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 7f7f718..26e1271 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]);