[RFC PATCH 05/28] x86: Define the stack protector guard symbol explicitly

[Ard Biesheuvel <ardb+git@google.com>]

From: Ard Biesheuvel <ardb@kernel.org>

Specify the guard symbol for the stack cookie explicitly, rather than
positioning it exactly 40 bytes into the per-CPU area. Doing so removes
the need for the per-CPU region to be absolute rather than relative to
the placement of the per-CPU template region in the kernel image, and
this allows the special handling for absolute per-CPU symbols to be
removed entirely.

This is a worthwhile cleanup in itself, but it is also a prerequisite
for PIE codegen and PIE linking, which can replace our bespoke and
rather clunky runtime relocation handling.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
---
 arch/x86/Makefile                     |  4 ++++
 arch/x86/include/asm/init.h           |  2 +-
 arch/x86/include/asm/processor.h      | 11 +++--------
 arch/x86/include/asm/stackprotector.h |  4 ----
 tools/perf/util/annotate.c            |  4 ++--
 5 files changed, 10 insertions(+), 15 deletions(-)

diff --git a/arch/x86/Makefile b/arch/x86/Makefile
index 6b3fe6e2aadd..b78b7623a4a9 100644
--- a/arch/x86/Makefile
+++ b/arch/x86/Makefile
@@ -193,6 +193,10 @@ else
         KBUILD_RUSTFLAGS += -Cno-redzone=y
         KBUILD_RUSTFLAGS += -Ccode-model=kernel
 
+        ifeq ($(CONFIG_STACKPROTECTOR),y)
+                KBUILD_CFLAGS += -mstack-protector-guard-symbol=fixed_percpu_data
+        endif
+
         # Don't emit relaxable GOTPCREL relocations
         KBUILD_AFLAGS_KERNEL += -Wa,-mrelax-relocations=no
         KBUILD_CFLAGS_KERNEL += -Wa,-mrelax-relocations=no
diff --git a/arch/x86/include/asm/init.h b/arch/x86/include/asm/init.h
index 14d72727d7ee..3ed0e8ec973f 100644
--- a/arch/x86/include/asm/init.h
+++ b/arch/x86/include/asm/init.h
@@ -2,7 +2,7 @@
 #ifndef _ASM_X86_INIT_H
 #define _ASM_X86_INIT_H
 
-#define __head	__section(".head.text")
+#define __head	__section(".head.text") __no_stack_protector
 
 struct x86_mapping_info {
 	void *(*alloc_pgt_page)(void *); /* allocate buf for page table */
diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h
index 4a686f0e5dbf..56bc36116814 100644
--- a/arch/x86/include/asm/processor.h
+++ b/arch/x86/include/asm/processor.h
@@ -402,14 +402,9 @@ struct irq_stack {
 #ifdef CONFIG_X86_64
 struct fixed_percpu_data {
 	/*
-	 * GCC hardcodes the stack canary as %gs:40.  Since the
-	 * irq_stack is the object at %gs:0, we reserve the bottom
-	 * 48 bytes of the irq stack for the canary.
-	 *
-	 * Once we are willing to require -mstack-protector-guard-symbol=
-	 * support for x86_64 stackprotector, we can get rid of this.
+	 * Since the irq_stack is the object at %gs:0, the bottom 8 bytes of
+	 * the irq stack are reserved for the canary.
 	 */
-	char		gs_base[40];
 	unsigned long	stack_canary;
 };
 
@@ -418,7 +413,7 @@ DECLARE_INIT_PER_CPU(fixed_percpu_data);
 
 static inline unsigned long cpu_kernelmode_gs_base(int cpu)
 {
-	return (unsigned long)per_cpu(fixed_percpu_data.gs_base, cpu);
+	return (unsigned long)&per_cpu(fixed_percpu_data, cpu);
 }
 
 extern asmlinkage void entry_SYSCALL32_ignore(void);
diff --git a/arch/x86/include/asm/stackprotector.h b/arch/x86/include/asm/stackprotector.h
index 00473a650f51..d1dcd22a0a4c 100644
--- a/arch/x86/include/asm/stackprotector.h
+++ b/arch/x86/include/asm/stackprotector.h
@@ -51,10 +51,6 @@ static __always_inline void boot_init_stack_canary(void)
 {
 	unsigned long canary = get_random_canary();
 
-#ifdef CONFIG_X86_64
-	BUILD_BUG_ON(offsetof(struct fixed_percpu_data, stack_canary) != 40);
-#endif
-
 	current->stack_canary = canary;
 #ifdef CONFIG_X86_64
 	this_cpu_write(fixed_percpu_data.stack_canary, canary);
diff --git a/tools/perf/util/annotate.c b/tools/perf/util/annotate.c
index 37ce43c4eb8f..7ecfedf5edb9 100644
--- a/tools/perf/util/annotate.c
+++ b/tools/perf/util/annotate.c
@@ -2485,10 +2485,10 @@ static bool is_stack_operation(struct arch *arch, struct disasm_line *dl)
 
 static bool is_stack_canary(struct arch *arch, struct annotated_op_loc *loc)
 {
-	/* On x86_64, %gs:40 is used for stack canary */
+	/* On x86_64, %gs:0 is used for stack canary */
 	if (arch__is(arch, "x86")) {
 		if (loc->segment == INSN_SEG_X86_GS && loc->imm &&
-		    loc->offset == 40)
+		    loc->offset == 0)
 			return true;
 	}
 
-- 
2.46.0.792.g87dc391469-goog