1 files changed, 17 insertions, 2 deletions

diff --git a/lib/libc/stdio/stdio.c b/lib/libc/stdio/stdio.c
index 57be5bb42140..58fafcbc170f 100644
--- a/lib/libc/stdio/stdio.c
+++ b/lib/libc/stdio/stdio.c
@@ -43,6 +43,7 @@ static const char rcsid[] =
 #endif /* LIBC_SCCS and not lint */
 
 #include "namespace.h"
+#include <errno.h>
 #include <fcntl.h>
 #include <unistd.h>
 #include <stdio.h>
@@ -94,10 +95,24 @@ __sseek(cookie, offset, whence)
 	register FILE *fp = cookie;
 	register off_t ret;
 
+	/*
+	 * Disallow negative seeks per POSIX.
+	 * It is needed here to help upper level caller
+	 * (fseek) in the cases it can't detect.
+	 */
+	if (whence == SEEK_SET && (off_t)offset < 0) {
+		errno = EINVAL;
+		return (-1);
+	}
 	ret = lseek(fp->_file, (off_t)offset, whence);
-	if (ret == -1)
+	if (ret < 0) {
+		if (ret != -1) {
+			/* Resulting seek is negative! */
+			ret = -1;
+			errno = EINVAL;
+		}
 		fp->_flags &= ~__SOFF;
-	else {
+	} else {
 		fp->_flags |= __SOFF;
 		fp->_offset = ret;
 	}