Avoid mangled output in BIO_debug_callback

Instead of blindly skipping 14 characters, we can use the return
value of snprintf() to determine how much we should skip.

From Martin Vahlensieck with minor tweaks by me

1 files changed, 12 insertions, 4 deletions

diff --git a/src/lib/libcrypto/bio/bio_cb.c b/src/lib/libcrypto/bio/bio_cb.c
index ab0e3a92ce..52cdd24177 100644
--- a/src/lib/libcrypto/bio/bio_cb.c
+++ b/src/lib/libcrypto/bio/bio_cb.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: bio_cb.c,v 1.16 2014/12/08 03:54:19 bcook Exp $ */
+/* $OpenBSD: bio_cb.c,v 1.17 2021/03/25 09:26:17 tb Exp $ */
 /* Copyright (C) 1995-1998 Eric Young (eay@cryptsoft.com)
  * All rights reserved.
  *
@@ -70,15 +70,22 @@ BIO_debug_callback(BIO *bio, int cmd, const char *argp, int argi, long argl,
         BIO *b;
         char buf[256];
         char *p;
+        int nbuf;
         long r = 1;
         size_t p_maxlen;
 
         if (BIO_CB_RETURN & cmd)
                 r = ret;
 
-        snprintf(buf, sizeof buf, "BIO[%p]:", bio);
-        p = &(buf[14]);
-        p_maxlen = sizeof buf - 14;
+        nbuf = snprintf(buf, sizeof(buf), "BIO[%p]: ", bio);
+        if (nbuf < 0)
+                nbuf = 0;       /* Ignore error; continue printing. */
+        if (nbuf >= sizeof(buf))
+                goto out;
+
+        p = buf + nbuf;
+        p_maxlen = sizeof(buf) - nbuf;
+
         switch (cmd) {
         case BIO_CB_FREE:
                 snprintf(p, p_maxlen, "Free - %s\n", bio->method->name);
@@ -136,6 +143,7 @@ BIO_debug_callback(BIO *bio, int cmd, const char *argp, int argi, long argl,
                 break;
         }
 
+ out:
         b = (BIO *)bio->cb_arg;
         if (b != NULL)
                 BIO_write(b, buf, strlen(buf));