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Subject: Re: [PATCH v7 1/3] riscv: Introduce CONFIG_RELOCATABLE
To: Alexandre Ghiti <alexandre.ghiti@canonical.com>,
 Michael Ellerman <mpe@ellerman.id.au>,
 Benjamin Herrenschmidt <benh@kernel.crashing.org>,
 Paul Mackerras <paulus@samba.org>, Paul Walmsley <paul.walmsley@sifive.com>,
 Palmer Dabbelt <palmer@dabbelt.com>, Albert Ou <aou@eecs.berkeley.edu>,
 linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org,
 linux-riscv@lists.infradead.org
References: <20211009171259.2515351-1-alexandre.ghiti@canonical.com>
 <20211009171259.2515351-2-alexandre.ghiti@canonical.com>
From: Alexandre ghiti <alex@ghiti.fr>
Message-ID: <a6223864-7263-5450-0890-0f05a137d8c2@ghiti.fr>
Date: Sat, 9 Oct 2021 19:20:20 +0200
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 <linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org>

Arf, I have sent this patchset with the wrong email address. @Palmer
tell me if you want me to resend it correctly.

Thanks,

Alex

On 10/9/21 7:12 PM, Alexandre Ghiti wrote:
> From: Alexandre Ghiti <alex@ghiti.fr>
>
> This config allows to compile 64b kernel as PIE and to relocate it at
> any virtual address at runtime: this paves the way to KASLR.
> 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             |  7 +++--
>  arch/riscv/kernel/vmlinux.lds.S |  6 ++++
>  arch/riscv/mm/Makefile          |  4 +++
>  arch/riscv/mm/init.c            | 54 ++++++++++++++++++++++++++++++++-
>  5 files changed, 80 insertions(+), 3 deletions(-)
>
> diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
> index ea16fa2dd768..043ba92559fa 100644
> --- a/arch/riscv/Kconfig
> +++ b/arch/riscv/Kconfig
> @@ -213,6 +213,18 @@ config PGTABLE_LEVELS
>  config LOCKDEP_SUPPORT
>  	def_bool y
>  
> +config RELOCATABLE
> +	bool
> +	depends on MMU && 64BIT && !XIP_KERNEL
> +	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"
>  source "arch/riscv/Kconfig.erratas"
>  
> diff --git a/arch/riscv/Makefile b/arch/riscv/Makefile
> index 0eb4568fbd29..2f509915f246 100644
> --- a/arch/riscv/Makefile
> +++ b/arch/riscv/Makefile
> @@ -9,9 +9,12 @@
>  #
>  
>  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
> +	LDFLAGS_vmlinux += --no-relax
>  	KBUILD_CPPFLAGS += -DCC_USING_PATCHABLE_FUNCTION_ENTRY
>  	CC_FLAGS_FTRACE := -fpatchable-function-entry=8
>  endif
> diff --git a/arch/riscv/kernel/vmlinux.lds.S b/arch/riscv/kernel/vmlinux.lds.S
> index 5104f3a871e3..862a8c09723c 100644
> --- a/arch/riscv/kernel/vmlinux.lds.S
> +++ b/arch/riscv/kernel/vmlinux.lds.S
> @@ -133,6 +133,12 @@ SECTIONS
>  
>  	BSS_SECTION(PAGE_SIZE, PAGE_SIZE, 0)
>  
> +	.rela.dyn : ALIGN(8) {
> +		__rela_dyn_start = .;
> +		*(.rela .rela*)
> +		__rela_dyn_end = .;
> +	}
> +
>  #ifdef CONFIG_EFI
>  	. = ALIGN(PECOFF_SECTION_ALIGNMENT);
>  	__pecoff_data_virt_size = ABSOLUTE(. - __pecoff_text_end);
> diff --git a/arch/riscv/mm/Makefile b/arch/riscv/mm/Makefile
> index 7ebaef10ea1b..2d33ec574bbb 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 = $(CC_FLAGS_FTRACE)
>  CFLAGS_REMOVE_cacheflush.o = $(CC_FLAGS_FTRACE)
> diff --git a/arch/riscv/mm/init.c b/arch/riscv/mm/init.c
> index c0cddf0fc22d..42041c12d496 100644
> --- a/arch/riscv/mm/init.c
> +++ b/arch/riscv/mm/init.c
> @@ -20,6 +20,9 @@
>  #include <linux/dma-map-ops.h>
>  #include <linux/crash_dump.h>
>  #include <linux/hugetlb.h>
> +#ifdef CONFIG_RELOCATABLE
> +#include <linux/elf.h>
> +#endif
>  
>  #include <asm/fixmap.h>
>  #include <asm/tlbflush.h>
> @@ -103,7 +106,7 @@ static void __init print_vm_layout(void)
>  	print_mlm("lowmem", (unsigned long)PAGE_OFFSET,
>  		  (unsigned long)high_memory);
>  #ifdef CONFIG_64BIT
> -	print_mlm("kernel", (unsigned long)KERNEL_LINK_ADDR,
> +	print_mlm("kernel", (unsigned long)kernel_map.virt_addr,
>  		  (unsigned long)ADDRESS_SPACE_END);
>  #endif
>  }
> @@ -518,6 +521,44 @@ static __init pgprot_t pgprot_from_va(uintptr_t va)
>  #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;
> +
> +static void __init relocate_kernel(void)
> +{
> +	Elf64_Rela *rela = (Elf64_Rela *)&__rela_dyn_start;
> +	/*
> +	 * This holds the offset between the linked virtual address and the
> +	 * relocated virtual address.
> +	 */
> +	uintptr_t reloc_offset = kernel_map.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 - kernel_map.phys_addr;
> +
> +	for ( ; rela < (Elf64_Rela *)&__rela_dyn_end; rela++) {
> +		Elf64_Addr addr = (rela->r_offset - va_kernel_link_pa_offset);
> +		Elf64_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;
> +
> +		*(Elf64_Addr *)addr = relocated_addr;
> +	}
> +}
> +#endif /* CONFIG_RELOCATABLE */
> +
>  #ifdef CONFIG_XIP_KERNEL
>  static void __init create_kernel_page_table(pgd_t *pgdir,
>  					    __always_unused bool early)
> @@ -625,6 +666,17 @@ asmlinkage void __init setup_vm(uintptr_t dtb_pa)
>  	BUG_ON((kernel_map.virt_addr + kernel_map.size) > ADDRESS_SPACE_END - SZ_4K);
>  #endif
>  
> +#ifdef CONFIG_RELOCATABLE
> +	/*
> +	 * 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.
> +	 */
> +	BUG_ON(PGDIR_SIZE - (kernel_map.virt_addr & (PGDIR_SIZE - 1)) < kernel_map.size);
> +	relocate_kernel();
> +#endif
> +
>  	pt_ops.alloc_pte = alloc_pte_early;
>  	pt_ops.get_pte_virt = get_pte_virt_early;
>  #ifndef __PAGETABLE_PMD_FOLDED