Hopefully fix Coverity CID 1008328 (Out-of-bounds write) in /bin/sh.

Replace the magic constant 127 in the loop interation count with
"PROMPTLEN - 1".

gethostname() is not guaranteed to NUL terminate the destination
string if it is too short. Decrease the length passed to gethostname()
by one, and add a NUL at the end of the buffer to make sure the
following loop to find the end of the name properly terminates.

The default: case is the likely cause of Coverity CID 1008328.  If
i is 126 at the top of the loop interation where the default case
is triggered, i will be incremented to 127 by the default case,
then incremented to 128 at the top of the loop before being compared
to 127 (PROMPTLENT - 1) and terminating the loop. Then the NUL
termination code after the loop will write to ps[128].  Fix by
checking for overflow before incrementing the index and storing the
second character in the buffer.

These fixes are not guaranteed to satisfy Coverity. The code that
increments i in the 'h'/'H' and 'w'/'W' cases may be beyond its
capability to analyze, but the code appears to be safe.

Reported by:	Coverity
CID:		1008328
Reviewed by:	jilles, cem
MFC after:	1 week
Differential Revision:	https://reviews.freebsd.org/D6482

1 files changed, 6 insertions, 4 deletions

diff --git a/bin/sh/parser.c b/bin/sh/parser.c
index f3cc98d..cc04a2a 100644
--- a/bin/sh/parser.c
+++ b/bin/sh/parser.c
@@ -1998,7 +1998,7 @@ getprompt(void *unused __unused)
 	/*
 	 * Format prompt string.
 	 */
-	for (i = 0; (i < 127) && (*fmt != '\0'); i++, fmt++)
+	for (i = 0; (i < PROMPTLEN - 1) && (*fmt != '\0'); i++, fmt++)
 		if (*fmt == '\\')
 			switch (*++fmt) {
 
@@ -2011,7 +2011,8 @@ getprompt(void *unused __unused)
 			case 'h':
 			case 'H':
 				ps[i] = '\0';
-				gethostname(&ps[i], PROMPTLEN - i);
+				gethostname(&ps[i], PROMPTLEN - i - 1);
+				ps[PROMPTLEN - 1] = '\0';
 				/* Skip to end of hostname. */
 				trim = (*fmt == 'h') ? '.' : '\0';
 				while ((ps[i] != '\0') && (ps[i] != trim))
@@ -2061,8 +2062,9 @@ getprompt(void *unused __unused)
 				 * Emit unrecognized formats verbatim.
 				 */
 			default:
-				ps[i++] = '\\';
-				ps[i] = *fmt;
+				ps[i] = '\\';
+				if (i < PROMPTLEN - 1)
+					ps[++i] = *fmt;
 				break;
 			}
 		else