Atmosphere/libraries/libvapours/source/crypto/crypto_memory_compare.arch.arm64.cpp

/*
 * Copyright (c) 2018-2020 Atmosphère-NX
 *
 * This program is free software; you can redistribute it and/or modify it
 * under the terms and conditions of the GNU General Public License,
 * version 2, as published by the Free Software Foundation.
 *
 * This program is distributed in the hope it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License for
 * more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program.  If not, see <http://www.gnu.org/licenses/>.
 */
#include <vapours.hpp>

namespace ams::crypto {

    bool IsSameBytes(const void *lhs, const void *rhs, size_t size) {
        bool result;
        u8 xor_acc, ltmp, rtmp;
        size_t index;

        __asm__ __volatile__(
            /* Clear registers and prepare for comparison. */
            "   mov     %w[xor_acc], #0\n"
            "   mov     %w[index], #0\n"
            "   b       1f\n"

            /* Compare one byte in constant time. */
            "0:\n"
            "   ldrb    %w[ltmp], [%[lhs]]\n"
            "   ldrb    %w[rtmp], [%[rhs]]\n"
            "   adds    %[lhs], %[lhs], #1\n"
            "   adds    %[rhs], %[rhs], #1\n"
            "   eor     %w[ltmp], %w[ltmp], %w[rtmp]\n"
            "   orr     %w[xor_acc], %w[xor_acc], %w[ltmp]\n"
            "   adds    %[index], %[index], #1\n"

            /* Check if there is still data to compare. */
            "1:\n"
            "   cmp     %[index], %[size]\n"
            "   bcc     0b\n"

            /* We're done, set result. */
            "   cmp     %w[xor_acc], #0\n"
            "   cset    %w[result], eq\n"
            : [result]"=r"(result), [lhs]"+r"(lhs), [rhs]"+r"(rhs), [xor_acc]"=&r"(xor_acc), [index]"=&r"(index), [ltmp]"=&r"(ltmp), [rtmp]"=&r"(rtmp)
            : [size]"r"(size)
            : "cc"
        );

        return result;
    }

}