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bn_nist: replace pointer bit-fiddling with ternary
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Bit-fiddling pointers is technically implementation defined behavior
in the C specification so the following code is not supported in all
platforms:

    PTR_SIZE_INT mask;
    void * a, b, c;
    int boolean_flag;

    mask = 0 - boolean_flag;
    /* Not guaranteed to be a valid ptr to a or b on all platforms  */
    a = (void *)
        ((((PTR_SIZE_INT) b & ~mask) | (((PTR_SIZE_INT)) c & mask)));

Using a ternary conditional operator is supported on all platforms
(i.e. `a = boolean_flag ? b : c;`).

On most modern compilers/CPUs, this will be faster, since it will
get converted to a CMOV instruction.

Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Paul Dale <pauli@openssl.org>
(Merged from #20748)
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@aloisklink
aloisklink committed Apr 16, 2023
1 parent 8835940 commit 326af4a
Showing 1 changed file with 19 additions and 45 deletions.
64 changes: 19 additions & 45 deletions crypto/bn/bn_nist.c
View file
Expand Up @@ -338,7 +338,6 @@ int BN_nist_mod_192(BIGNUM *r, const BIGNUM *a, const BIGNUM *field,
sizeof(unsigned int)];
} buf;
BN_ULONG c_d[BN_NIST_192_TOP], *res;
PTR_SIZE_INT mask;
static const BIGNUM ossl_bignum_nist_p_192_sqr = {
(BN_ULONG *)_nist_p_192_sqr,
OSSL_NELEM(_nist_p_192_sqr),
Expand Down Expand Up @@ -439,13 +438,9 @@ int BN_nist_mod_192(BIGNUM *r, const BIGNUM *a, const BIGNUM *field,
* 'tmp=result-modulus; if (!carry || !borrow) result=tmp;'
* this is what happens below, but without explicit if:-) a.
*/
mask =
0 - (PTR_SIZE_INT) bn_sub_words(c_d, r_d, _nist_p_192[0],
BN_NIST_192_TOP);
mask &= 0 - (PTR_SIZE_INT) carry;
res = c_d;
res = (BN_ULONG *)
(((PTR_SIZE_INT) res & ~mask) | ((PTR_SIZE_INT) r_d & mask));
res = (bn_sub_words(c_d, r_d, _nist_p_192[0], BN_NIST_192_TOP) && carry)
? r_d
: c_d;
nist_cp_bn(r_d, res, BN_NIST_192_TOP);
r->top = BN_NIST_192_TOP;
bn_correct_top(r);
Expand Down Expand Up @@ -479,7 +474,6 @@ int BN_nist_mod_224(BIGNUM *r, const BIGNUM *a, const BIGNUM *field,
sizeof(unsigned int)];
} buf;
BN_ULONG c_d[BN_NIST_224_TOP], *res;
PTR_SIZE_INT mask;
union {
bn_addsub_f f;
PTR_SIZE_INT p;
Expand Down Expand Up @@ -616,19 +610,14 @@ int BN_nist_mod_224(BIGNUM *r, const BIGNUM *a, const BIGNUM *field,
carry =
(int)bn_add_words(r_d, r_d, _nist_p_224[-carry - 1],
BN_NIST_224_TOP);
mask = 0 - (PTR_SIZE_INT) carry;
u.p = ((PTR_SIZE_INT) bn_sub_words & mask) |
((PTR_SIZE_INT) bn_add_words & ~mask);
u.f = carry ? bn_sub_words : bn_add_words;
} else
carry = 1;

/* otherwise it's effectively same as in BN_nist_mod_192... */
mask =
0 - (PTR_SIZE_INT) (*u.f) (c_d, r_d, _nist_p_224[0], BN_NIST_224_TOP);
mask &= 0 - (PTR_SIZE_INT) carry;
res = c_d;
res = (BN_ULONG *)(((PTR_SIZE_INT) res & ~mask) |
((PTR_SIZE_INT) r_d & mask));
res = ((*u.f) (c_d, r_d, _nist_p_224[0], BN_NIST_224_TOP) && carry)
? r_d
: c_d;
nist_cp_bn(r_d, res, BN_NIST_224_TOP);
r->top = BN_NIST_224_TOP;
bn_correct_top(r);
Expand Down Expand Up @@ -660,7 +649,6 @@ int BN_nist_mod_256(BIGNUM *r, const BIGNUM *a, const BIGNUM *field,
sizeof(unsigned int)];
} buf;
BN_ULONG c_d[BN_NIST_256_TOP], *res;
PTR_SIZE_INT mask;
union {
bn_addsub_f f;
PTR_SIZE_INT p;
Expand Down Expand Up @@ -859,18 +847,13 @@ int BN_nist_mod_256(BIGNUM *r, const BIGNUM *a, const BIGNUM *field,
carry =
(int)bn_add_words(r_d, r_d, _nist_p_256[-carry - 1],
BN_NIST_256_TOP);
mask = 0 - (PTR_SIZE_INT) carry;
u.p = ((PTR_SIZE_INT) bn_sub_words & mask) |
((PTR_SIZE_INT) bn_add_words & ~mask);
u.f = carry ? bn_sub_words : bn_add_words;
} else
carry = 1;

mask =
0 - (PTR_SIZE_INT) (*u.f) (c_d, r_d, _nist_p_256[0], BN_NIST_256_TOP);
mask &= 0 - (PTR_SIZE_INT) carry;
res = c_d;
res = (BN_ULONG *)(((PTR_SIZE_INT) res & ~mask) |
((PTR_SIZE_INT) r_d & mask));
res = ((*u.f) (c_d, r_d, _nist_p_256[0], BN_NIST_256_TOP) && carry)
? r_d
: c_d;
nist_cp_bn(r_d, res, BN_NIST_256_TOP);
r->top = BN_NIST_256_TOP;
bn_correct_top(r);
Expand Down Expand Up @@ -906,7 +889,6 @@ int BN_nist_mod_384(BIGNUM *r, const BIGNUM *a, const BIGNUM *field,
sizeof(unsigned int)];
} buf;
BN_ULONG c_d[BN_NIST_384_TOP], *res;
PTR_SIZE_INT mask;
union {
bn_addsub_f f;
PTR_SIZE_INT p;
Expand Down Expand Up @@ -1140,18 +1122,13 @@ int BN_nist_mod_384(BIGNUM *r, const BIGNUM *a, const BIGNUM *field,
carry =
(int)bn_add_words(r_d, r_d, _nist_p_384[-carry - 1],
BN_NIST_384_TOP);
mask = 0 - (PTR_SIZE_INT) carry;
u.p = ((PTR_SIZE_INT) bn_sub_words & mask) |
((PTR_SIZE_INT) bn_add_words & ~mask);
u.f = carry ? bn_sub_words : bn_add_words;
} else
carry = 1;

mask =
0 - (PTR_SIZE_INT) (*u.f) (c_d, r_d, _nist_p_384[0], BN_NIST_384_TOP);
mask &= 0 - (PTR_SIZE_INT) carry;
res = c_d;
res = (BN_ULONG *)(((PTR_SIZE_INT) res & ~mask) |
((PTR_SIZE_INT) r_d & mask));
res = ((*u.f) (c_d, r_d, _nist_p_384[0], BN_NIST_384_TOP) && carry)
? r_d
: c_d;
nist_cp_bn(r_d, res, BN_NIST_384_TOP);
r->top = BN_NIST_384_TOP;
bn_correct_top(r);
Expand All @@ -1168,7 +1145,6 @@ int BN_nist_mod_521(BIGNUM *r, const BIGNUM *a, const BIGNUM *field,
{
int top = a->top, i;
BN_ULONG *r_d, *a_d = a->d, t_d[BN_NIST_521_TOP], val, tmp, *res;
PTR_SIZE_INT mask;
static const BIGNUM ossl_bignum_nist_p_521_sqr = {
(BN_ULONG *)_nist_p_521_sqr,
OSSL_NELEM(_nist_p_521_sqr),
Expand Down Expand Up @@ -1221,12 +1197,10 @@ int BN_nist_mod_521(BIGNUM *r, const BIGNUM *a, const BIGNUM *field,
r_d[i] &= BN_NIST_521_TOP_MASK;

bn_add_words(r_d, r_d, t_d, BN_NIST_521_TOP);
mask =
0 - (PTR_SIZE_INT) bn_sub_words(t_d, r_d, _nist_p_521,
BN_NIST_521_TOP);
res = t_d;
res = (BN_ULONG *)(((PTR_SIZE_INT) res & ~mask) |
((PTR_SIZE_INT) r_d & mask));
res = bn_sub_words(t_d, r_d, _nist_p_521,
BN_NIST_521_TOP)
? r_d
: t_d;
nist_cp_bn(r_d, res, BN_NIST_521_TOP);
r->top = BN_NIST_521_TOP;
bn_correct_top(r);
Expand Down

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