[PATCH v8 1/3] riscv: Introduce CONFIG_RELOCATABLE
Alexandre Ghiti
alex at ghiti.fr
Wed Feb 22 23:29:11 AEDT 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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