stdio: fix sclose() buffer overrun when terminating string, realloc() error handling (thanks porlock)

theres a bug is in sclose() where it doesnt check if wp is beyond
the buffer. also wp was not updated after realloc().

bug was reported by porlock on 9fans:

Plan 9's implementation of the standard C functions snprintf and
vsnprintf have a buffer overrun bug.

If the buffer length equals the output length (without the terminating
null), then one too many characters is written to the buffer.

For example,
              snprintf(buf, 4, "ABCD");

will write 5 characters to buf.

1 files changed, 15 insertions, 7 deletions

diff --git a/sys/src/libstdio/sclose.c b/sys/src/libstdio/sclose.c
index 940b80227..b32f27c38 100644
--- a/sys/src/libstdio/sclose.c
+++ b/sys/src/libstdio/sclose.c
@@ -5,27 +5,35 @@
 char *sclose(FILE *f){
 	switch(f->state){
 	default:	/* ERR CLOSED */
+	Error:
 		if(f->buf && f->flags&BALLOC)
 			free(f->buf);
-		f->flags=0;
-		return NULL;
+		f->buf=0;
+		break;
 	case OPEN:
 		f->buf=malloc(1);
 		f->buf[0]='\0';
 		break;
 	case RD:
 	case END:
-		f->flags=0;
 		break;
 	case RDWR:
 	case WR:
+		f->rp=f->buf+f->bufl;
 		if(f->wp==f->rp){
-			if(f->flags&BALLOC)
-				f->buf=realloc(f->buf, f->bufl+1);
-			if(f->buf==NULL) return NULL;
+			if(f->flags&BALLOC){
+				char *t = realloc(f->buf, f->bufl+1);
+				if(t==NULL)
+					goto Error;
+				f->buf=t;
+				f->wp=t+f->bufl;
+			} else {
+				if(f->wp > f->buf)
+					*(f->wp-1) = '\0';
+				goto Error;
+			}
 		}
 		*f->wp='\0';
-		f->flags=0;
 		break;
 	}
 	f->state=CLOSED;