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Message-ID: <5846825d-cd7e-5085-569e-17cfaf36630f@ghiti.fr>
Date: Wed, 27 Oct 2021 07:04:42 +0200
From: Alexandre ghiti <alex@...ti.fr>
To: Palmer Dabbelt <palmer@...belt.com>
Cc: alexandre.ghiti@...onical.com, mpe@...erman.id.au,
benh@...nel.crashing.org, paulus@...ba.org,
Paul Walmsley <paul.walmsley@...ive.com>,
aou@...s.berkeley.edu, linuxppc-dev@...ts.ozlabs.org,
linux-kernel@...r.kernel.org, linux-riscv@...ts.infradead.org
Subject: Re: [PATCH v7 1/3] riscv: Introduce CONFIG_RELOCATABLE
Hi Palmer,
On 10/26/21 11:29 PM, Palmer Dabbelt wrote:
> On Sat, 09 Oct 2021 10:20:20 PDT (-0700), alex@...ti.fr wrote:
>> Arf, I have sent this patchset with the wrong email address. @Palmer
>> tell me if you want me to resend it correctly.
>
> Sorry for being kind of slow here. It's fine: there's a "From:" in
> the patch, and git picks those up so it'll match the signed-off-by
> line. I send pretty much all my patches that way, as I never managed
> to get my Google address working correctly.
>
>>
>> Thanks,
>>
>> Alex
>>
>> On 10/9/21 7:12 PM, Alexandre Ghiti wrote:
>>> From: Alexandre Ghiti <alex@...ti.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.
>
> IMO this should really be user selectable, at a bare minimum so it's
> testable.
> I just sent along a patch to do that (my power's off at home, so email
> is a bit
> wacky right now).
>
> I haven't put this on for-next yet as I'm not sure if you had a fix
> for the
> kasan issue (which IIUC would conflict with this).
The kasan issue only revealed that I need to move the kasan shadow
memory around with sv48 support, that's not related to the relocatable
kernel.
Thanks,
Alex
>
>>> 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@...ti.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
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