tracing: Handle %.*s in trace_check_vprintf()

If a trace event uses the %*.s notation, the trace_check_vprintf() will
fail and will warn about a bad processing of strings, because it does not
take into account the length field when processing the star (*) part.
Have it handle this case as well.

Link: https://lore.kernel.org/linux-nfs/238C0E2D-C2A4-4578-ADD2-C565B3B99842@oracle.com/Reported-by: Chuck Lever III <chuck.lever@oracle.com>
Fixes: 9a6944fe ("tracing: Add a verifier to check string pointers for trace events")
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

kernel/trace/trace.c

@@ -3704,6 +3704,9 @@ void trace_check_vprintf(struct trace_iterator *iter, const char *fmt,
 		goto print;
 
 	while (*p) {
+		bool star = false;
+		int len = 0;
+
 		j = 0;
 
 		/* We only care about %s and variants */
@@ -3725,13 +3728,17 @@ void trace_check_vprintf(struct trace_iterator *iter, const char *fmt,
 				/* Need to test cases like %08.*s */
 				for (j = 1; p[i+j]; j++) {
 					if (isdigit(p[i+j]) ||
-					    p[i+j] == '*' ||
 					    p[i+j] == '.')
 						continue;
+					if (p[i+j] == '*') {
+						star = true;
+						continue;
+					}
 					break;
 				}
 				if (p[i+j] == 's')
 					break;
+				star = false;
 			}
 			j = 0;
 		}
@@ -3744,6 +3751,9 @@ void trace_check_vprintf(struct trace_iterator *iter, const char *fmt,
 		iter->fmt[i] = '\0';
 		trace_seq_vprintf(&iter->seq, iter->fmt, ap);
 
+		if (star)
+			len = va_arg(ap, int);
+
 		/* The ap now points to the string data of the %s */
 		str = va_arg(ap, const char *);
 
@@ -3762,8 +3772,18 @@ void trace_check_vprintf(struct trace_iterator *iter, const char *fmt,
 			int ret;
 
 			/* Try to safely read the string */
-			ret = strncpy_from_kernel_nofault(iter->fmt, str,
-							  iter->fmt_size);
+			if (star) {
+				if (len + 1 > iter->fmt_size)
+					len = iter->fmt_size - 1;
+				if (len < 0)
+					len = 0;
+				ret = copy_from_kernel_nofault(iter->fmt, str, len);
+				iter->fmt[len] = 0;
+				star = false;
+			} else {
+				ret = strncpy_from_kernel_nofault(iter->fmt, str,
+								  iter->fmt_size);
+			}
 			if (ret < 0)
 				trace_seq_printf(&iter->seq, "(0x%px)", str);
 			else
@@ -3775,7 +3795,10 @@ void trace_check_vprintf(struct trace_iterator *iter, const char *fmt,
 			strncpy(iter->fmt, p + i, j + 1);
 			iter->fmt[j+1] = '\0';
 		}
-		trace_seq_printf(&iter->seq, iter->fmt, str);
+		if (star)
+			trace_seq_printf(&iter->seq, iter->fmt, len, str);
+		else
+			trace_seq_printf(&iter->seq, iter->fmt, str);
 
 		p += i + j + 1;
 	}