[kvm-unit-tests PATCH v2 4/8] x86: fix ISR thunk to use absolute indirect jump

[Giacomo Mazzola <gmazz@amazon.de>]

Replace the relative jmp (e9 rel32) with an absolute indirect
jmp *0(%rip) (ff 25 00000000 + addr64).  The rel32 displacement
is sign-extended to 64 bits, so when the displacement between
the thunk and isr_entry_point exceeds 2 GiB the sign extension
corrupts the target address.

Signed-off-by: Giacomo Mazzola <gmazz@amazon.de>
---
 lib/x86/isr.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/lib/x86/isr.c b/lib/x86/isr.c
index 53c0c2b3..4f474931 100644
--- a/lib/x86/isr.c
+++ b/lib/x86/isr.c
@@ -79,9 +79,11 @@ void handle_irq(unsigned vec, void (*func)(isr_regs_t *regs))
     /* mov $func_high, %(rsp+4) */
     *thunk++ = 0xc7; *thunk++ = 0x44; *thunk++ = 0x24; *thunk++ = 0x04;
     *(u32 *)thunk = (ulong)func >> 32; thunk += 4;
-    /* jmp isr_entry_point */
-    *thunk ++ = 0xe9;
-    *(u32 *)thunk = (ulong)isr_entry_point - (ulong)(thunk + 4);
+    /* jmp *0(%rip): absolute indirect jump to isr_entry_point */
+    *thunk++ = 0xff; *thunk++ = 0x25;
+    /* disp32 = 0: address follows */
+    *(u32 *)thunk = 0; thunk += 4;
+    *(u64 *)thunk = (ulong)isr_entry_point;
 #else
     /* push $func */
     *thunk++ = 0x68;
-- 
2.47.3