Re: [PATCH] riscv: Introduce CONFIG_RELOCATABLE

From: Alex Ghiti
Date: Mon Jan 27 2020 - 08:29:49 EST

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Hi Anup,

On 1/24/20 11:57 PM, Anup Patel wrote:

On Fri, Jan 24, 2020 at 1:44 AM Alexandre Ghiti <alex@xxxxxxxx> wrote:

This config allows to compile the kernel as PIE and to relocate it at any
virtual address at runtime: this paves the way to KASLR and to 4-level
page table folding at runtime. 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@xxxxxxxx>
---
arch/riscv/Kconfig | 11 ++++
arch/riscv/Makefile | 5 +-
arch/riscv/boot/loader.lds.S | 2 +-
arch/riscv/include/asm/page.h | 5 +-
arch/riscv/kernel/head.S | 3 +-
arch/riscv/kernel/vmlinux.lds.S | 10 ++--
arch/riscv/mm/Makefile | 4 ++
arch/riscv/mm/init.c | 92 ++++++++++++++++++++++++++++-----
8 files changed, 110 insertions(+), 22 deletions(-)

diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
index fa7dc03459e7..c652b4b850ce 100644
--- a/arch/riscv/Kconfig
+++ b/arch/riscv/Kconfig
@@ -163,6 +163,17 @@ config PGTABLE_LEVELS
default 3 if 64BIT
default 2

+config RELOCATABLE
+ bool
+ 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.
+

I am sure this cannot be used for NOMMU support because of the
page table mappings.

I guess this should depend on MMU.

Yes you're right, I'll add a dependency on MMU in v2, thanks.

source "arch/riscv/Kconfig.socs"

menu "Platform type"
diff --git a/arch/riscv/Makefile b/arch/riscv/Makefile
index b9009a2fbaf5..5a115cf6a9c1 100644
--- a/arch/riscv/Makefile
+++ b/arch/riscv/Makefile
@@ -9,7 +9,10 @@
#

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
endif
diff --git a/arch/riscv/boot/loader.lds.S b/arch/riscv/boot/loader.lds.S
index 47a5003c2e28..a9ed218171aa 100644
--- a/arch/riscv/boot/loader.lds.S
+++ b/arch/riscv/boot/loader.lds.S
@@ -7,7 +7,7 @@ ENTRY(_start)

SECTIONS
{
- . = PAGE_OFFSET;
+ . = CONFIG_PAGE_OFFSET;

.payload : {
*(.payload)

The loader.lds.S is used by NOMMU support so this should be separate patch.

This patch defines PAGE_OFFSET as a variable which will make NOMMU support not compilable,
I'm not sure I should move that out of this patch, are you ?

diff --git a/arch/riscv/include/asm/page.h b/arch/riscv/include/asm/page.h
index ac699246ae7e..27c95da68ecb 100644
--- a/arch/riscv/include/asm/page.h
+++ b/arch/riscv/include/asm/page.h
@@ -31,9 +31,9 @@
* When not using MMU this corresponds to the first free page in
* physical memory (aligned on a page boundary).
*/
-#define PAGE_OFFSET _AC(CONFIG_PAGE_OFFSET, UL)
+#define PAGE_OFFSET kernel_load_addr

-#define KERN_VIRT_SIZE (-PAGE_OFFSET)
+#define KERN_VIRT_SIZE (-_AC(CONFIG_PAGE_OFFSET, UL))

#ifndef __ASSEMBLY__

@@ -97,6 +97,7 @@ extern unsigned long pfn_base;
#define ARCH_PFN_OFFSET (PAGE_OFFSET >> PAGE_SHIFT)
#endif /* CONFIG_MMU */

+extern unsigned long kernel_load_addr;

Having this variable named kernel_virt_addr seems more
logical to me.

Is it mandatory to use kernel_load_addr name ?

Not mandatory at all, I decided the naming. I agree with your suggestion, "load" refers better
to physical addressing. I'll change to kernel_virt_addr in v2, thanks.

extern unsigned long max_low_pfn;
extern unsigned long min_low_pfn;

diff --git a/arch/riscv/kernel/head.S b/arch/riscv/kernel/head.S
index 2227db63f895..5042b2b48a06 100644
--- a/arch/riscv/kernel/head.S
+++ b/arch/riscv/kernel/head.S
@@ -126,7 +126,8 @@ clear_bss_done:
#ifdef CONFIG_MMU
relocate:
/* Relocate return address */
- li a1, PAGE_OFFSET
+ la a1, kernel_load_addr
+ REG_L a1, 0(a1)
la a2, _start
sub a1, a1, a2
add ra, ra, a1
diff --git a/arch/riscv/kernel/vmlinux.lds.S b/arch/riscv/kernel/vmlinux.lds.S
index 12f42f96d46e..5095aee7c37e 100644
--- a/arch/riscv/kernel/vmlinux.lds.S
+++ b/arch/riscv/kernel/vmlinux.lds.S
@@ -4,7 +4,7 @@
* Copyright (C) 2017 SiFive
*/

-#define LOAD_OFFSET PAGE_OFFSET
+#define LOAD_OFFSET CONFIG_PAGE_OFFSET
#include <asm/vmlinux.lds.h>
#include <asm/page.h>
#include <asm/cache.h>
@@ -70,9 +70,11 @@ SECTIONS

EXCEPTION_TABLE(0x10)

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

_end = .;

diff --git a/arch/riscv/mm/Makefile b/arch/riscv/mm/Makefile
index a1bd95c8047a..dcd3d806243f 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 = -pg
endif
diff --git a/arch/riscv/mm/init.c b/arch/riscv/mm/init.c
index 965a8cf4829c..ac9a9f69abc0 100644
--- a/arch/riscv/mm/init.c
+++ b/arch/riscv/mm/init.c
@@ -12,6 +12,9 @@
#include <linux/sizes.h>
#include <linux/of_fdt.h>
#include <linux/libfdt.h>
+#ifdef CONFIG_RELOCATABLE
+#include <linux/elf.h>
+#endif

#include <asm/fixmap.h>
#include <asm/tlbflush.h>
@@ -28,6 +31,9 @@ EXPORT_SYMBOL(empty_zero_page);
extern char _start[];
void *dtb_early_va;

+unsigned long kernel_load_addr = _AC(CONFIG_PAGE_OFFSET, UL);
+EXPORT_SYMBOL(kernel_load_addr);
+
static void __init zone_sizes_init(void)
{
unsigned long max_zone_pfns[MAX_NR_ZONES] = { 0, };
@@ -132,7 +138,8 @@ void __init setup_bootmem(void)
phys_addr_t end = reg->base + reg->size;

if (reg->base <= vmlinux_end && vmlinux_end <= end) {
- mem_size = min(reg->size, (phys_addr_t)-PAGE_OFFSET);
+ mem_size = min(reg->size,
+ (phys_addr_t)-kernel_load_addr);

/*
* Remove memblock from the end of usable area to the
@@ -269,7 +276,7 @@ static phys_addr_t __init alloc_pmd(uintptr_t va)
if (mmu_enabled)
return memblock_phys_alloc(PAGE_SIZE, PAGE_SIZE);

- pmd_num = (va - PAGE_OFFSET) >> PGDIR_SHIFT;
+ pmd_num = (va - kernel_load_addr) >> PGDIR_SHIFT;
BUG_ON(pmd_num >= NUM_EARLY_PMDS);
return (uintptr_t)&early_pmd[pmd_num * PTRS_PER_PMD];
}
@@ -370,6 +377,54 @@ static uintptr_t __init best_map_size(phys_addr_t base, phys_addr_t size)
#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;
+
+#ifdef CONFIG_64BIT
+#define Elf_Rela Elf64_Rela
+#define Elf_Addr Elf64_Addr
+#else
+#define Elf_Rela Elf32_Rela
+#define Elf_Addr Elf32_Addr
+#endif
+
+void __init relocate_kernel(uintptr_t load_pa)
+{
+ Elf_Rela *rela = (Elf_Rela *)&__rela_dyn_start;
+ uintptr_t link_addr = _AC(CONFIG_PAGE_OFFSET, UL);
+ /*
+ * This holds the offset between the linked virtual address and the
+ * relocated virtual address.
+ */
+ uintptr_t reloc_offset = kernel_load_addr - link_addr;
+ /*
+ * This holds the offset between linked virtual address and physical
+ * address whereas va_pa_offset holds the offset between relocated
+ * virtual address and physical address.
+ */
+ uintptr_t va_link_pa_offset = link_addr - load_pa;
+
+ for ( ; rela < (Elf_Rela *)&__rela_dyn_end; rela++) {
+ Elf_Addr addr = (rela->r_offset - va_link_pa_offset);
+ Elf_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 >= link_addr)
+ relocated_addr += reloc_offset;
+
+ *(Elf_Addr *)addr = relocated_addr;
+ }
+}
+#endif
+
asmlinkage void __init setup_vm(uintptr_t dtb_pa)
{
uintptr_t va, end_va;
@@ -377,9 +432,20 @@ asmlinkage void __init setup_vm(uintptr_t dtb_pa)
uintptr_t load_sz = (uintptr_t)(&_end) - load_pa;
uintptr_t map_size = best_map_size(load_pa, MAX_EARLY_MAPPING_SIZE);

- va_pa_offset = PAGE_OFFSET - load_pa;
+ va_pa_offset = kernel_load_addr - load_pa;
pfn_base = PFN_DOWN(load_pa);

+#ifdef CONFIG_RELOCATABLE
+ /*
+ * Early page table uses only one PGDIR, which makes it possible
+ * to map 1GB aligned on 1GB: if the relocation offset makes the kernel
+ * cross over a 1G boundary, raise a bug since a part of the kernel
+ * would not get mapped.
+ */
+ BUG_ON(SZ_1G - (kernel_load_addr & (SZ_1G - 1)) < load_sz);
+ relocate_kernel(load_pa);
+#endif
+
/*
* Enforce boot alignment requirements of RV32 and
* RV64 by only allowing PMD or PGD mappings.
@@ -387,7 +453,7 @@ asmlinkage void __init setup_vm(uintptr_t dtb_pa)
BUG_ON(map_size == PAGE_SIZE);

/* Sanity check alignment and size */
- BUG_ON((PAGE_OFFSET % PGDIR_SIZE) != 0);
+ BUILD_BUG_ON((_AC(CONFIG_PAGE_OFFSET, UL) % PGDIR_SIZE) != 0);
BUG_ON((load_pa % map_size) != 0);
BUG_ON(load_sz > MAX_EARLY_MAPPING_SIZE);

@@ -400,13 +466,13 @@ asmlinkage void __init setup_vm(uintptr_t dtb_pa)
create_pmd_mapping(fixmap_pmd, FIXADDR_START,
(uintptr_t)fixmap_pte, PMD_SIZE, PAGE_TABLE);
/* Setup trampoline PGD and PMD */
- create_pgd_mapping(trampoline_pg_dir, PAGE_OFFSET,
+ create_pgd_mapping(trampoline_pg_dir, kernel_load_addr,
(uintptr_t)trampoline_pmd, PGDIR_SIZE, PAGE_TABLE);
- create_pmd_mapping(trampoline_pmd, PAGE_OFFSET,
+ create_pmd_mapping(trampoline_pmd, kernel_load_addr,
load_pa, PMD_SIZE, PAGE_KERNEL_EXEC);
#else
/* Setup trampoline PGD */
- create_pgd_mapping(trampoline_pg_dir, PAGE_OFFSET,
+ create_pgd_mapping(trampoline_pg_dir, kernel_load_addr,
load_pa, PGDIR_SIZE, PAGE_KERNEL_EXEC);
#endif

@@ -415,10 +481,10 @@ asmlinkage void __init setup_vm(uintptr_t dtb_pa)
* us to reach paging_init(). We map all memory banks later
* in setup_vm_final() below.
*/
- end_va = PAGE_OFFSET + load_sz;
- for (va = PAGE_OFFSET; va < end_va; va += map_size)
+ end_va = kernel_load_addr + load_sz;
+ for (va = kernel_load_addr; va < end_va; va += map_size)
create_pgd_mapping(early_pg_dir, va,
- load_pa + (va - PAGE_OFFSET),
+ load_pa + (va - kernel_load_addr),
map_size, PAGE_KERNEL_EXEC);

/* Create fixed mapping for early FDT parsing */
@@ -457,9 +523,9 @@ static void __init setup_vm_final(void)
break;
if (memblock_is_nomap(reg))
continue;
- if (start <= __pa(PAGE_OFFSET) &&
- __pa(PAGE_OFFSET) < end)
- start = __pa(PAGE_OFFSET);
+ if (start <= __pa(kernel_load_addr) &&
+ __pa(kernel_load_addr) < end)
+ start = __pa(kernel_load_addr);

map_size = best_map_size(start, end - start);
for (pa = start; pa < end; pa += map_size) {
--
2.20.1

Apart from minor comments above, I will certainly try this patch
my end.

Great, more testing is always welcome, thanks.

Regards,
Anup

Alex

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