Re: [PATCH RESEND v2] riscv: Introduce CONFIG_RELOCATABLE

From: Alex Ghiti
Date: Sun Mar 22 2020 - 03:11:27 EST

Hi Zong,

Sorry for the response delay, please find below my comments.

On 3/12/20 1:57 AM, Zong Li wrote:
On Sat, Mar 7, 2020 at 1:58 AM Palmer Dabbelt <palmer@xxxxxxxxxxx> wrote:

On Mon, 02 Mar 2020 21:44:37 PST (-0800), 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>
Reviewed-by: Zong Li <zong.li@xxxxxxxxxx>
Reviewed-by: Anup Patel <anup@xxxxxxxxxxxxxx>
Tested-by: Zong Li <zong.li@xxxxxxxxxx>
---
Changes in v2:
- Make RELOCATABLE depend on MMU as suggested by Anup
- Rename kernel_load_addr into kernel_virt_addr as suggested by Anup
- Use __pa_symbol instead of __pa, as suggested by Zong
- Rebased on top of v5.6-rc3
- Tested with sv48 patchset
- Add Reviewed/Tested-by from Zong and Anup

arch/riscv/Kconfig | 12 +++++
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, 111 insertions(+), 22 deletions(-)

diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
index 73f029eae0cc..f5f3d474504d 100644
--- a/arch/riscv/Kconfig
+++ b/arch/riscv/Kconfig
@@ -163,6 +163,18 @@ config PGTABLE_LEVELS
default 3 if 64BIT
default 2

+config RELOCATABLE
+ bool
+ depends on MMU
+ 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"

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)
diff --git a/arch/riscv/include/asm/page.h b/arch/riscv/include/asm/page.h
index 8ca1930caa44..af5810f9aebd 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_virt_addr

I assume we want to keep PAGE_OFFSET a constant for the non-relocatable
systems. As it currently stands this is imposing a performance hit even when


I had almost done the KASLR implementation on top of this patch.
Actually, PAGE_OFFSET change is unnecessary in KASLR , because we
would move kernel image to a random physical address as well, so the
$pc will go to the relevant random virtual address. We need

I don't understand what you mean here, can you explain it a bit more ?

kernel_virt_addr to record the new destination, but keep PAGE_OFFSET
to be CONFIG_PAGE_OFFSET is enough.

From my understanding, PAGE_OFFSET should represent the start of the direct mapping, so in case of a relocatable kernel, its value should reflect the offset too.

Is there any issue with having PAGE_OFFSET equal to kernel_virt_addr ?

If we use both, we will need to know precisely when we should use kernel_virt_addr or PAGE_OFFSET, which I think will be painful and error prone.


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

This seems like it would cause issues if the kernel is relocated to high enough
addresses that "kernel_virt_addr+KERN_VIRT_SIZE" overflows.


Based on the same reason, keep KERN_VIRT_SIZE to be -PAGE_OFFSET is good.

#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_virt_addr;
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 271860fc2c3f..d792912c2da3 100644
--- a/arch/riscv/kernel/head.S
+++ b/arch/riscv/kernel/head.S
@@ -131,7 +131,8 @@ clear_bss_done:
#ifdef CONFIG_MMU
relocate:
/* Relocate return address */
- li a1, PAGE_OFFSET
+ la a1, kernel_virt_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 1e0193ded420..5bf69e9b91e6 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>
@@ -71,9 +71,11 @@ SECTIONS

EXCEPTION_TABLE(0x10)

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

It looks like the indentation is screwed up here: I see a mix of tabs/spaces
that doesn't match the rest of the file.


_end = .;

diff --git a/arch/riscv/mm/Makefile b/arch/riscv/mm/Makefile
index 50b7af58c566..27593d362248 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..428aee2669aa 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_virt_addr = _AC(CONFIG_PAGE_OFFSET, UL);
+EXPORT_SYMBOL(kernel_virt_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_virt_addr);

PAGE_OFFSET is kernel_virt_addr, so I don't see any reason to change these --
they account for a significant fraction of the diff.


kernel_virt_addr would be assigned to a random destination by KASLR,
but here still should be PAGE_OFFSET rather than kernel_virt_addr as
mentioned above.

/*
* 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_virt_addr) >> PGDIR_SHIFT;

Here is the same, please use PAGE_OFFSET instead of kernel_virt_addr.

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_virt_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;

This should at least provide a warning when it encounters an unresolvable
relocation. Is it currently stands this just ignores all other runtime
relocations, and while I can buy the argument there shouldn't be any (though
I'd expect R_RISCV_{32,64} to show up?) we certainly shouldn't just silently
skip them.

+
+ /*
+ * 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_virt_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_virt_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_virt_addr,
(uintptr_t)trampoline_pmd, PGDIR_SIZE, PAGE_TABLE);
- create_pmd_mapping(trampoline_pmd, PAGE_OFFSET,
+ create_pmd_mapping(trampoline_pmd, kernel_virt_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_virt_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_virt_addr + load_sz;
+ for (va = kernel_virt_addr; va < end_va; va += map_size)
create_pgd_mapping(early_pg_dir, va,
- load_pa + (va - PAGE_OFFSET),
+ load_pa + (va - kernel_virt_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_symbol(kernel_virt_addr) &&
+ __pa(kernel_virt_addr) < end)
+ start = __pa_symbol(kernel_virt_addr);

Here is the same, please use PAGE_OFFSET instead of kernel_virt_addr.


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

Thanks,

Alex