ftrace: use dynamic patching for updating mcount calls

This patch replaces the indirect call to the mcount function
pointer with a direct call that will be patched by the
dynamic ftrace routines.

On boot up, the mcount function calls the ftace_stub function.
When the dynamic ftrace code is initialized, the ftrace_stub
is replaced with a call to the ftrace_record_ip, which records
the instruction pointers of the locations that call it.

Later, the ftraced daemon will call kstop_machine and patch all
the locations to nops.

When a ftrace is enabled, the original calls to mcount will now
be set top call ftrace_caller, which will do a direct call
to the registered ftrace function. This direct call is also patched
when the function that should be called is updated.

All patching is performed by a kstop_machine routine to prevent any
type of race conditions that is associated with modifying code
on the fly.

Signed-off-by: Steven Rostedt <srostedt@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

1 files changed, 40 insertions, 1 deletions

diff --git a/arch/x86/kernel/ftrace.c b/arch/x86/kernel/ftrace.c
index b69795efa226..9f44623e0072 100644
--- a/arch/x86/kernel/ftrace.c
+++ b/arch/x86/kernel/ftrace.c
@@ -109,10 +109,49 @@ ftrace_modify_code(unsigned long ip, unsigned char *old_code,
         return faulted;
 }
 
-int __init ftrace_dyn_arch_init(void)
+notrace int ftrace_update_ftrace_func(ftrace_func_t func)
+{
+        unsigned long ip = (unsigned long)(&ftrace_call);
+        unsigned char old[5], *new;
+        int ret;
+
+        ip += CALL_BACK;
+
+        memcpy(old, &ftrace_call, 5);
+        new = ftrace_call_replace(ip, (unsigned long)func);
+        ret = ftrace_modify_code(ip, old, new);
+
+        return ret;
+}
+
+notrace int ftrace_mcount_set(unsigned long *data)
+{
+        unsigned long ip = (long)(&mcount_call);
+        unsigned long *addr = data;
+        unsigned char old[5], *new;
+
+        /* ip is at the location, but modify code will subtact this */
+        ip += CALL_BACK;
+
+        /*
+         * Replace the mcount stub with a pointer to the
+         * ip recorder function.
+         */
+        memcpy(old, &mcount_call, 5);
+        new = ftrace_call_replace(ip, *addr);
+        *addr = ftrace_modify_code(ip, old, new);
+
+        return 0;
+}
+
+int __init ftrace_dyn_arch_init(void *data)
 {
         const unsigned char *const *noptable = find_nop_table();
 
+        /* This is running in kstop_machine */
+
+        ftrace_mcount_set(data);
+
         ftrace_nop = (unsigned long *)noptable[CALL_BACK];
 
         return 0;