[PATCH v2 1/2] x86: introduce kaslr_offset()

[Jiri Kosina]

Offset that has been chosen for kaslr during kernel decompression can be 
easily computed as a difference between _text and __START_KERNEL. We are 
already making use of this in dump_kernel_offset() notifier and in 
arch_crash_save_vmcoreinfo().

Introduce kaslr_offset() that makes this computation instead of 
hard-coding it, so that other kernel code (such as live patching) can make 
use of it. Also convert existing users to make use of it.

Signed-off-by: Jiri Kosina <jkosina@xxxxxxx>
---

It'd be great to potentially have Ack from x86 guys for this patch so that 
I could take it through livepatching.git with the depending 2/2 patch. 
Thanks.

v1 -> v2: convert arch_crash_save_vmcoreinfo(), as spotted by Josh 
	  Poimboeuf.

 arch/x86/include/asm/setup.h       | 6 ++++++
 arch/x86/kernel/machine_kexec_64.c | 3 ++-
 arch/x86/kernel/setup.c            | 2 +-
 3 files changed, 9 insertions(+), 2 deletions(-)

diff --git a/arch/x86/include/asm/setup.h b/arch/x86/include/asm/setup.h
index f69e06b..785ac2f 100644
--- a/arch/x86/include/asm/setup.h
+++ b/arch/x86/include/asm/setup.h
@@ -65,12 +65,18 @@ static inline void x86_ce4100_early_setup(void) { }
  * This is set up by the setup-routine at boot-time
  */
 extern struct boot_params boot_params;
+extern char _text[];
 
 static inline bool kaslr_enabled(void)
 {
 	return !!(boot_params.hdr.loadflags & KASLR_FLAG);
 }
 
+static inline unsigned long kaslr_offset(void)
+{
+	return (unsigned long)&_text - __START_KERNEL;
+}
+
 /*
  * Do NOT EVER look at the BIOS memory size location.
  * It does not work on many machines.
diff --git a/arch/x86/kernel/machine_kexec_64.c b/arch/x86/kernel/machine_kexec_64.c
index 415480d..e102963 100644
--- a/arch/x86/kernel/machine_kexec_64.c
+++ b/arch/x86/kernel/machine_kexec_64.c
@@ -25,6 +25,7 @@
 #include <asm/io_apic.h>
 #include <asm/debugreg.h>
 #include <asm/kexec-bzimage64.h>
+#include <asm/setup.h>
 
 #ifdef CONFIG_KEXEC_FILE
 static struct kexec_file_ops *kexec_file_loaders[] = {
@@ -334,7 +335,7 @@ void arch_crash_save_vmcoreinfo(void)
 	VMCOREINFO_LENGTH(node_data, MAX_NUMNODES);
 #endif
 	vmcoreinfo_append_str("KERNELOFFSET=%lx\n",
-			      (unsigned long)&_text - __START_KERNEL);
+			      kaslr_offset());
 }
 
 /* arch-dependent functionality related to kexec file-based syscall */
diff --git a/arch/x86/kernel/setup.c b/arch/x86/kernel/setup.c
index d74ac33..5056d3c 100644
--- a/arch/x86/kernel/setup.c
+++ b/arch/x86/kernel/setup.c
@@ -834,7 +834,7 @@ dump_kernel_offset(struct notifier_block *self, unsigned long v, void *p)
 {
 	if (kaslr_enabled()) {
 		pr_emerg("Kernel Offset: 0x%lx from 0x%lx (relocation range: 0x%lx-0x%lx)\n",
-			 (unsigned long)&_text - __START_KERNEL,
+			 kaslr_offset(),
 			 __START_KERNEL,
 			 __START_KERNEL_map,
 			 MODULES_VADDR-1);

-- 
Jiri Kosina
SUSE Labs
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