tracing/probe: Add immediate parameter support

Add immediate value parameter (\1234) support to
probe events. This allows you to specify an immediate
(or dummy) parameter instead of fetching from memory
or register.

This feature looks odd, but imagine when you put a probe
on a code to trace some data. If the code is compiled into
2 instructions and 1 instruction has a value but other has
nothing since it is optimized out.
In that case, you can not fold those into one event, even
if ftrace supported multiple probes on one event.
With this feature, you can set a dummy value like
foo=\deadbeef instead of something like foo=%di.

Link: http://lkml.kernel.org/r/156095690733.28024.13258186548822649469.stgit@devnote2

Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

Documentation/trace/kprobetrace.rst

@@ -52,6 +52,7 @@ Synopsis of kprobe_events
   $retval	: Fetch return value.(\*2)
   $comm		: Fetch current task comm.
   +|-[u]OFFS(FETCHARG) : Fetch memory at FETCHARG +|- OFFS address.(\*3)(\*4)
+  \IMM		: Store an immediate value to the argument.
   NAME=FETCHARG : Set NAME as the argument name of FETCHARG.
   FETCHARG:TYPE : Set TYPE as the type of FETCHARG. Currently, basic types
 		  (u8/u16/u32/u64/s8/s16/s32/s64), hexadecimal types

Documentation/trace/uprobetracer.rst

@@ -45,6 +45,7 @@ Synopsis of uprobe_tracer
    $retval	: Fetch return value.(\*1)
    $comm	: Fetch current task comm.
    +|-[u]OFFS(FETCHARG) : Fetch memory at FETCHARG +|- OFFS address.(\*2)(\*3)
+   \IMM		: Store an immediate value to the argument.
    NAME=FETCHARG     : Set NAME as the argument name of FETCHARG.
    FETCHARG:TYPE     : Set TYPE as the type of FETCHARG. Currently, basic types
 		       (u8/u16/u32/u64/s8/s16/s32/s64), hexadecimal types

kernel/trace/trace.c

@@ -4848,7 +4848,7 @@ static const char readme_msg[] =
 #else
 	"\t           $stack<index>, $stack, $retval, $comm,\n"
 #endif
-	"\t           +|-[u]<offset>(<fetcharg>)\n"
+	"\t           +|-[u]<offset>(<fetcharg>), \\imm-value\n"
 	"\t     type: s8/16/32/64, u8/16/32/64, x8/16/32/64, string, symbol,\n"
 	"\t           b<bit-width>@<bit-offset>/<container-size>, ustring,\n"
 	"\t           <type>\\[<array-size>\\]\n"

kernel/trace/trace_probe.c

@@ -316,6 +316,17 @@ static int parse_probe_vars(char *arg, const struct fetch_type *t,
 	return -EINVAL;
 }
 
+static int str_to_immediate(char *str, unsigned long *imm)
+{
+	if (isdigit(str[0]))
+		return kstrtoul(str, 0, imm);
+	else if (str[0] == '-')
+		return kstrtol(str, 0, (long *)imm);
+	else if (str[0] == '+')
+		return kstrtol(str + 1, 0, (long *)imm);
+	return -EINVAL;
+}
+
 /* Recursive argument parser */
 static int
 parse_probe_arg(char *arg, const struct fetch_type *type,
@@ -444,6 +455,13 @@ parse_probe_arg(char *arg, const struct fetch_type *type,
 			code->offset = offset;
 		}
 		break;
+	case '\\':	/* Immediate value */
+		ret = str_to_immediate(arg + 1, &code->immediate);
+		if (ret)
+			trace_probe_log_err(offs + 1, BAD_IMM);
+		else
+			code->op = FETCH_OP_IMM;
+		break;
 	}
 	if (!ret && code->op == FETCH_OP_NOP) {
 		/* Parsed, but do not find fetch method */

kernel/trace/trace_probe.h

@@ -408,6 +408,7 @@ extern int traceprobe_define_arg_fields(struct trace_event_call *event_call,
 	C(BAD_VAR,		"Invalid $-valiable specified"),	\
 	C(BAD_REG_NAME,		"Invalid register name"),		\
 	C(BAD_MEM_ADDR,		"Invalid memory address"),		\
+	C(BAD_IMM,		"Invalid immediate value"),		\
 	C(FILE_ON_KPROBE,	"File offset is not available with kprobe"), \
 	C(BAD_FILE_OFFS,	"Invalid file offset value"),		\
 	C(SYM_ON_UPROBE,	"Symbol is not available with uprobe"),	\