gcc 動態連結

在今天 Jserv 大師 的

你所不知道的 C 語言

http://hackfoldr.org/dykc/

動態連結器篇

http://hackfoldr.org/dykc/VESe3UsncpM

https://embedded2015.hackpad.com/C-VESe3UsncpM

( Youtube: https://www.youtube.com/c/GUTS4tech/live , 正播放中 )


有提到一個 malloc 動態連結置換 原程式 的方式.

我在 mac os x 上做個實驗.

1. 寫一個簡單測試程式 :

#include <stdio.h>
#include <stdlib.h>

#include <string.h>

int main(int argc, char const *argv[])

{

    char *buf;

    printf("Hello World\n");

    buf=malloc(50);

    sprintf(buf, "Hi, This is test\r\n");

    printf("%s", buf);

    free(buf);

    return 0;

}

Compile and test

gcc -Wall -o test test.c 

$ ./test

Hello World

Hi, This is test

2. 用原範例撰寫動態連結庫

void *malloc(size_t size)

{

    char buf[32];

    static void *(*real_malloc)(size_t) = NULL;

    if (real_malloc == NULL) {

        real_malloc = dlsym(RTLD_NEXT, "malloc");

    }

    sprintf(buf, "malloc called, size = %zu\n", size);

    write(2, buf, strlen(buf));

    return real_malloc(size);

}

Compiler

$ gcc -Wall -dynamiclib -o /tmp/libmcount.dylib malloc_count.c

測試 : 

$ DYLD_FORCE_FLAT_NAMESPACE=1  DYLD_INSERT_LIBRARIES=/tmp/libmcount.dylib ./test

malloc called, size = 8

malloc called, size = 16

malloc called, size = 32

malloc called, size = 32

malloc called, size = 32

malloc called, size = 32

malloc called, size = 8

malloc called, size = 27

malloc called, size = 22

malloc called, size = 11

malloc called, size = 45

malloc called, size = 99

malloc called, size = 45

malloc called, size = 50

malloc called, size = 47

malloc called, size = 15

malloc called, size = 64

malloc called, size = 49

malloc called, size = 67

malloc called, size = 52

malloc called, size = 54

malloc called, size = 12

malloc called, size = 45

malloc called, size = 32

malloc called, size = 16

malloc called, size = 64

malloc called, size = 14

malloc called, size = 4096

Hello World

malloc called, size = 16

malloc called, size = 50

Hi, This is test

不過很奇怪, mac 上這方式用在 /bin/ls 沒有用.
 找天 來深入 trace 一下 ls 看看原因 

--
延伸閱讀 :

TCMalloc : http://goog-perftools.sourceforge.net/doc/tcmalloc.html

Elf : 

• 《Binary Hacks》

• Executable and Linkable Format (非常詳盡)

• ELF Hacks

• Hacking Your ELF For Fun And Profit

回來繼續聽課.

個人 hackpad: