lib/ukdebug: Fix overrun in outf for OUTDEV_BUFFER

This patch fixes two bugs:
1) The return value of vsnprintf does not include the null-terminator.
Subtracting one is thus not necessary.
2) In case the buffer is too small, vsnprintf returns the number of
characters that would have been written if the buffer had
enough space. The return value must therefore be tested against
the remaining buffer space.

Signed-off-by: Marc Rittinghaus <[email protected]>
Reviewed-by: Vlad-Andrei Badoiu <[email protected]>
Tested-by: Unikraft CI <[email protected]>
GitHub-Pull-Request: unikraft#232

Author: marcrittinghaus

lib/ukdebug/outf.c

@@ -43,6 +43,7 @@
 int outf(struct out_dev *dev, const char *fmt, ...)
 {
 	int ret = 0;
+	size_t rem;
 	va_list ap;
 
 	UK_ASSERT(dev);
@@ -57,11 +58,9 @@ int outf(struct out_dev *dev, const char *fmt, ...)
 		ret = __uk_vsnprintf(dev->buffer.pos, dev->buffer.left, fmt, ap);
 
 		if (ret > 0) {
-			/* in order to overwrite '\0' by successive calls,
-			 * we move the buffer pointer by (ret-1) characters
-			 */
-			dev->buffer.pos  += (ret - 1);
-			dev->buffer.left -= (ret - 1);
+			rem = MIN(dev->buffer.left, (size_t)ret);
+			dev->buffer.pos  += rem;
+			dev->buffer.left -= rem;
 		}
 		break;
 	case OUTDEV_DEBUG: