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110 lines
3.3 KiB
Plaintext
110 lines
3.3 KiB
Plaintext
=pod
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=head1 NAME
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OSSL_SAFE_MATH_SIGNED, OSSL_SAFE_MATH_UNSIGNED,
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safe_add_TYPE, safe_sub_TYPE, safe_mul_TYPE, safe_div_TYPE, safe_mod_TYPE,
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safe_div_round_up_TYPE, safe_neg_TYPE
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- create helper functions to safely perform non-overflowing integer operations
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=head1 SYNOPSIS
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=for openssl generic
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#include "internal/safe_math.h"
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OSSL_SAFE_MATH_SIGNED(NAME, TYPE)
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OSSL_SAFE_MATH_UNSIGNED(NAME, TYPE)
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TYPE safe_add_TYPE(TYPE a, TYPE b, int *err);
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TYPE safe_sub_TYPE(TYPE a, TYPE b, int *err);
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TYPE safe_mul_TYPE(TYPE a, TYPE b, int *err);
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TYPE safe_div_TYPE(TYPE a, TYPE b, int *err);
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TYPE safe_mod_TYPE(TYPE a, TYPE b, int *err);
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TYPE safe_div_round_up_TYPE(TYPE a, TYPE b, int *err);
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TYPE safe_muldiv_TYPE(TYPE a, TYPE b, TYPE c, int *err);
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TYPE safe_neg_TYPE(TYPE a, int *err);
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TYPE safe_abs_TYPE(TYPE a, int *err);
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=head1 DESCRIPTION
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Define helper functions to assist with handling integer overflow detection.
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All of these functions perform an arithmetic operation on its arguments and
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return the result of the operation. If the operation cannot be
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correctly represented, the error I<err> flag is set. No behaviour that is
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undefined as per the C standard will take place.
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OSSL_SAFE_MATH_SIGNED() creates helper functions for the B<I<TYPE>> with the
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suffix B<I<NAME>>.
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OSSL_SAFE_MATH_UNSIGNED() creates helper functions for the B<I<TYPE>> with the
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suffix B<I<NAME>>.
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safe_add_TYPE() adds the two arguments I<a> and I<b> together.
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safe_sub_TYPE() subtracts I<b> from I<a>.
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safe_mul_TYPE() multiplies the two arguments I<a> and I<b> together.
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safe_div_TYPE() divides I<a> by I<b>.
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safe_mod_TYPE() calculates the remainder when I<a> is divided by I<b>.
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safe_div_round_up_TYPE() calculates I<a> / I<b> + (I<a> % I<b> != 0).
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I.e. it computes the quotient of I<a> and I<b> rounding any remainder towards
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positive infinity.
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safe_muldiv_TYPE() multiplies I<a> and I<b> together and divides the
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result by I<c>.
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safe_neg_TYPE() calculates the negation of I<a>.
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safe_abs_TYPE() calculates the absolute value of I<a>.
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=head1 NOTES
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The safe_muldiv_TYPE() function is not perfect. There exist inputs where
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a valid result could be computed with infinite length integers but this
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function returns an error condition. Such instances should, however,
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be rare in practice. The converse is not true. An invalid result will
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always be flagged as an error.
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=head1 RETURN VALUES
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All these functions return the result of the operation, if the operation
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is well defined. They return an arbitrary value if not.
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=head1 EXAMPLES
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This example is of a function that computes the size of a record that
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has a four byte element count which is followed by that many elements.
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It returns zero on overflow.
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OSSL_SAFE_MATH_UNSIGNED(sizet, size_t, SIZE_MAX)
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size_t compute_record_size(uint32_t n)
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{
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int err = 0;
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size_t result, product;
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product = safe_mul_sizet(n, sizeof(struct widget), &err);
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result = safe_add_sizet(product, sizeof(n), &err);
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return err ? 0 : result;
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}
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=head1 HISTORY
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The functions described here were all added in OpenSSL 3.1.
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=head1 COPYRIGHT
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Copyright 2021-2022 The OpenSSL Project Authors. All Rights Reserved.
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Licensed under the Apache License 2.0 (the "License"). You may not use
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this file except in compliance with the License. You can obtain a copy
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in the file LICENSE in the source distribution or at
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L<https://www.openssl.org/source/license.html>.
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=cut
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