[PATCH v8 1/3] riscv: Introduce CONFIG_RELOCATABLE

Alexandre Ghiti alex at ghiti.fr
Wed Feb 22 04:29:11 PST 2023


+cc linux-kbuild, llvm, Nathan, Nick

On 2/15/23 15:36, Alexandre Ghiti wrote:
> From: Alexandre Ghiti <alex at 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 at ghiti.fr>
> ---
>   arch/riscv/Kconfig              | 14 +++++++++
>   arch/riscv/Makefile             |  7 +++--
>   arch/riscv/kernel/efi-header.S  |  6 ++--
>   arch/riscv/kernel/vmlinux.lds.S | 10 ++++--
>   arch/riscv/mm/Makefile          |  4 +++
>   arch/riscv/mm/init.c            | 54 ++++++++++++++++++++++++++++++++-
>   6 files changed, 87 insertions(+), 8 deletions(-)
>
> diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
> index e2b656043abf..e0ee7ce4b2e3 100644
> --- a/arch/riscv/Kconfig
> +++ b/arch/riscv/Kconfig
> @@ -544,6 +544,20 @@ config COMPAT
>   
>   	  If you want to execute 32-bit userspace applications, say Y.
>   
> +config RELOCATABLE
> +	bool "Build a relocatable kernel"
> +	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.
> +
> +          If unsure, say N.
> +
>   endmenu # "Kernel features"
>   
>   menu "Boot options"
> diff --git a/arch/riscv/Makefile b/arch/riscv/Makefile
> index 82153960ac00..97c34136b027 100644
> --- a/arch/riscv/Makefile
> +++ b/arch/riscv/Makefile
> @@ -7,9 +7,12 @@
>   #
>   
>   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/efi-header.S b/arch/riscv/kernel/efi-header.S
> index 8e733aa48ba6..f7ee09c4f12d 100644
> --- a/arch/riscv/kernel/efi-header.S
> +++ b/arch/riscv/kernel/efi-header.S
> @@ -33,7 +33,7 @@ optional_header:
>   	.byte	0x02					// MajorLinkerVersion
>   	.byte	0x14					// MinorLinkerVersion
>   	.long	__pecoff_text_end - efi_header_end	// SizeOfCode
> -	.long	__pecoff_data_virt_size			// SizeOfInitializedData
> +	.long	__pecoff_data_virt_end - __pecoff_text_end	// SizeOfInitializedData
>   	.long	0					// SizeOfUninitializedData
>   	.long	__efistub_efi_pe_entry - _start		// AddressOfEntryPoint
>   	.long	efi_header_end - _start			// BaseOfCode
> @@ -91,9 +91,9 @@ section_table:
>   		IMAGE_SCN_MEM_EXECUTE			// Characteristics
>   
>   	.ascii	".data\0\0\0"
> -	.long	__pecoff_data_virt_size			// VirtualSize
> +	.long	__pecoff_data_virt_end - __pecoff_text_end	// VirtualSize
>   	.long	__pecoff_text_end - _start		// VirtualAddress
> -	.long	__pecoff_data_raw_size			// SizeOfRawData
> +	.long	__pecoff_data_raw_end - __pecoff_text_end	// SizeOfRawData
>   	.long	__pecoff_text_end - _start		// PointerToRawData
>   
>   	.long	0					// PointerToRelocations
> diff --git a/arch/riscv/kernel/vmlinux.lds.S b/arch/riscv/kernel/vmlinux.lds.S
> index 4e6c88aa4d87..8be2de3be08c 100644
> --- a/arch/riscv/kernel/vmlinux.lds.S
> +++ b/arch/riscv/kernel/vmlinux.lds.S
> @@ -122,9 +122,15 @@ SECTIONS
>   		*(.sdata*)
>   	}
>   
> +	.rela.dyn : ALIGN(8) {
> +		__rela_dyn_start = .;
> +		*(.rela .rela*)
> +		__rela_dyn_end = .;
> +	}
> +


So I realized those relocations would be better in the init section so 
we can get rid of them at some point. So I tried the following:

diff --git a/arch/riscv/kernel/vmlinux.lds.S 
b/arch/riscv/kernel/vmlinux.lds.S
index 7ac215467fd5..6111023a89ef 100644
--- a/arch/riscv/kernel/vmlinux.lds.S
+++ b/arch/riscv/kernel/vmlinux.lds.S
@@ -93,6 +93,12 @@ SECTIONS
                 *(.rel.dyn*)
         }

+       .rela.dyn : ALIGN(8) {
+               __rela_dyn_start = .;
+               *(.rela .rela*)
+               __rela_dyn_end = .;
+       }
+
         __init_data_end = .;

         . = ALIGN(8);
@@ -119,12 +125,6 @@ SECTIONS
                 *(.sdata*)
         }

-       .rela.dyn : ALIGN(8) {
-               __rela_dyn_start = .;
-               *(.rela .rela*)
-               __rela_dyn_end = .;
-       }
-
  #ifdef CONFIG_EFI
         .pecoff_edata_padding : { BYTE(0); . = 
ALIGN(PECOFF_FILE_ALIGNMENT); }
         __pecoff_data_raw_end = ABSOLUTE(.);


But then all the relocations in vmlinux end up being null:

vmlinux:     file format elf64-littleriscv

$ riscv64-linux-gnu-objdump -R vmlinux

DYNAMIC RELOCATION RECORDS
OFFSET           TYPE              VALUE
0000000000000000 R_RISCV_NONE      *ABS*
0000000000000000 R_RISCV_NONE      *ABS*
....

  I also noticed that re-linking vmlinux with the same command right 
after works (ie, the relocations are now valid):

$ riscv64-linux-gnu-objdump -R vmlinux

vmlinux:     file format elf64-littleriscv

DYNAMIC RELOCATION RECORDS
OFFSET           TYPE              VALUE
ffffffff82600718 R_RISCV_RELATIVE  *ABS*-0x000000007d9ff8e8
ffffffff82600720 R_RISCV_RELATIVE  *ABS*-0x000000007d9ff8e8
...

Below is the command used to generate this working vmlinux:

riscv64-unknown-linux-gnu-ld -melf64lriscv -z noexecstack 
--no-warn-rwx-segments -shared -Bsymbolic -z notext -z norelro 
--no-relax --build-id=sha1 --script=./arch/riscv/kernel/vmlinux.lds 
-Map=vmlinux.map -o vmlinux --whole-archive vmlinux.a .vmlinux.export.o 
init/version-timestamp.o --no-whole-archive --start-group 
./drivers/firmware/efi/libstub/lib.a --end-group .tmp_vmlinux.kallsyms3.o

I tried a lot of things, but I struggle to understand, does anyone have 
any idea? FYI, the same problem happens with LLVM.

Thanks,

Alex


>   #ifdef CONFIG_EFI
>   	.pecoff_edata_padding : { BYTE(0); . = ALIGN(PECOFF_FILE_ALIGNMENT); }
> -	__pecoff_data_raw_size = ABSOLUTE(. - __pecoff_text_end);
> +	__pecoff_data_raw_end = ABSOLUTE(.);
>   #endif
>   
>   	/* End of data section */
> @@ -134,7 +140,7 @@ SECTIONS
>   
>   #ifdef CONFIG_EFI
>   	. = ALIGN(PECOFF_SECTION_ALIGNMENT);
> -	__pecoff_data_virt_size = ABSOLUTE(. - __pecoff_text_end);
> +	__pecoff_data_virt_end = ABSOLUTE(.);
>   #endif
>   	_end = .;
>   
> diff --git a/arch/riscv/mm/Makefile b/arch/riscv/mm/Makefile
> index 2ac177c05352..b85e9e82f082 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 7f01c2e56efe..3862696c2ac9 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>
> @@ -146,7 +149,7 @@ static void __init print_vm_layout(void)
>   		print_ml("kasan", KASAN_SHADOW_START, KASAN_SHADOW_END);
>   #endif
>   
> -		print_ml("kernel", (unsigned long)KERNEL_LINK_ADDR,
> +		print_ml("kernel", (unsigned long)kernel_map.virt_addr,
>   			 (unsigned long)ADDRESS_SPACE_END);
>   	}
>   }
> @@ -854,6 +857,44 @@ static __init void set_satp_mode(uintptr_t dtb_pa)
>   #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)
> @@ -1039,6 +1080,17 @@ 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 PUD, which makes it possible
> +	 * to map PUD_SIZE aligned on PUD_SIZE: if the relocation offset
> +	 * makes the kernel cross over a PUD_SIZE boundary, raise a bug
> +	 * since a part of the kernel would not get mapped.
> +	 */
> +	BUG_ON(PUD_SIZE - (kernel_map.virt_addr & (PUD_SIZE - 1)) < kernel_map.size);
> +	relocate_kernel();
> +#endif
> +
>   	apply_early_boot_alternatives();
>   	pt_ops_set_early();
>   



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