tracing: uprobes: Re-enable $comm support for uprobe events

Since commit 53305928 ("tracing: probeevent: Introduce new
argument fetching code") dropped the $comm support from uprobe
events, this re-enables it.

For $comm support, uses strlcpy() instead of strncpy_from_user()
to copy current task's comm. Because it is in the kernel space,
strncpy_from_user() always fails to copy the comm.
This also uses strlen() instead of strnlen_user() to measure the
length of the comm.

Note that this uses -ECOMM as a token value to fetch the comm
string. If the user-space pointer points -ECOMM, it will be
translated to task->comm.

Link: http://lkml.kernel.org/r/155723734162.9149.4042756162201097965.stgit@devnote2

Fixes: 53305928 ("tracing: probeevent: Introduce new argument fetching code")
Reported-by: Andreas Ziegler <andreas.ziegler@fau.de>
Acked-by: Andreas Ziegler <andreas.ziegler@fau.de>
Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

kernel/trace/trace_probe.h

@@ -124,6 +124,7 @@ struct fetch_insn {
 
 /* fetch + deref*N + store + mod + end <= 16, this allows N=12, enough */
 #define FETCH_INSN_MAX	16
+#define FETCH_TOKEN_COMM	(-ECOMM)
 
 /* Fetch type information table */
 struct fetch_type {

kernel/trace/trace_uprobe.c

@@ -156,6 +156,9 @@ fetch_store_string(unsigned long addr, void *dest, void *base)
 	if (unlikely(!maxlen))
 		return -ENOMEM;
 
+	if (addr == FETCH_TOKEN_COMM)
+		ret = strlcpy(dst, current->comm, maxlen);
+	else
 		ret = strncpy_from_user(dst, src, maxlen);
 	if (ret >= 0) {
 		if (ret == maxlen)
@@ -180,6 +183,9 @@ fetch_store_strlen(unsigned long addr)
 	int len;
 	void __user *vaddr = (void __force __user *) addr;
 
+	if (addr == FETCH_TOKEN_COMM)
+		len = strlen(current->comm) + 1;
+	else
 		len = strnlen_user(vaddr, MAX_STRING_SIZE);
 
 	return (len > MAX_STRING_SIZE) ? 0 : len;
@@ -220,6 +226,9 @@ process_fetch_insn(struct fetch_insn *code, struct pt_regs *regs, void *dest,
 	case FETCH_OP_IMM:
 		val = code->immediate;
 		break;
+	case FETCH_OP_COMM:
+		val = FETCH_TOKEN_COMM;
+		break;
 	case FETCH_OP_FOFFS:
 		val = translate_user_vaddr(code->immediate);
 		break;