[Xen-devel] [RFC v2 15/23] x86/boot/64: Use _text in a global for PIE support

[Thomas Garnier <thgarnie@xxxxxxxxxx>]

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@xxxxxxxxxx>
---
 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 925b2928f377..e71f27a20576 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;
@@ -64,7 +70,7 @@ 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)
-- 
2.14.0.434.g98096fd7a8-goog


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