[PATCH v4] tracing/probe: add a char type to show the character value of traced arguments

[Donglin Peng]

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. Add a new type named 'char' to do it.

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[2]' > kprobe_events

we will get the following trace log:

... 95.451350: myprobe: (demo_func+0x0/0x29) arg1=A arg2={b,p}

Signed-off-by: Donglin Peng <dolinux.peng@xxxxxxxxx>
Acked-by: Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>
Reported-by: kernel test robot <lkp@xxxxxxxxx>
---
Changes in v4:
 - update the example in the commit log

Changes in v3:
 - update readme_msg

Changes in v2:
 - fix build warnings reported by kernel test robot
 - modify commit log
---
 Documentation/trace/kprobetrace.rst | 3 ++-
 kernel/trace/trace.c                | 2 +-
 kernel/trace/trace_probe.c          | 2 ++
 kernel/trace/trace_probe.h          | 1 +
 4 files changed, 6 insertions(+), 2 deletions(-)

diff --git a/Documentation/trace/kprobetrace.rst b/Documentation/trace/kprobetrace.rst
index 4274cc6a2f94..007972a3c5c4 100644
--- a/Documentation/trace/kprobetrace.rst
+++ b/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" and bitfield
+		  (x8/x16/x32/x64), "char", "string", "ustring" 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.
diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index 5cfc95a52bc3..a64e206f94e6 100644
--- a/kernel/trace/trace.c
+++ b/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           <type>\\[<array-size>\\]\n"
 #ifdef CONFIG_HIST_TRIGGERS
diff --git a/kernel/trace/trace_probe.c b/kernel/trace/trace_probe.c
index 36dff277de46..a4abf7f6c295 100644
--- a/kernel/trace/trace_probe.c
+++ b/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)
 {
@@ -93,6 +94,7 @@ 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
diff --git a/kernel/trace/trace_probe.h b/kernel/trace/trace_probe.h
index de38f1c03776..8c86aaa8b0c9 100644
--- a/kernel/trace/trace_probe.h
+++ b/kernel/trace/trace_probe.h
@@ -164,6 +164,7 @@ 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);
 
-- 
2.25.1