET;
>
> .payload : {
> *(.payload)

The loader.lds.S is used by NOMMU support so this should be separate patch.

> diff --git a/arch/riscv/include/asm/page.h b/arch/riscv/include/asm/page.h
> index ac699246ae7e..27c95da68ecb 100644
> --- a/arch/riscv/include/asm/page.h
> +++ b/arch/riscv/include/asm/page.h
> @@ -31,9 +31,9 @@
> * When not using MMU this corresponds to the first free page in
> * physical memory (aligned on a page boundary).
> */
> -#define PAGE_OFFSET _AC(CONFIG_PAGE_OFFSET, UL)
> +#define PAGE_OFFSET kernel_load_addr
>
> -#define KERN_VIRT_SIZE (-PAGE_OFFSET)
> +#define KERN_VIRT_SIZE (-_AC(CONFIG_PAGE_OFFSET, UL))
>
> #ifndef __ASSEMBLY__
>
> @@ -97,6 +97,7 @@ extern unsigned long pfn_base;
> #define ARCH_PFN_OFFSET (PAGE_OFFSET >> PAGE_SHIFT)
> #endif /* CONFIG_MMU */
>
> +extern unsigned long kernel_load_addr;

Having this variable named kernel_virt_addr seems more
logical to me.

Is it mandatory to use kernel_load_addr name ?

> extern unsigned long max_low_pfn;
> extern unsigned long min_low_pfn;
>
> diff --git a/arch/riscv/kernel/head.S b/arch/riscv/kernel/head.S
> index 2227db63f895..5042b2b48a06 100644
> --- a/arch/riscv/kernel/head.S
> +++ b/arch/riscv/kernel/head.S
> @@ -126,7 +126,8 @@ clear_bss_done:
> #ifdef CONFIG_MMU
> relocate:
> /* Relocate return address */
> - li a1, PAGE_OFFSET
> + la a1, kernel_load_addr
> + REG_L a1, 0(a1)
> la a2, _start
> sub a1, a1, a2
> add ra, ra, a1
> diff --git a/arch/riscv/kernel/vmlinux.lds.S b/arch/riscv/kernel/vmlinux.lds.S
> index 12f42f96d46e..5095aee7c37e 100644
> --- a/arch/riscv/kernel/vmlinux.lds.S
> +++ b/arch/riscv/kernel/vmlinux.lds.S
> @@ -4,7 +4,7 @@
> * Copyright (C) 2017 SiFive
> */
>
> -#define LOAD_OFFSET PAGE_OFFSET
> +#define LOAD_OFFSET CONFIG_PAGE_OFFSET
> #include <asm/vmlinux.lds.h>
> #include <asm/page.h>
> #include <asm/cache.h>
> @@ -70,9 +70,11 @@ SECTIONS
>
> EXCEPTION_TABLE(0x10)
>
> - .rel.dyn : {
> - *(.rel.dyn*)
> - }
> + .rela.dyn : ALIGN(8) {
> + __rela_dyn_start = .;
> + *(.rela .rela*)
> + __rela_dyn_end = .;
> + }
>
> _end = .;
>
> diff --git a/arch/riscv/mm/Makefile b/arch/riscv/mm/Makefile
> index a1bd95c8047a..dcd3d806243f 100644
> --- a/arch/riscv/mm/Makefile
> +++ b/arch/riscv/mm/Makefile
> @@ -1,6 +1,10 @@
> # SPDX-License-Identifier: GPL-2.0-only
>
> CFLAGS_init.o := -mcmodel=medany
> +ifdef CONFIG_RELOCATABLE
> +CFLAGS_init.o += -fno-pie
> +endif
> +
> ifdef CONFIG_FTRACE
> CFLAGS_REMOVE_init.o = -pg
> endif
> diff --git a/arch/riscv/mm/init.c b/arch/riscv/mm/init.c
> index 965a8cf4829c..ac9a9f69abc0 100644
> --- a/arch/riscv/mm/init.c
> +++ b/arch/riscv/mm/init.c
> @@ -12,6 +12,9 @@
> #include <linux/sizes.h>
> #include <linux/of_fdt.h>
> #include <linux/libfdt.h>
> +#ifdef CONFIG_RELOCATABLE
> +#include <linux/elf.h>
> +#endif
>
> #include <asm/fixmap.h>
> #include <asm/tlbflush.h>
> @@ -28,6 +31,9 @@ EXPORT_SYMBOL(empty_zero_page);
> extern char _start[];
> void *dtb_early_va;
>
> +unsigned long kernel_load_addr = _AC(CONFIG_PAGE_OFFSET, UL);
> +EXPORT_SYMBOL(kernel_load_addr);
> +
> static void __init zone_sizes_init(void)
> {
> unsigned long max_zone_pfns[MAX_NR_ZONES] = { 0, };
> @@ -132,7 +138,8 @@ void __init setup_bootmem(void)
> phys_addr_t end = reg->base + reg->size;
>
> if (reg->base <= vmlinux_end && vmlinux_end <= end) {
> - mem_size = min(reg->size, (phys_addr_t)-PAGE_OFFSET);
> + mem_size = min(reg->size,
> + (phys_addr_t)-kernel_load_addr);
>
> /*
> * Remove memblock from the end of usable area to the
> @@ -269,7 +276,7 @@ static phys_addr_t __init alloc_pmd(uintptr_t va)
> if (mmu_enabled)
> return memblock_phys_alloc(PAGE_SIZE, PAGE_SIZE);
>
> - pmd_num = (va - PAGE_OFFSET) >> PGDIR_SHIFT;
> + pmd_num = (va - kernel_load_addr) >> PGDIR_SHIFT;
> BUG_ON(pmd_num >= NUM_EARLY_PMDS);
> return (uintptr_t)&early_pmd[pmd_num * PTRS_PER_PMD];
> }
> @@ -370,6 +377,54 @@ static uintptr_t __init best_map_size(phys_addr_t base, phys_addr_t size)
> #error "setup_vm() is called from head.S before relocate so it should not use absolute addressing."
> #endif
>
> +#ifdef CONFIG_RELOCATABLE
> +extern unsigned long __rela_dyn_start, __rela_dyn_end;
> +
> +#ifdef CONFIG_64BIT
> +#define Elf_Rela Elf64_Rela
> +#define Elf_Addr Elf64_Addr
> +#else
> +#define Elf_Rela Elf32_Rela
> +#define Elf_Addr Elf32_Addr
> +#endif
> +
> +void __init relocate_kernel(uintptr_t load_pa)
> +{
> + Elf_Rela *rela = (Elf_Rela *)&__rela_dyn_start;
> + uintptr_t link_addr = _AC(CONFIG_PAGE_OFFSET, UL);
> + /*
> + * This holds the offset between the linked virtual address and the
> + * relocated virtual address.
> + */
> + uintptr_t reloc_offset = kernel_load_addr - link_addr;
> + /*
> + * This holds the offset between linked virtual address and physical
> + * address whereas va_pa_offset holds the offset between relocated
> + * virtual address and physical address.
> + */
> + uintptr_t va_link_pa_offset = link_addr - load_pa;
> +
> + for ( ; rela < (Elf_Rela *)&__rela_dyn_end; rela++) {
> + Elf_Addr addr = (rela->r_offset - va_link_pa_offset);
> + Elf_Addr relocated_addr = rela->r_addend;
> +
> + if (rela->r_info != R_RISCV_RELATIVE)
> + continue;
> +
> + /*
> + * Make sure to not relocate vdso symbols like rt_sigreturn
> + * which are linked from the address 0 in vmlinux since
> + * vdso symbol addresses are actually used as an offset from
> + * mm->context.vdso in VDSO_OFFSET macro.
> + */
> + if (relocated_addr >= link_addr)
> + relocated_addr += reloc_offset;
> +
> + *(Elf_Addr *)addr = relocated_addr;
> + }
> +}
> +#endif
> +
> asmlinkage void __init setup_vm(uintptr_t dtb_pa)
> {
> uintptr_t va, end_va;
> @@ -377,9 +432,20 @@ asmlinkage void __init setup_vm(uintptr_t dtb_pa)
> uintptr_t load_sz = (uintptr_t)(&_end) - load_pa;
> uintptr_t map_size = best_map_size(load_pa, MAX_EARLY_MAPPING_SIZE);
>
> - va_pa_offset = PAGE_OFFSET - load_pa;
> + va_pa_offset = kernel_load_addr - load_pa;
> pfn_base = PFN_DOWN(load_pa);
>
> +#ifdef CONFIG_RELOCATABLE
> + /*
> + * Early page table uses only one PGDIR, which makes it possible
> + * to map 1GB aligned on 1GB: if the relocation offset makes the kernel
> + * cross over a 1G boundary, raise a bug since a part of the kernel
> + * would not get mapped.
> + */
> + BUG_ON(SZ_1G - (kernel_load_addr & (SZ_1G - 1)) < load_sz);
> + relocate_kernel(load_pa);
> +#endif
> +
> /*
> * Enforce boot alignment requirements of RV32 and
> * RV64 by only allowing PMD or PGD mappings.
> @@ -387,7 +453,7 @@ asmlinkage void __init setup_vm(uintptr_t dtb_pa)
> BUG_ON(map_size == PAGE_SIZE);
>
> /* Sanity check alignment and size */
> - BUG_ON((PAGE_OFFSET % PGDIR_SIZE) != 0);
> + BUILD_BUG_ON((_AC(CONFIG_PAGE_OFFSET, UL) % PGDIR_SIZE) != 0);
> BUG_ON((load_pa % map_size) != 0);
> BUG_ON(load_sz > MAX_EARLY_MAPPING_SIZE);
>
> @@ -400,13 +466,13 @@ asmlinkage void __init setup_vm(uintptr_t dtb_pa)
> create_pmd_mapping(fixmap_pmd, FIXADDR_START,
> (uintptr_t)fixmap_pte, PMD_SIZE, PAGE_TABLE);
> /* Setup trampoline PGD and PMD */
> - create_pgd_mapping(trampoline_pg_dir, PAGE_OFFSET,
> + create_pgd_mapping(trampoline_pg_dir, kernel_load_addr,
> (uintptr_t)trampoline_pmd, PGDIR_SIZE, PAGE_TABLE);
> - create_pmd_mapping(trampoline_pmd, PAGE_OFFSET,
> + create_pmd_mapping(trampoline_pmd, kernel_load_addr,
> load_pa, PMD_SIZE, PAGE_KERNEL_EXEC);
> #else
> /* Setup trampoline PGD */
> - create_pgd_mapping(trampoline_pg_dir, PAGE_OFFSET,
> + create_pgd_mapping(trampoline_pg_dir, kernel_load_addr,
> load_pa, PGDIR_SIZE, PAGE_KERNEL_EXEC);
> #endif
>
> @@ -415,10 +481,10 @@ asmlinkage void __init setup_vm(uintptr_t dtb_pa)
> * us to reach paging_init(). We map all memory banks later
> * in setup_vm_final() below.
> */
> - end_va = PAGE_OFFSET + load_sz;
> - for (va = PAGE_OFFSET; va < end_va; va += map_size)
> + end_va = kernel_load_addr + load_sz;
> + for (va = kernel_load_addr; va < end_va; va += map_size)
> create_pgd_mapping(early_pg_dir, va,
> - load_pa + (va - PAGE_OFFSET),
> + load_pa + (va - kernel_load_addr),
> map_size, PAGE_KERNEL_EXEC);
>
> /* Create fixed mapping for early FDT parsing */
> @@ -457,9 +523,9 @@ static void __init setup_vm_final(void)
> break;
> if (memblock_is_nomap(reg))
> continue;
> - if (start <= __pa(PAGE_OFFSET) &&
> - __pa(PAGE_OFFSET) < end)
> - start = __pa(PAGE_OFFSET);
> + if (start <= __pa(kernel_load_addr) &&
> + __pa(kernel_load_addr) < end)
> + start = __pa(kernel_load_addr);
>
> map_size = best_map_size(start, end - start);
> for (pa = start; pa < end; pa += map_size) {
> --
> 2.20.1
>

Apart from minor comments above, I will certainly try this patch
my end.

Regards,
Anup

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