OSSL_FN: Add internal functions to acquire the OSSL_FN from a BIGNUM

bn_acquire_ossl_fn() returns the OSSL_FN of a BIGNUM if there is one,
expanded to a given number of limbs.
bn_release() makes necessary adjustments to the BIGNUM after a run of
OSSL_FN operations on the OSSL instance it's backed by.

These two functions are most useful with BIGNUMs used to store operation
results, and are meant to help refactoring BN operators to be wrappers
around corresponding OSSL_FN operators.

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/29028)

diff --git a/crypto/bn/bn_local.h b/crypto/bn/bn_local.h
index 93a8fbbbbf6a44fdac5d2d52f1ee4e8b0c951b91..a0c7f848105ad17bdfc63ae9130aeb5c42e20057 100644 (file)
--- a/crypto/bn/bn_local.h
+++ b/crypto/bn/bn_local.h
@@ -721,4 +721,58 @@ static ossl_inline int bn_set_top(BIGNUM *b, int newtop)
     return b->top;
 }
 
+/*
+ * bn_acquire_ossl_fn() and bn_release() work in tandem, and are
+ * most useful in OSSL_FN_ wrapper functions, specifically with the
+ * BIGNUM in which an operation result is written.
+ */
+
+/**
+ * Acquire the OSSL_FN from a BIGNUM.  The BIGNUM should no longer be
+ * used until bn_release() is called.
+ *
+ * @param[in]   b       The BIGNUM instance to get an OSSL_FN from
+ * @param[in]   limbs   The minimum number of limbs 'b' should be expanded to.
+ *                      Note that this doesn't set 'top', that's done by
+ *                      bn_release().
+ * @returns     the OSSL_FN instance of the BIGNUM, if there is one
+ * @pre         b and b->data must not be NULL
+ */
+static ossl_inline OSSL_FN *bn_acquire_ossl_fn(BIGNUM *b, int limbs)
+{
+    if (ossl_unlikely(b == NULL || b->data == NULL))
+        return NULL;
+
+    if (bn_wexpand(b, limbs) == NULL)
+        return NULL;
+    /* TODO(FIXNUM): should we add a flag bit for this in b->flags ? */
+    return b->data;
+}
+
+/**
+ * Release the BIGNUM from which the OSSL_FN was acquired.  This will
+ * adjust the BIGNUM to what was done with its OSSL_FN, and the BIGNUM
+ * can again be used with BN_ functions.
+ *
+ * @param[in]   b       The BIGNUM instance to release
+ * @param[in]   limbs   The maximum number of significant limbs.  This sets
+ *                      'top'.
+ * @pre         b and b->data must not be NULL
+ */
+static ossl_inline void bn_release(BIGNUM *b, int limbs)
+{
+    if (ossl_unlikely(b == NULL || b->data == NULL))
+        return;
+
+    int fixed_top = (b->flags & BN_FLG_FIXED_TOP) != 0;
+
+    bn_set_top(b, limbs);
+
+    /* Don't correct top if BN_FLG_FIXED_TOP was set */
+    if (fixed_top)
+        return;
+
+    bn_correct_top(b);
+}
+
 #endif