Start cleaning up BN_div_reciprocal() a bit

The fast path where no division is performed can be dealt with without
BN_CTX, so do that up front so there's no need to clean up before return.
Error check BN_CTX_get() on each use asd simplify the logic for optional
input parameters. KNF for an ugly comment.

ok jsing

1 files changed, 23 insertions, 24 deletions

diff --git a/src/lib/libcrypto/bn/bn_recp.c b/src/lib/libcrypto/bn/bn_recp.c
index 8f917e95db..757ed0c3d2 100644
--- a/src/lib/libcrypto/bn/bn_recp.c
+++ b/src/lib/libcrypto/bn/bn_recp.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: bn_recp.c,v 1.30 2025/01/22 12:53:16 tb Exp $ */
+/* $OpenBSD: bn_recp.c,v 1.31 2025/02/04 05:09:53 tb Exp $ */
 /* Copyright (C) 1995-1998 Eric Young (eay@cryptsoft.com)
  * All rights reserved.
  *
@@ -139,34 +139,33 @@ BN_div_reciprocal(BIGNUM *dv, BIGNUM *rem, const BIGNUM *m, BN_RECP_CTX *recp,
         int i, j, ret = 0;
         BIGNUM *a, *b, *d, *r;
 
+        if (BN_ucmp(m, recp->N) < 0) {
+                if (dv != NULL)
+                        BN_zero(dv);
+                if (rem != NULL)
+                        return bn_copy(rem, m);
+                return 1;
+        }
+
         BN_CTX_start(ctx);
-        a = BN_CTX_get(ctx);
-        b = BN_CTX_get(ctx);
-        if (dv != NULL)
-                d = dv;
-        else
+        if ((a = BN_CTX_get(ctx)) == NULL)
+                goto err;
+        if ((b = BN_CTX_get(ctx)) == NULL)
+                goto err;
+
+        if ((d = dv) == NULL)
                 d = BN_CTX_get(ctx);
-        if (rem != NULL)
-                r = rem;
-        else
-                r = BN_CTX_get(ctx);
-        if (a == NULL || b == NULL || d == NULL || r == NULL)
+        if (d == NULL)
                 goto err;
 
-        if (BN_ucmp(m, recp->N) < 0) {
-                BN_zero(d);
-                if (!bn_copy(r, m)) {
-                        BN_CTX_end(ctx);
-                        return 0;
-                }
-                BN_CTX_end(ctx);
-                return 1;
-        }
+        if ((r = rem) == NULL)
+                r = BN_CTX_get(ctx);
+        if (r == NULL)
+                goto err;
 
-        /* We want the remainder
-         * Given input of ABCDEF / ab
-         * we need multiply ABCDEF by 3 digests of the reciprocal of ab
-         *
+        /*
+         * We want the remainder. Given input of ABCDEF / ab we need to
+         * multiply ABCDEF by 3 digits of the reciprocal of ab.
          */
 
         /* i := max(BN_num_bits(m), 2*BN_num_bits(N)) */