[PATCH v2 07/11] x86/percpu/64: Use relative percpu offsets

Brian Gerst
2023-10-26T16:01:59Z

The percpu section is currently linked at virtual address 0, because
older compilers hardcoded the stack protector canary value at a fixed
offset from the start of the GS segment. Now that the canary is a
normal percpu variable, the percpu section can be linked normally.
This means that x86-64 will calculate percpu offsets like most other
architectures, as the delta between the initial percpu address and the
dynamically allocated memory.

Signed-off-by: Brian Gerst <brgerst@gmail.com>
---
arch/x86/kernel/head_64.S | 6 ------
arch/x86/kernel/setup_percpu.c | 12 ++----------
arch/x86/kernel/vmlinux.lds.S | 24 +-----------------------
arch/x86/platform/pvh/head.S | 6 ------
arch/x86/tools/relocs.c | 10 +++-------
arch/x86/xen/xen-head.S | 6 ------
init/Kconfig | 2 +-
7 files changed, 7 insertions(+), 59 deletions(-)

diff --git a/arch/x86/kernel/head_64.S b/arch/x86/kernel/head_64.S
index f2453eb38417..b35f74e58dd7 100644
--- a/arch/x86/kernel/head_64.S
+++ b/arch/x86/kernel/head_64.S
@@ -72,14 +72,8 @@ SYM_CODE_START_NOALIGN(startup_64)

/* Setup GSBASE to allow stack canary access for C code */
movl $MSR_GS_BASE, %ecx
-#ifdef CONFIG_SMP
- leaq __per_cpu_load(%rip), %rdx
- movl %edx, %eax
- shrq $32, %rdx
-#else
xorl %eax, %eax
xorl %edx, %edx
-#endif
wrmsr

call startup_64_setup_env
diff --git a/arch/x86/kernel/setup_percpu.c b/arch/x86/kernel/setup_percpu.c
index 2c97bf7b56ae..8707dd07b9ce 100644
--- a/arch/x86/kernel/setup_percpu.c
+++ b/arch/x86/kernel/setup_percpu.c
@@ -23,18 +23,10 @@
#include <asm/cpumask.h>
#include <asm/cpu.h>

-#ifdef CONFIG_X86_64
-#define BOOT_PERCPU_OFFSET ((unsigned long)__per_cpu_load)
-#else
-#define BOOT_PERCPU_OFFSET 0
-#endif
-
-DEFINE_PER_CPU_READ_MOSTLY(unsigned long, this_cpu_off) = BOOT_PERCPU_OFFSET;
+DEFINE_PER_CPU_READ_MOSTLY(unsigned long, this_cpu_off);
EXPORT_PER_CPU_SYMBOL(this_cpu_off);

-unsigned long __per_cpu_offset[NR_CPUS] __ro_after_init = {
- [0 ... NR_CPUS-1] = BOOT_PERCPU_OFFSET,
-};
+unsigned long __per_cpu_offset[NR_CPUS] __ro_after_init;
EXPORT_SYMBOL(__per_cpu_offset);

/*
diff --git a/arch/x86/kernel/vmlinux.lds.S b/arch/x86/kernel/vmlinux.lds.S
index e6126cd21615..efa4885060b5 100644
--- a/arch/x86/kernel/vmlinux.lds.S
+++ b/arch/x86/kernel/vmlinux.lds.S
@@ -103,12 +103,6 @@ const_pcpu_hot = pcpu_hot;
PHDRS {
text PT_LOAD FLAGS(5); /* R_E */
data PT_LOAD FLAGS(6); /* RW_ */
-#ifdef CONFIG_X86_64
-#ifdef CONFIG_SMP
- percpu PT_LOAD FLAGS(6); /* RW_ */
-#endif
- init PT_LOAD FLAGS(7); /* RWE */
-#endif
note PT_NOTE FLAGS(0); /* ___ */
}

@@ -224,21 +218,7 @@ SECTIONS
__init_begin = .; /* paired with __init_end */
}

-#if defined(CONFIG_X86_64) && defined(CONFIG_SMP)
- /*
- * percpu offsets are zero-based on SMP. PERCPU_VADDR() changes the
- * output PHDR, so the next output section - .init.text - should
- * start another segment - init.
- */
- PERCPU_VADDR(INTERNODE_CACHE_BYTES, 0, :percpu)
- ASSERT(SIZEOF(.data..percpu) < CONFIG_PHYSICAL_START,
- "per-CPU data too large - increase CONFIG_PHYSICAL_START")
-#endif
-
INIT_TEXT_SECTION(PAGE_SIZE)
-#ifdef CONFIG_X86_64
- :init
-#endif

/*
* Section for code used exclusively before alternatives are run. All
@@ -368,9 +348,7 @@ SECTIONS
EXIT_DATA
}

-#if !defined(CONFIG_X86_64) || !defined(CONFIG_SMP)
PERCPU_SECTION(INTERNODE_CACHE_BYTES)
-#endif

. = ALIGN(PAGE_SIZE);

@@ -508,7 +486,7 @@ SECTIONS
* Per-cpu symbols which need to be offset from __per_cpu_load
* for the boot processor.
*/
-#define INIT_PER_CPU(x) init_per_cpu__##x = ABSOLUTE(x) + __per_cpu_load
+#define INIT_PER_CPU(x) init_per_cpu__##x = ABSOLUTE(x)
INIT_PER_CPU(gdt_page);
INIT_PER_CPU(irq_stack_backing_store);

diff --git a/arch/x86/platform/pvh/head.S b/arch/x86/platform/pvh/head.S
index d215b16bf89f..4bd925b23436 100644
--- a/arch/x86/platform/pvh/head.S
+++ b/arch/x86/platform/pvh/head.S
@@ -96,14 +96,8 @@ SYM_CODE_START_LOCAL(pvh_start_xen)
1:
/* Set base address in stack canary descriptor. */
mov $MSR_GS_BASE,%ecx
-#ifdef CONFIG_SMP
- lea __per_cpu_load(%rip), %rdx
- mov %edx, %eax
- shr $32, %rdx
-#else
xor %eax, %eax
xor %edx, %edx
-#endif
wrmsr

call xen_prepare_pvh
diff --git a/arch/x86/tools/relocs.c b/arch/x86/tools/relocs.c
index 3ccd9d4fcf9c..01efbfdd3eb3 100644
--- a/arch/x86/tools/relocs.c
+++ b/arch/x86/tools/relocs.c
@@ -815,12 +815,7 @@ static void percpu_init(void)
*/
static int is_percpu_sym(ElfW(Sym) *sym, const char *symname)
{
- int shndx = sym_index(sym);
-
- return (shndx == per_cpu_shndx) &&
- strcmp(symname, "__init_begin") &&
- strcmp(symname, "__per_cpu_load") &&
- strncmp(symname, "init_per_cpu_", 13);
+ return 0;
}

@@ -1043,7 +1038,8 @@ static int cmp_relocs(const void *va, const void *vb)

static void sort_relocs(struct relocs *r)
{
- qsort(r->offset, r->count, sizeof(r->offset[0]), cmp_relocs);
+ if (r->count)
+ qsort(r->offset, r->count, sizeof(r->offset[0]), cmp_relocs);
}

static int write32(uint32_t v, FILE *f)
diff --git a/arch/x86/xen/xen-head.S b/arch/x86/xen/xen-head.S
index 9ce0d9d268bb..c1d9c92b417a 100644
--- a/arch/x86/xen/xen-head.S
+++ b/arch/x86/xen/xen-head.S
@@ -57,14 +57,8 @@ SYM_CODE_START(startup_xen)
* the per cpu areas are set up.
*/
movl $MSR_GS_BASE,%ecx
-#ifdef CONFIG_SMP
- leaq __per_cpu_load(%rip), %rdx
- movl %edx, %eax
- shrq $32, %rdx
-#else
xorl %eax, %eax
xorl %edx, %edx
-#endif
wrmsr

mov %rsi, %rdi
diff --git a/init/Kconfig b/init/Kconfig
index 6d35728b94b2..1af31b23e376 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -1718,7 +1718,7 @@ config KALLSYMS_ALL
config KALLSYMS_ABSOLUTE_PERCPU
bool
depends on KALLSYMS
- default X86_64 && SMP
+ default n

config KALLSYMS_BASE_RELATIVE
bool
--
2.41.0