[kernel-hardening] [RFC v3 13/27] x86/boot/64: Use _text in a global for PIE support

[Thomas Garnier <thgarnie@google.com>]

By default PIE generated code create only relative references so _text
points to the temporary virtual address. Instead use a global variable
so the relocation is done as expected.

Position Independent Executable (PIE) support will allow to extended the
KASLR randomization range below the -2G memory limit.

Signed-off-by: Thomas Garnier <thgarnie@google.com>
---
 arch/x86/kernel/head64.c | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kernel/head64.c b/arch/x86/kernel/head64.c
index bab4fa579450..675f1dba3b21 100644
--- a/arch/x86/kernel/head64.c
+++ b/arch/x86/kernel/head64.c
@@ -45,8 +45,14 @@ static void __head *fixup_pointer(void *ptr, unsigned long physaddr)
 	return ptr - (void *)_text + (void *)physaddr;
 }
 
-unsigned long __head __startup_64(unsigned long physaddr,
-				  struct boot_params *bp)
+/*
+ * Use a global variable to properly calculate _text delta on PIE. By default
+ * a PIE binary do a RIP relative difference instead of the relocated address.
+ */
+unsigned long _text_offset = (unsigned long)(_text - __START_KERNEL_map);
+
+unsigned long __head notrace __startup_64(unsigned long physaddr,
+					  struct boot_params *bp)
 {
 	unsigned long load_delta, *p;
 	unsigned long pgtable_flags;
@@ -65,7 +71,7 @@ unsigned long __head __startup_64(unsigned long physaddr,
 	 * Compute the delta between the address I am compiled to run at
 	 * and the address I am actually running at.
 	 */
-	load_delta = physaddr - (unsigned long)(_text - __START_KERNEL_map);
+	load_delta = physaddr - _text_offset;
 
 	/* Is the address not 2M aligned? */
 	if (load_delta & ~PMD_PAGE_MASK)
-- 
2.14.2.920.gcf0c67979c-goog