[PATCH] tracing: expose current->comm to kprobe events

[Omar Sandoval]

From: Omar Sandoval <osandov@xxxxxx>

ftrace is very quick to give up on saving the task command line (see
`trace_save_cmdline()`). The workaround for events which really care
about the command line is to explicitly assign it as part of the entry.
However, this doesn't work for kprobe events, as there's no
straightforward way to get access to current->comm. Add a kprobe event
variable $comm which provides exactly that.

Signed-off-by: Omar Sandoval <osandov@xxxxxx>
---
This is a problem I ran into while trying to debug an unrelated issue. I
was considering trying to make ftrace try harder to save the command
line, but that would most likely be at the expense at most events which
don't care about the task comm. This is much less work.

 Documentation/trace/kprobetrace.txt |  2 ++
 kernel/trace/trace_kprobe.c         | 13 +++++++++++++
 kernel/trace/trace_probe.c          |  5 +++++
 kernel/trace/trace_probe.h          |  2 ++
 kernel/trace/trace_uprobe.c         |  7 +++++++
 5 files changed, 29 insertions(+)

diff --git a/Documentation/trace/kprobetrace.txt b/Documentation/trace/kprobetrace.txt
index d68ea5fc812b..90914cbc1e51 100644
--- a/Documentation/trace/kprobetrace.txt
+++ b/Documentation/trace/kprobetrace.txt
@@ -40,6 +40,7 @@ Synopsis of kprobe_events
   $stackN	: Fetch Nth entry of stack (N >= 0)
   $stack	: Fetch stack address.
   $retval	: Fetch return value.(*)
+  $comm		: Fetch current task comm.(***)
   +|-offs(FETCHARG) : Fetch memory at FETCHARG +|- offs address.(**)
   NAME=FETCHARG : Set NAME as the argument name of FETCHARG.
   FETCHARG:TYPE : Set TYPE as the type of FETCHARG. Currently, basic types
@@ -48,6 +49,7 @@ Synopsis of kprobe_events
 
   (*) only for return probe.
   (**) this is useful for fetching a field of data structures.
+  (***) you probably want this as a string, i.e., $comm:string
 
 Types
 -----
diff --git a/kernel/trace/trace_kprobe.c b/kernel/trace/trace_kprobe.c
index 5546eec0505f..e3a087552d49 100644
--- a/kernel/trace/trace_kprobe.c
+++ b/kernel/trace/trace_kprobe.c
@@ -214,6 +214,18 @@ static void FETCH_FUNC_NAME(memory, string_size)(struct pt_regs *regs,
 }
 NOKPROBE_SYMBOL(FETCH_FUNC_NAME(memory, string_size));
 
+#define DEFINE_FETCH_comm(type)					\
+void FETCH_FUNC_NAME(comm, type)(struct pt_regs *regs,		\
+				 void *data, void *dest)	\
+{								\
+	fetch_memory_##type(regs, current->comm, dest);		\
+}								\
+NOKPROBE_SYMBOL(FETCH_FUNC_NAME(comm, type));
+
+DEFINE_BASIC_FETCH_FUNCS(comm)
+DEFINE_FETCH_comm(string)
+DEFINE_FETCH_comm(string_size)
+
 #define DEFINE_FETCH_symbol(type)					\
 void FETCH_FUNC_NAME(symbol, type)(struct pt_regs *regs, void *data, void *dest)\
 {									\
@@ -587,6 +599,7 @@ static int create_trace_kprobe(int argc, char **argv)
 	 *  $retval	: fetch return value
 	 *  $stack	: fetch stack address
 	 *  $stackN	: fetch Nth of stack (N:0-)
+	 *  $comm       : fetch current task comm
 	 *  @ADDR	: fetch memory at ADDR (ADDR should be in kernel)
 	 *  @SYM[+|-offs] : fetch memory at SYM +|- offs (SYM is a data symbol)
 	 *  %REG	: fetch register REG
diff --git a/kernel/trace/trace_probe.c b/kernel/trace/trace_probe.c
index 1d372fa6fefb..918775fcacd3 100644
--- a/kernel/trace/trace_probe.c
+++ b/kernel/trace/trace_probe.c
@@ -348,6 +348,11 @@ static int parse_probe_vars(char *arg, const struct fetch_type *t,
 			}
 		} else
 			ret = -EINVAL;
+	} else if (strcmp(arg, "comm") == 0) {
+		if (is_kprobe)
+			f->fn = t->fetch[FETCH_MTD_comm];
+		else
+			ret = -EINVAL;
 	} else
 		ret = -EINVAL;
 
diff --git a/kernel/trace/trace_probe.h b/kernel/trace/trace_probe.h
index f6398db09114..3f1a987e6dc1 100644
--- a/kernel/trace/trace_probe.h
+++ b/kernel/trace/trace_probe.h
@@ -102,6 +102,7 @@ enum {
 	FETCH_MTD_reg = 0,
 	FETCH_MTD_stack,
 	FETCH_MTD_retval,
+	FETCH_MTD_comm,
 	FETCH_MTD_memory,
 	FETCH_MTD_symbol,
 	FETCH_MTD_deref,
@@ -213,6 +214,7 @@ DEFINE_FETCH_##method(u64)
 ASSIGN_FETCH_FUNC(reg, ftype),				\
 ASSIGN_FETCH_FUNC(stack, ftype),			\
 ASSIGN_FETCH_FUNC(retval, ftype),			\
+ASSIGN_FETCH_FUNC(comm, ftype),				\
 ASSIGN_FETCH_FUNC(memory, ftype),			\
 ASSIGN_FETCH_FUNC(symbol, ftype),			\
 ASSIGN_FETCH_FUNC(deref, ftype),			\
diff --git a/kernel/trace/trace_uprobe.c b/kernel/trace/trace_uprobe.c
index c53485441c88..f7eedb191d1f 100644
--- a/kernel/trace/trace_uprobe.c
+++ b/kernel/trace/trace_uprobe.c
@@ -172,6 +172,13 @@ static void FETCH_FUNC_NAME(memory, string_size)(struct pt_regs *regs,
 		*(u32 *)dest = len;
 }
 
+#define fetch_comm_u8		NULL
+#define fetch_comm_u16		NULL
+#define fetch_comm_u32		NULL
+#define fetch_comm_u64		NULL
+#define fetch_comm_string	NULL
+#define fetch_comm_string_size	NULL
+
 static unsigned long translate_user_vaddr(void *file_offset)
 {
 	unsigned long base_addr;
-- 
2.8.3