[PATCH v8 1/3] riscv: Introduce CONFIG_RELOCATABLE
Björn Töpel
bjorn at kernel.org
Sat Feb 25 02:58:52 AEDT 2023
Alexandre Ghiti <alex at ghiti.fr> writes:
> +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.
Don't ask me *why*, but adding --emit-relocs to your linker flags solves
"the NULL .rela.dyn" both for GCC and LLVM.
The downside is that you end up with a bunch of .rela cruft in your
vmlinux.
Björn
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