[Xen-devel] [PATCH 10/13] x86/alternative: Support indirect call replacement

[Josh Poimboeuf <jpoimboe@xxxxxxxxxx>]

Add alternative patching support for replacing an instruction with an
indirect call.  This will be needed for the paravirt alternatives.

Signed-off-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
---
 arch/x86/kernel/alternative.c | 22 +++++++++++++++-------
 1 file changed, 15 insertions(+), 7 deletions(-)

diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
index 3344d3382e91..81c577c7deba 100644
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -410,20 +410,28 @@ void __init_or_module noinline apply_alternatives(struct 
alt_instr *start,
                insnbuf_sz = a->replacementlen;
 
                /*
-                * 0xe8 is a relative jump; fix the offset.
-                *
-                * Instruction length is checked before the opcode to avoid
-                * accessing uninitialized bytes for zero-length replacements.
+                * Fix the address offsets for call and jump instructions which
+                * use PC-relative addressing.
                 */
                if (a->replacementlen == 5 && *insnbuf == 0xe8) {
+                       /* direct call */
                        *(s32 *)(insnbuf + 1) += replacement - instr;
-                       DPRINTK("Fix CALL offset: 0x%x, CALL 0x%lx",
+                       DPRINTK("Fix direct CALL offset: 0x%x, CALL 0x%lx",
                                *(s32 *)(insnbuf + 1),
                                (unsigned long)instr + *(s32 *)(insnbuf + 1) + 
5);
-               }
 
-               if (a->replacementlen && is_jmp(replacement[0]))
+               } else if (a->replacementlen == 6 && *insnbuf == 0xff &&
+                          *(insnbuf+1) == 0x15) {
+                       /* indirect call */
+                       *(s32 *)(insnbuf + 2) += replacement - instr;
+                       DPRINTK("Fix indirect CALL offset: 0x%x, CALL *0x%lx",
+                               *(s32 *)(insnbuf + 2),
+                               (unsigned long)instr + *(s32 *)(insnbuf + 2) + 
6);
+
+               } else if (a->replacementlen && is_jmp(replacement[0])) {
+                       /* direct jump */
                        recompute_jump(a, instr, replacement, insnbuf);
+               }
 
                if (a->instrlen > a->replacementlen) {
                        add_nops(insnbuf + a->replacementlen,
-- 
2.13.6


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