Revert bio_prev removal

As schwarze points out, you can pop any BIO in a chain, not just the first
one (bonus points for a great name for this API).

The internal doubly linked was used to fix up the BIO chain bio was part
of when you BIO_pop() a bio that wasn't in the first position, which is
explicitly allowed in our documentation and implied by OpenSSL's.

Author: tb

src/lib/libcrypto/bio/bio_lib.c

@@ -1,4 +1,4 @@
-/* $OpenBSD: bio_lib.c,v 1.38 2022/11/30 01:56:18 jsing Exp $ */
+/* $OpenBSD: bio_lib.c,v 1.39 2022/12/02 19:44:04 tb Exp $ */
 /* Copyright (C) 1995-1998 Eric Young (eay@cryptsoft.com)
  * All rights reserved.
  *
@@ -161,6 +161,7 @@ BIO_set(BIO *bio, const BIO_METHOD *method)
 	bio->retry_reason = 0;
 	bio->num = 0;
 	bio->ptr = NULL;
+	bio->prev_bio = NULL;
 	bio->next_bio = NULL;
 	bio->references = 1;
 	bio->num_read = 0L;
@@ -636,6 +637,8 @@ BIO_push(BIO *b, BIO *bio)
 	while (lb->next_bio != NULL)
 		lb = lb->next_bio;
 	lb->next_bio = bio;
+	if (bio != NULL)
+		bio->prev_bio = lb;
 	/* called to do internal processing */
 	BIO_ctrl(b, BIO_CTRL_PUSH, 0, lb);
 	return (b);
@@ -653,7 +656,13 @@ BIO_pop(BIO *b)
 
 	BIO_ctrl(b, BIO_CTRL_POP, 0, b);
 
+	if (b->prev_bio != NULL)
+		b->prev_bio->next_bio = b->next_bio;
+	if (b->next_bio != NULL)
+		b->next_bio->prev_bio = b->prev_bio;
+
 	b->next_bio = NULL;
+	b->prev_bio = NULL;
 	return (ret);
 }
 

src/lib/libcrypto/bio/bio_local.h

@@ -1,4 +1,4 @@
-/* $OpenBSD: bio_local.h,v 1.4 2022/11/28 07:50:00 tb Exp $ */
+/* $OpenBSD: bio_local.h,v 1.5 2022/12/02 19:44:04 tb Exp $ */
 /* Copyright (C) 1995-1998 Eric Young (eay@cryptsoft.com)
  * All rights reserved.
  *
@@ -87,6 +87,7 @@ struct bio_st {
 	int num;
 	void *ptr;
 	struct bio_st *next_bio;	/* used by filter BIOs */
+	struct bio_st *prev_bio;	/* used by filter BIOs */
 	int references;
 	unsigned long num_read;
 	unsigned long num_write;