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.

2 files changed, 12 insertions, 2 deletions

diff --git a/src/lib/libcrypto/bio/bio_lib.c b/src/lib/libcrypto/bio/bio_lib.c
index b33ebe167b..c09ad8fe8e 100644
--- a/src/lib/libcrypto/bio/bio_lib.c
+++ b/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);
 }
 
diff --git a/src/lib/libcrypto/bio/bio_local.h b/src/lib/libcrypto/bio/bio_local.h
index 94dd460bc5..4eecf7e04a 100644
--- a/src/lib/libcrypto/bio/bio_local.h
+++ b/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;