x86: Make alternative instruction pointers relative

This save a few bytes on x86-64 and means that future patches can
apply alternatives to unrelocated code.

Signed-off-by: Andy Lutomirski <luto@mit.edu>
Link: http://lkml.kernel.org/r/ff64a6b9a1a3860ca4a7b8b6dc7b4754f9491cd7.1310563276.git.luto@mit.edu
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>

1 files changed, 13 insertions, 8 deletions

diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
index a81f2d52f869..ddb207bb5f91 100644
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -263,6 +263,7 @@ void __init_or_module apply_alternatives(struct alt_instr *start,
                                          struct alt_instr *end)
 {
         struct alt_instr *a;
+        u8 *instr, *replacement;
         u8 insnbuf[MAX_PATCH_LEN];
 
         DPRINTK("%s: alt table %p -> %p\n", __func__, start, end);
@@ -276,25 +277,29 @@ void __init_or_module apply_alternatives(struct alt_instr *start,
          * order.
          */
         for (a = start; a < end; a++) {
-                u8 *instr = a->instr;
+                instr = (u8 *)&a->instr_offset + a->instr_offset;
+                replacement = (u8 *)&a->repl_offset + a->repl_offset;
                 BUG_ON(a->replacementlen > a->instrlen);
                 BUG_ON(a->instrlen > sizeof(insnbuf));
                 BUG_ON(a->cpuid >= NCAPINTS*32);
                 if (!boot_cpu_has(a->cpuid))
                         continue;
+
+                memcpy(insnbuf, replacement, a->replacementlen);
+
+                /* 0xe8 is a relative jump; fix the offset. */
+                if (*insnbuf == 0xe8 && a->replacementlen == 5)
+                    *(s32 *)(insnbuf + 1) += replacement - instr;
+
+                add_nops(insnbuf + a->replacementlen,
+                         a->instrlen - a->replacementlen);
+
 #ifdef CONFIG_X86_64
                 /* vsyscall code is not mapped yet. resolve it manually. */
                 if (instr >= (u8 *)VSYSCALL_START && instr < (u8*)VSYSCALL_END) {
                         instr = __va(instr - (u8*)VSYSCALL_START + (u8*)__pa_symbol(&__vsyscall_0));
-                        DPRINTK("%s: vsyscall fixup: %p => %p\n",
-                                __func__, a->instr, instr);
                 }
 #endif
-                memcpy(insnbuf, a->replacement, a->replacementlen);
-                if (*insnbuf == 0xe8 && a->replacementlen == 5)
-                    *(s32 *)(insnbuf + 1) += a->replacement - a->instr;
-                add_nops(insnbuf + a->replacementlen,
-                         a->instrlen - a->replacementlen);
                 text_poke_early(instr, insnbuf, a->instrlen);
         }
 }