ftrace: Do not process kernel/trace/ftrace.o with C recordmcount program

The file kernel/trace/ftrace.c references the mcount() call to
convert the mcount() callers to nops. But because it references
mcount(), the mcount() address is placed in the relocation table.

The C version of recordmcount reads the relocation table of all
object files, and it will add all references to mcount to the
__mcount_loc table that is used to find the places that call mcount()
and change the call to a nop. When recordmcount finds the mcount reference
in kernel/trace/ftrace.o, it saves that location even though the code
is not a call, but references mcount as data.

On boot up, when all calls are converted to nops, the code has a safety
check to determine what op code it is actually replacing before it
replaces it. If that op code at the address does not match, then
a warning is printed and the function tracer is disabled.

The reference to mcount in ftrace.c, causes this warning to trigger,
since the reference is not a call to mcount(). The ftrace.c file is
not compiled with the -pg flag, so no calls to mcount() should be
expected.

This patch simply makes recordmcount.c skip the kernel/trace/ftrace.c
file. This was the same solution used by the perl version of
recordmcount.

Reported-by: Ingo Molnar <mingo@elte.hu>
Cc: John Reiser <jreiser@bitwagon.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

1 files changed, 20 insertions, 2 deletions

diff --git a/scripts/recordmcount.c b/scripts/recordmcount.c
index 7f7f7180fe24..26e1271259ba 100644
--- a/scripts/recordmcount.c
+++ b/scripts/recordmcount.c
@@ -313,12 +313,30 @@ do_file(char const *const fname)
 int
 main(int argc, char const *argv[])
 {
+        const char ftrace[] = "kernel/trace/ftrace.o";
+        int ftrace_size = sizeof(ftrace) - 1;
         int n_error = 0;  /* gcc-4.3.0 false positive complaint */
-        if (argc <= 1)
+
+        if (argc <= 1) {
                 fprintf(stderr, "usage: recordmcount file.o...\n");
-        else  /* Process each file in turn, allowing deep failure. */
+                return 0;
+        }
+
+        /* Process each file in turn, allowing deep failure. */
         for (--argc, ++argv; 0 < argc; --argc, ++argv) {
                 int const sjval = setjmp(jmpenv);
+                int len;
+
+                /*
+                 * The file kernel/trace/ftrace.o references the mcount
+                 * function but does not call it. Since ftrace.o should
+                 * not be traced anyway, we just skip it.
+                 */
+                len = strlen(argv[0]);
+                if (len >= ftrace_size &&
+                    strcmp(argv[0] + (len - ftrace_size), ftrace) == 0)
+                        continue;
+
                 switch (sjval) {
                 default: {
                         fprintf(stderr, "internal error: %s\n", argv[0]);