tracing/probe: add a char type to show the character value of traced arguments

There are scenes that we want to show the character value of traced
arguments other than a decimal or hexadecimal or string value for debug
convinience. I add a new type named 'char' to do it and a new test case
file named 'kprobe_args_char.tc' to do selftest for char type.

For example:

The to be traced function is 'void demo_func(char type, char *name);', we
can add a kprobe event as follows to show argument values as we want:

echo  'p:myprobe demo_func $arg1:char +0($arg2):char[5]' > kprobe_events

we will get the following trace log:

... myprobe: (demo_func+0x0/0x29) arg1='A' arg2={'b','p','f','1',''}

Link: https://lore.kernel.org/all/20221219110613.367098-1-dolinux.peng@gmail.com/Signed-off-by: Donglin Peng <dolinux.peng@gmail.com>
Acked-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>

Documentation/trace/kprobetrace.rst

@@ -58,7 +58,7 @@ Synopsis of kprobe_events
   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
-		  (x8/x16/x32/x64), "string", "ustring", "symbol", "symstr"
+		  (x8/x16/x32/x64), "char", "string", "ustring", "symbol", "symstr"
                   and bitfield are supported.
 
   (\*1) only for the probe on function entry (offs == 0).
@@ -80,6 +80,7 @@ E.g. 'x16[4]' means an array of x16 (2bytes hex) with 4 elements.
 Note that the array can be applied to memory type fetchargs, you can not
 apply it to registers/stack-entries etc. (for example, '$stack1:x8[8]' is
 wrong, but '+8($stack):x8[8]' is OK.)
+Char type can be used to show the character value of traced arguments.
 String type is a special type, which fetches a "null-terminated" string from
 kernel space. This means it will fail and store NULL if the string container
 has been paged out. "ustring" type is an alternative of string for user-space.

kernel/trace/trace.c

@@ -5615,7 +5615,7 @@ static const char readme_msg[] =
 	"\t           $stack<index>, $stack, $retval, $comm,\n"
 #endif
 	"\t           +|-[u]<offset>(<fetcharg>), \\imm-value, \\\"imm-string\"\n"
-	"\t     type: s8/16/32/64, u8/16/32/64, x8/16/32/64, string, symbol,\n"
+	"\t     type: s8/16/32/64, u8/16/32/64, x8/16/32/64, char, string, symbol,\n"
 	"\t           b<bit-width>@<bit-offset>/<container-size>, ustring,\n"
 	"\t           symstr, <type>\\[<array-size>\\]\n"
 #ifdef CONFIG_HIST_TRIGGERS

kernel/trace/trace_probe.c

@@ -50,6 +50,7 @@ DEFINE_BASIC_PRINT_TYPE_FUNC(x8,  u8,  "0x%x")
 DEFINE_BASIC_PRINT_TYPE_FUNC(x16, u16, "0x%x")
 DEFINE_BASIC_PRINT_TYPE_FUNC(x32, u32, "0x%x")
 DEFINE_BASIC_PRINT_TYPE_FUNC(x64, u64, "0x%Lx")
+DEFINE_BASIC_PRINT_TYPE_FUNC(char, u8, "'%c'")
 
 int PRINT_TYPE_FUNC_NAME(symbol)(struct trace_seq *s, void *data, void *ent)
 {
@@ -95,6 +96,7 @@ static const struct fetch_type probe_fetch_types[] = {
 	ASSIGN_FETCH_TYPE_ALIAS(x16, u16, u16, 0),
 	ASSIGN_FETCH_TYPE_ALIAS(x32, u32, u32, 0),
 	ASSIGN_FETCH_TYPE_ALIAS(x64, u64, u64, 0),
+	ASSIGN_FETCH_TYPE_ALIAS(char, u8, u8,  0),
 	ASSIGN_FETCH_TYPE_ALIAS(symbol, ADDR_FETCH_TYPE, ADDR_FETCH_TYPE, 0),
 
 	ASSIGN_FETCH_TYPE_END

kernel/trace/trace_probe.h

@@ -166,6 +166,7 @@ DECLARE_BASIC_PRINT_TYPE_FUNC(x16);
 DECLARE_BASIC_PRINT_TYPE_FUNC(x32);
 DECLARE_BASIC_PRINT_TYPE_FUNC(x64);
 
+DECLARE_BASIC_PRINT_TYPE_FUNC(char);
 DECLARE_BASIC_PRINT_TYPE_FUNC(string);
 DECLARE_BASIC_PRINT_TYPE_FUNC(symbol);
 

tools/testing/selftests/ftrace/test.d/kprobe/kprobe_args_char.tc

+#!/bin/sh
+# SPDX-License-Identifier: GPL-2.0
+# description: Kprobe event char type argument
+# requires: kprobe_events
+
+case `uname -m` in
+x86_64)
+  ARG1=%di
+;;
+i[3456]86)
+  ARG1=%ax
+;;
+aarch64)
+  ARG1=%x0
+;;
+arm*)
+  ARG1=%r0
+;;
+ppc64*)
+  ARG1=%r3
+;;
+ppc*)
+  ARG1=%r3
+;;
+s390*)
+  ARG1=%r2
+;;
+mips*)
+  ARG1=%r4
+;;
+*)
+  echo "Please implement other architecture here"
+  exit_untested
+esac
+
+: "Test get argument (1)"
+echo "p:testprobe tracefs_create_dir arg1=+0(${ARG1}):char" > kprobe_events
+echo 1 > events/kprobes/testprobe/enable
+echo "p:test $FUNCTION_FORK" >> kprobe_events
+grep -qe "testprobe.* arg1='t'" trace
+
+echo 0 > events/kprobes/testprobe/enable
+: "Test get argument (2)"
+echo "p:testprobe tracefs_create_dir arg1=+0(${ARG1}):char arg2=+0(${ARG1}):char[4]" > kprobe_events
+echo 1 > events/kprobes/testprobe/enable
+echo "p:test $FUNCTION_FORK" >> kprobe_events
+grep -qe "testprobe.* arg1='t' arg2={'t','e','s','t'}" trace