Re: [PATCH v3 2/3] riscv: Introduce CONFIG_RELOCATABLE

[Zong Li <zong.li@sifive.com>]

On Sun, May 24, 2020 at 4:55 PM Alexandre Ghiti <alex@ghiti.fr> 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@ghiti.fr>
> ---
>  arch/riscv/Kconfig              | 12 +++++++
>  arch/riscv/Makefile             |  5 ++-
>  arch/riscv/kernel/vmlinux.lds.S |  6 ++--
>  arch/riscv/mm/Makefile          |  4 +++
>  arch/riscv/mm/init.c            | 63 +++++++++++++++++++++++++++++++++
>  5 files changed, 87 insertions(+), 3 deletions(-)
>
> diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
> index a31e1a41913a..93127d5913fe 100644
> --- a/arch/riscv/Kconfig
> +++ b/arch/riscv/Kconfig
> @@ -170,6 +170,18 @@ config PGTABLE_LEVELS
>         default 3 if 64BIT
>         default 2
>
> +config RELOCATABLE
> +       bool
> +       depends on MMU
> +       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.
> +
>  source "arch/riscv/Kconfig.socs"
>
>  menu "Platform type"
> diff --git a/arch/riscv/Makefile b/arch/riscv/Makefile
> index fb6e37db836d..1406416ea743 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/kernel/vmlinux.lds.S b/arch/riscv/kernel/vmlinux.lds.S
> index a9abde62909f..e8ffba8c2044 100644
> --- a/arch/riscv/kernel/vmlinux.lds.S
> +++ b/arch/riscv/kernel/vmlinux.lds.S
> @@ -85,8 +85,10 @@ SECTIONS
>
>         BSS_SECTION(PAGE_SIZE, PAGE_SIZE, 0)
>
> -       .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 363ef01c30b1..dc5cdaa80bc1 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 17f108baec4f..7074522d40c6 100644
> --- a/arch/riscv/mm/init.c
> +++ b/arch/riscv/mm/init.c
> @@ -13,6 +13,9 @@
>  #include <linux/of_fdt.h>
>  #include <linux/libfdt.h>
>  #include <linux/set_memory.h>
> +#ifdef CONFIG_RELOCATABLE
> +#include <linux/elf.h>
> +#endif
>
>  #include <asm/fixmap.h>
>  #include <asm/tlbflush.h>
> @@ -379,6 +382,53 @@ 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;
> +       /*
> +        * This holds the offset between the linked virtual address and the
> +        * relocated virtual address.
> +        */
> +       uintptr_t reloc_offset = kernel_virt_addr - KERNEL_LINK_ADDR;
> +       /*
> +        * This holds the offset between kernel linked virtual address and
> +        * physical address.
> +        */
> +       uintptr_t va_kernel_link_pa_offset = KERNEL_LINK_ADDR - load_pa;
> +
> +       for ( ; rela < (Elf_Rela *)&__rela_dyn_end; rela++) {
> +               Elf_Addr addr = (rela->r_offset - va_kernel_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 >= KERNEL_LINK_ADDR)
> +                       relocated_addr += reloc_offset;
> +
> +               *(Elf_Addr *)addr = relocated_addr;
> +       }
> +}
> +
> +#endif
> +
>  static uintptr_t load_pa, load_sz;
>
>  void create_kernel_page_table(pgd_t *pgdir, uintptr_t map_size)
> @@ -405,6 +455,19 @@ asmlinkage void __init setup_vm(uintptr_t dtb_pa)
>
>         pfn_base = PFN_DOWN(load_pa);
>
> +#ifdef CONFIG_RELOCATABLE
> +#ifdef CONFIG_64BIT
> +       /*
> +        * Early page table uses only one PGDIR, which makes it possible
> +        * to map PGDIR_SIZE aligned on PGDIR_SIZE: if the relocation offset
> +        * makes the kernel cross over a PGDIR_SIZE boundary, raise a bug
> +        * since a part of the kernel would not get mapped.
> +        * This cannot happen on rv32 as we use the entire page directory level.
> +        */
> +       BUG_ON(PGDIR_SIZE - (kernel_virt_addr & (PGDIR_SIZE - 1)) < load_sz);
> +#endif
> +       relocate_kernel(load_pa);
> +#endif
>         /*
>          * Enforce boot alignment requirements of RV32 and
>          * RV64 by only allowing PMD or PGD mappings.
> --
> 2.20.1
>

Looks good to me.

Reviewed-by: Zong Li <zong.li@sifive.com>