Re: [PATCH v7 1/3] riscv: Introduce CONFIG_RELOCATABLE
From: Alexandre ghiti
Date: Mon Jan 10 2022 - 03:05:22 EST
Hi Palmer,
Do you think this could go in for-next?
Thanks,
Alex
On 12/6/21 10:44, Alexandre ghiti wrote:
@Palmer, can I do anything for that to be pulled in 5.17?
Thanks,
Alex
On 10/27/21 07:04, Alexandre ghiti wrote:
Hi Palmer,
On 10/26/21 11:29 PM, Palmer Dabbelt wrote:
On Sat, 09 Oct 2021 10:20:20 PDT (-0700), alex@xxxxxxxx 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@xxxxxxxx>
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@xxxxxxxx>
---
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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