c - C 中的 malloc() 内存损坏

Question

malloc() 在以下代码中给我一个错误。我用了 valgrind 还是没用。我是 C 语言的初学者，我和我的队友正在尝试实现 SHA1 算法，但有一些东西我们谁都无法理解，并且为此花费了数小时。这是有错误的函数 -

/*
 * Returns an array of chunks on the heap from the message
 */

static unsigned char **sha1_chunkify(const unsigned char *message, const uint64_t message_length)
{
    long num_chunks = message_length / 64;    //breaking it down into 64 byte chunks
    printf("%lu %ld\n", message_length, num_chunks);
    unsigned char **chunks = malloc (num_chunks * sizeof(*chunks)); //Error is coming over here

    for (int i = 0; i < num_chunks; i++) {
        chunks[i] = malloc (64 * sizeof(*chunks[i])); //Or sometimes over here.

        for (int j = 0; j < 64; j++) {
            chunks[i][j] = message[64 * i + j];
        }
    }

    return chunks;
}

这是这些行的 gdb 输出 -

117     printf("%lu %ld\n", message_length, num_chunks);
(gdb) 
1472 23
118     unsigned char **chunks = (unsigned char **) malloc (num_chunks * sizeof(*chunks));
(gdb) 
*** Error in `/home/username/Desktop/Project_VCS/a.out': malloc(): memory corruption: 0x000000000060cac0 ***

Program received signal SIGABRT, Aborted.
0x00007ffff7a4bcc9 in __GI_raise (sig=sig@entry=6)
at ../nptl/sysdeps/unix/sysv/linux/raise.c:56
56  ../nptl/sysdeps/unix/sysv/linux/raise.c: No such file or directory.

Valgrind 显示以下内容 -

3
mytry.c
==16688== Invalid write of size 1
==16688==    at 0x401EA6: append_zeroes (sha1.c:155)
==16688==    by 0x401F1D: sha1_pad (sha1.c:177)
==16688==    by 0x401FB7: sha1 (sha1.c:200)
==16688==    by 0x400C72: snap_file (snap.c:40)
==16688==    by 0x4012F7: snap (snap.c:143)
==16688==    by 0x401852: snap_all (snap.c:220)
==16688==    by 0x40222E: main (nako.c:24)
==16688==  Address 0x5220720 is 0 bytes after a block of size 1,408 alloc'd
==16688==    at 0x4C2AB80: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==16688==    by 0x401F7B: sha1 (sha1.c:195)
==16688==    by 0x400C72: snap_file (snap.c:40)
==16688==    by 0x4012F7: snap (snap.c:143)
==16688==    by 0x401852: snap_all (snap.c:220)
==16688==    by 0x40222E: main (nako.c:24)
==16688== 
==16688== Invalid write of size 1
==16688==    at 0x401E6A: append_msg_len (sha1.c:142)
==16688==    by 0x401F34: sha1_pad (sha1.c:179)
==16688==    by 0x401FB7: sha1 (sha1.c:200)
==16688==    by 0x400C72: snap_file (snap.c:40)
==16688==    by 0x4012F7: snap (snap.c:143)
==16688==    by 0x401852: snap_all (snap.c:220)
==16688==    by 0x40222E: main (nako.c:24)
==16688==  Address 0x5220758 is not stack'd, malloc'd or (recently) free'd
==16688== 
1472 23

valgrind: m_mallocfree.c:277 (mk_plain_bszB): Assertion 'bszB != 0' failed.
valgrind: This is probably caused by your program erroneously writing past the
end of a heap block and corrupting heap metadata.  If you fix any
invalid writes reported by Memcheck, this assertion failure will
probably go away.  Please try that before reporting this as a bug.

==16688==    at 0x38050BAC: ??? (in /usr/lib/valgrind/memcheck-amd64-linux)
==16688==    by 0x38050D06: ??? (in /usr/lib/valgrind/memcheck-amd64-linux)
==16688==    by 0x3805B36A: ??? (in /usr/lib/valgrind/memcheck-amd64-linux)
==16688==    by 0x3805D2D7: ??? (in /usr/lib/valgrind/memcheck-amd64-linux)
==16688==    by 0x380216D4: ??? (in /usr/lib/valgrind/memcheck-amd64-linux)
==16688==    by 0x380218A2: ??? (in /usr/lib/valgrind/memcheck-amd64-linux)
==16688==    by 0x3809DC03: ??? (in /usr/lib/valgrind/memcheck-amd64-linux)
==16688==    by 0x380AC87C: ??? (in /usr/lib/valgrind/memcheck-amd64-linux)

sched status:
running_tid=1

Thread 1: status = VgTs_Runnable
==16688==    at 0x4C2AB80: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==16688==    by 0x401D94: sha1_chunkify (sha1.c:118)
==16688==    by 0x401FCA: sha1 (sha1.c:202)
==16688==    by 0x400C72: snap_file (snap.c:40)
==16688==    by 0x4012F7: snap (snap.c:143)
==16688==    by 0x401852: snap_all (snap.c:220)
==16688==    by 0x40222E: main (nako.c:24)

我已经经历了几个线程，但错误仍然存​​在。如果之前有人问过，我深表歉意。我找不到相同的。预先感谢您。

这里是函数append_zeroes 和append_msg_len -

/*
 * Pad the message with its length.
 * The input lengths are in bytes, while the padding, in accordance with
 * the SHA1 algorithm, is done in bits.
 */
static inline void append_msg_len(unsigned char *message,
                  uint64_t *message_length,
              uint64_t original_msglen)
{
    int shift = 56;
    while (shift >= 0) {
        /* Add the next 8 bits. */
         message[*message_length] = (8 * original_msglen >> shift) & 0xff;
         shift -= 8;
    *message_length += 1;
    }
}

/*
 * Appends enough zeroes until the message has just enough room for appending
 * the message length, i.e, length 448 mod 512
 */
static inline void append_zeroes(unsigned char *message, uint64_t *message_length)
{
    while (*message_length % 64 != 56) {
        message[*message_length] = (unsigned char) 0x00;
        *message_length += 1;
    }
}

Answer 1

查看这段代码:

long num_chunks = message_length / 64;    //breaking it down into 64 byte chunks

例如，如果我们使用 message_length = 63，num_chunks 就会变成零，这显然是错误的。如果您使用:

long num_chunks = (message_length + 63) / 64;    //breaking it down into 64 byte chunks

您将获得正确数量的 block ，并且希望其余代码表现良好。