ftrace: add ftrace_init_nop()

Architectures may need to perform special initialization of ftrace
callsites, and today they do so by special-casing ftrace_make_nop() when
the expected branch address is MCOUNT_ADDR. In some cases (e.g. for
patchable-function-entry), we don't have an mcount-like symbol and don't
want a synthetic MCOUNT_ADDR, but we may need to perform some
initialization of callsites.

To make it possible to separate initialization from runtime
modification, and to handle cases without an mcount-like symbol, this
patch adds an optional ftrace_init_nop() function that architectures can
implement, which does not pass a branch address.

Where an architecture does not provide ftrace_init_nop(), we will fall
back to the existing behaviour of calling ftrace_make_nop() with
MCOUNT_ADDR.

At the same time, ftrace_code_disable() is renamed to
ftrace_nop_initialize() to make it clearer that it is intended to
intialize a callsite into a disabled state, and is not for disabling a
callsite that has been runtime enabled. The kerneldoc description of rec
arguments is updated to cover non-mcount callsites.
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Reviewed-by: Amit Daniel Kachhap <amit.kachhap@arm.com>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Miroslav Benes <mbenes@suse.cz>
Reviewed-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Reviewed-by: Torsten Duwe <duwe@suse.de>
Tested-by: Amit Daniel Kachhap <amit.kachhap@arm.com>
Tested-by: Sven Schnelle <svens@stackframe.org>
Tested-by: Torsten Duwe <duwe@suse.de>
Cc: Ingo Molnar <mingo@redhat.com>

include/linux/ftrace.h

@@ -499,7 +499,7 @@ static inline int ftrace_disable_ftrace_graph_caller(void) { return 0; }
 /**
  * ftrace_make_nop - convert code into nop
  * @mod: module structure if called by module load initialization
- * @rec: the mcount call site record
+ * @rec: the call site record (e.g. mcount/fentry)
  * @addr: the address that the call site should be calling
  *
  * This is a very sensitive operation and great care needs
@@ -520,9 +520,38 @@ static inline int ftrace_disable_ftrace_graph_caller(void) { return 0; }
 extern int ftrace_make_nop(struct module *mod,
 			   struct dyn_ftrace *rec, unsigned long addr);
 
+
+/**
+ * ftrace_init_nop - initialize a nop call site
+ * @mod: module structure if called by module load initialization
+ * @rec: the call site record (e.g. mcount/fentry)
+ *
+ * This is a very sensitive operation and great care needs
+ * to be taken by the arch.  The operation should carefully
+ * read the location, check to see if what is read is indeed
+ * what we expect it to be, and then on success of the compare,
+ * it should write to the location.
+ *
+ * The code segment at @rec->ip should contain the contents created by
+ * the compiler
+ *
+ * Return must be:
+ *  0 on success
+ *  -EFAULT on error reading the location
+ *  -EINVAL on a failed compare of the contents
+ *  -EPERM  on error writing to the location
+ * Any other value will be considered a failure.
+ */
+#ifndef ftrace_init_nop
+static inline int ftrace_init_nop(struct module *mod, struct dyn_ftrace *rec)
+{
+	return ftrace_make_nop(mod, rec, MCOUNT_ADDR);
+}
+#endif
+
 /**
  * ftrace_make_call - convert a nop call site into a call to addr
- * @rec: the mcount call site record
+ * @rec: the call site record (e.g. mcount/fentry)
  * @addr: the address that the call site should call
  *
  * This is a very sensitive operation and great care needs
@@ -545,7 +574,7 @@ extern int ftrace_make_call(struct dyn_ftrace *rec, unsigned long addr);
 #ifdef CONFIG_DYNAMIC_FTRACE_WITH_REGS
 /**
  * ftrace_modify_call - convert from one addr to another (no nop)
- * @rec: the mcount call site record
+ * @rec: the call site record (e.g. mcount/fentry)
  * @old_addr: the address expected to be currently called to
  * @addr: the address to change to
  *

kernel/trace/ftrace.c

@@ -2494,14 +2494,14 @@ struct dyn_ftrace *ftrace_rec_iter_record(struct ftrace_rec_iter *iter)
 }
 
 static int
-ftrace_code_disable(struct module *mod, struct dyn_ftrace *rec)
+ftrace_nop_initialize(struct module *mod, struct dyn_ftrace *rec)
 {
 	int ret;
 
 	if (unlikely(ftrace_disabled))
 		return 0;
 
-	ret = ftrace_make_nop(mod, rec, MCOUNT_ADDR);
+	ret = ftrace_init_nop(mod, rec);
 	if (ret) {
 		ftrace_bug_type = FTRACE_BUG_INIT;
 		ftrace_bug(ret, rec);
@@ -2943,7 +2943,7 @@ static int ftrace_update_code(struct module *mod, struct ftrace_page *new_pgs)
 			 * to the NOP instructions.
 			 */
 			if (!__is_defined(CC_USING_NOP_MCOUNT) &&
-			    !ftrace_code_disable(mod, p))
+			    !ftrace_nop_initialize(mod, p))
 				break;
 
 			update_cnt++;