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Use top to size BN add/sub results

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Size the acquired result OSSL_FN from the significant operand limbs instead of
the allocated operand width. This keeps BN_uadd() and BN_usub() consistent
with the other wrappers that already use top for result sizing.

This follows a realization that applying OSSL_FN semantics onto BIGNUM too much
is pointless, and as demonstrated with BN_mul(), may be harmful, especially when
the result BIGNUM as the same as one of the operands.

Assisted-by: Pi:openai/gpt-5.5
Signed-off-by: Richard Levitte <levitte@openssl.foundation>

Reviewed-by: Tomas Mraz <tomas@openssl.foundation>
Reviewed-by: Igor Ustinov <igus@openssl.foundation>
MergeDate: Fri Jun 12 15:16:33 2026
(Merged from https://github.com/openssl/openssl/pull/31448)
This commit is contained in:
Richard Levitte 2026-06-10 19:47:18 +02:00 • committed by Tomas Mraz
parent 7e62ed1d3c
commit 3eb0b45e63
1 changed files with 10 additions and 15 deletions
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25
crypto/bn/bn_add.c
View file

@ -10,19 +10,14 @@
#include "internal/cryptlib.h"
#include "bn_local.h"
static size_t calculate_max_limbs(const BIGNUM *a, const BIGNUM *b)
static size_t calculate_max_top(const BIGNUM *a, const BIGNUM *b)
{
OSSL_FN *af = a->data;
OSSL_FN *bf = b->data;
return (af->dsize > bf->dsize) ? af->dsize : bf->dsize;
return (a->top > b->top) ? a->top : b->top;
}
static bool is_highest_bit_set(const BIGNUM *a)
{
OSSL_FN *af = a->data;
return (af->d[af->dsize - 1] & OSSL_FN_HIGH_BIT_MASK) != 0;
return a->top > 0 && (a->d[a->top - 1] & OSSL_FN_HIGH_BIT_MASK) != 0;
}
/* signed add of b to a. */
@ -147,18 +142,18 @@ int BN_uadd(BIGNUM *r, const BIGNUM *a, const BIGNUM *b)
bn_check_top(a);
bn_check_top(b);
size_t max = calculate_max_limbs(a, b);
size_t top = calculate_max_top(a, b);
/*
* If either operands have the highest bit set the result may become
* one limb larger.
*/
if (is_highest_bit_set(a) || is_highest_bit_set(b))
max++;
top++;
OSSL_FN *rf = bn_acquire_ossl_fn(r, (int)max);
OSSL_FN *rf = bn_acquire_ossl_fn(r, (int)top);
int ret = OSSL_FN_add(rf, a->data, b->data);
bn_release(r, (int)max);
bn_release(r, (int)top);
return ret;
}
@ -226,11 +221,11 @@ int BN_usub(BIGNUM *r, const BIGNUM *a, const BIGNUM *b)
return 0;
}
size_t max = calculate_max_limbs(a, b);
size_t top = calculate_max_top(a, b);
OSSL_FN *rf = bn_acquire_ossl_fn(r, (int)max);
OSSL_FN *rf = bn_acquire_ossl_fn(r, (int)top);
int ret = OSSL_FN_sub(rf, a->data, b->data);
bn_release(r, (int)max);
bn_release(r, (int)top);
return ret;
}