1 files changed, 40 insertions, 17 deletions

diff --git a/arch/s390/kernel/ftrace.c b/arch/s390/kernel/ftrace.c
index f908e42e11c4..fcb009d3edde 100644
--- a/arch/s390/kernel/ftrace.c
+++ b/arch/s390/kernel/ftrace.c
@@ -1,7 +1,7 @@
 /*
  * Dynamic function tracer architecture backend.
  *
- * Copyright IBM Corp. 2009
+ * Copyright IBM Corp. 2009,2014
  *
  *   Author(s): Heiko Carstens <heiko.carstens@de.ibm.com>,
  *              Martin Schwidefsky <schwidefsky@de.ibm.com>
@@ -17,6 +17,7 @@
 #include <asm/asm-offsets.h>
 #include "entry.h"
 
+void mcount_replace_code(void);
 void ftrace_disable_code(void);
 void ftrace_enable_insn(void);
 
@@ -24,38 +25,50 @@ void ftrace_enable_insn(void);
 /*
  * The 64-bit mcount code looks like this:
  *      stg     %r14,8(%r15)            # offset 0
- * >    larl    %r1,<&counter>          # offset 6
- * >    brasl   %r14,_mcount            # offset 12
+ *      larl    %r1,<&counter>          # offset 6
+ *      brasl   %r14,_mcount            # offset 12
  *      lg      %r14,8(%r15)            # offset 18
- * Total length is 24 bytes. The middle two instructions of the mcount
- * block get overwritten by ftrace_make_nop / ftrace_make_call.
+ * Total length is 24 bytes. The complete mcount block initially gets replaced
+ * by ftrace_make_nop. Subsequent calls to ftrace_make_call / ftrace_make_nop
+ * only patch the jg/lg instruction within the block.
+ * Note: we do not patch the first instruction to an unconditional branch,
+ * since that would break kprobes/jprobes. It is easier to leave the larl
+ * instruction in and only modify the second instruction.
  * The 64-bit enabled ftrace code block looks like this:
- *      stg     %r14,8(%r15)            # offset 0
+ *      larl    %r0,.+24                # offset 0
  * >    lg      %r1,__LC_FTRACE_FUNC    # offset 6
- * >    lgr     %r0,%r0                 # offset 12
- * >    basr    %r14,%r1                # offset 16
- *      lg      %r14,8(%15)             # offset 18
- * The return points of the mcount/ftrace function have the same offset 18.
- * The 64-bit disable ftrace code block looks like this:
- *      stg     %r14,8(%r15)            # offset 0
+ *      br      %r1                     # offset 12
+ *      brcl    0,0                     # offset 14
+ *      brc     0,0                     # offset 20
+ * The ftrace function gets called with a non-standard C function call ABI
+ * where r0 contains the return address. It is also expected that the called
+ * function only clobbers r0 and r1, but restores r2-r15.
+ * The return point of the ftrace function has offset 24, so execution
+ * continues behind the mcount block.
+ *      larl    %r0,.+24                # offset 0
  * >    jg      .+18                    # offset 6
- * >    lgr     %r0,%r0                 # offset 12
- * >    basr    %r14,%r1                # offset 16
- *      lg      %r14,8(%15)             # offset 18
+ *      br      %r1                     # offset 12
+ *      brcl    0,0                     # offset 14
+ *      brc     0,0                     # offset 20
  * The jg instruction branches to offset 24 to skip as many instructions
  * as possible.
  */
 asm(
         "       .align  4\n"
+        "mcount_replace_code:\n"
+        "       larl    %r0,0f\n"
         "ftrace_disable_code:\n"
         "       jg      0f\n"
-        "       lgr     %r0,%r0\n"
-        "       basr    %r14,%r1\n"
+        "       br      %r1\n"
+        "       brcl    0,0\n"
+        "       brc     0,0\n"
         "0:\n"
         "       .align  4\n"
         "ftrace_enable_insn:\n"
         "       lg      %r1,"__stringify(__LC_FTRACE_FUNC)"\n");
 
+#define MCOUNT_BLOCK_SIZE       24
+#define MCOUNT_INSN_OFFSET      6
 #define FTRACE_INSN_SIZE        6
 
 #else /* CONFIG_64BIT */
@@ -116,6 +129,16 @@ int ftrace_modify_call(struct dyn_ftrace *rec, unsigned long old_addr,
 int ftrace_make_nop(struct module *mod, struct dyn_ftrace *rec,
                     unsigned long addr)
 {
+#ifdef CONFIG_64BIT
+        /* Initial replacement of the whole mcount block */
+        if (addr == MCOUNT_ADDR) {
+                if (probe_kernel_write((void *) rec->ip - MCOUNT_INSN_OFFSET,
+                                       mcount_replace_code,
+                                       MCOUNT_BLOCK_SIZE))
+                        return -EPERM;
+                return 0;
+        }
+#endif
         if (probe_kernel_write((void *) rec->ip, ftrace_disable_code,
                                MCOUNT_INSN_SIZE))
                 return -EPERM;