c - 通过 `malloc` 替换 `calloc` 、 `realloc` 、 `free` 和 `LD

c - 通过 `malloc` 替换 `calloc` 、 `realloc` 、 `free` 和 `LD_PRELOAD` 时出现段错误

转载作者：行者123 更新时间：2023-12-01 23:28:00

背景

我正在尝试替换 malloc(3)/calloc(3)/realloc(3)/free(3 ) 通过 LD_PRELOAD 环境变量。我尝试使用静态链接的自定义函数，它们运行良好。

但是，当我将它作为共享库附加到 LD_PRELOAD 时，它总是会导致段错误。

关于功能的简短技术说明

我使用 Linux x86-64 mmap(2) 和 munmap(2) 系统调用 malloc(3) 和 free (3).
calloc(3) 只是调用带有乘法溢出检查的 malloc(3)。
realloc(3) 调用 malloc(3)，然后将旧数据复制到新分配的内存并取消映射旧内存。

问题

我的方法有什么问题，总是导致段错误？
如何调试它(gdb 和 valgrind 也是段错误)？
我错过了什么？

注意事项

我完全清楚每次调用 malloc 时始终使用 mmap 是一个坏主意，尤其是对于性能而言。我只想知道为什么我的方法不起作用。

输出

ammarfaizi2@integral:~$ gcc -shared mem.c -O3 -o my_mem.so
ammarfaizi2@integral:~$ LD_PRELOAD=$(pwd)/my_mem.so ls
Segmentation fault (core dumped)
ammarfaizi2@integral:~$ LD_PRELOAD=$(pwd)/my_mem.so cat
Segmentation fault (core dumped)
ammarfaizi2@integral:~$ LD_PRELOAD=$(pwd)/my_mem.so w
Segmentation fault (core dumped)
ammarfaizi2@integral:~$ LD_PRELOAD=$(pwd)/my_mem.so gdb ls
Segmentation fault (core dumped)
ammarfaizi2@integral:~$ LD_PRELOAD=$(pwd)/my_mem.so valgrind ls
Segmentation fault (core dumped)
ammarfaizi2@integral:~$

glibc 版本

ammarfaizi2@integral:~$ /lib/x86_64-linux-gnu/libc.so.6
GNU C Library (Ubuntu GLIBC 2.33-0ubuntu2) release release version 2.33.
Copyright (C) 2021 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.
There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A
PARTICULAR PURPOSE.
Compiled by GNU CC version 10.2.1 20210130.
libc ABIs: UNIQUE IFUNC ABSOLUTE
For bug reporting instructions, please see:
<https://bugs.launchpad.net/ubuntu/+source/glibc/+bugs>.
ammarfaizi2@integral:~$

代码内存.c


#include <errno.h>
#include <stdlib.h>
#include <stdint.h>
#include <sys/mman.h>
#include <string.h>


static inline void *my_mmap(void *addr, size_t length, int prot, int flags,
                            int fd, off_t offset)
{
    void *ret;
    register int _flags asm("r10") = flags;
    register int _fd asm("r8") = fd;
    register off_t _offset asm("r9") = offset;

    asm volatile(
        "syscall"
        : "=a"(ret)
        : "a"(9), "D"(addr), "S"(length), "d"(prot),
          "r"(_flags), "r"(_fd), "r"(_offset)
        : "memory", "r11", "rcx"
    );
    return ret;
}


static inline int my_munmap(void *addr, size_t length)
{
    int ret;

    asm volatile(
        "syscall"
        : "=a"(ret)
        : "a"(11), "D"(addr), "S"(length)
        : "memory", "r11", "rcx"
    );
    return ret;
}

#define unlikely(EXPR) __builtin_expect(EXPR, 0)

void * __attribute__((noinline)) malloc(size_t len)
{
    void *start_map;
    uintptr_t user_ptr, cmperr;
    size_t add_req = 0;

    add_req += sizeof(size_t);
    add_req += sizeof(uint8_t);
    add_req += 0x1full;

    start_map = my_mmap(NULL, add_req + len, PROT_READ | PROT_WRITE,
                        MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);

    cmperr = 0xffffffffffffff00ull;
    if (unlikely(((uintptr_t)start_map & cmperr) == cmperr)) {
        errno = ENOMEM;
        return NULL;
    }

    /* Align 32-byte and take space to save the length and diff */
    user_ptr = ((uintptr_t)start_map + add_req) & ~0x1full;
    *(size_t  *)(user_ptr - 8) = len;
    *(uint8_t *)(user_ptr - 9) = (uint8_t)(user_ptr - (uintptr_t)start_map);

    return (void *)user_ptr;
}


void free(void *__user_ptr)
{
    size_t len;
    uint8_t diff;
    uintptr_t user_ptr = (uintptr_t)__user_ptr;

    len  = *(size_t  *)(user_ptr - 8);
    diff = *(uint8_t *)(user_ptr - 9);
    my_munmap((void *)(user_ptr - diff), len);
}


void *realloc(void *__user_ptr, size_t new_len)
{
    void *new_mem;
    size_t len;
    uint8_t diff;
    uintptr_t user_ptr = (uintptr_t)__user_ptr;

    len  = *(size_t  *)(user_ptr - 8);
    diff = *(uint8_t *)(user_ptr - 9);

    new_mem = malloc(new_len);
    if (unlikely(new_mem == NULL))
        return NULL;

    memcpy(new_mem, __user_ptr, (new_len < len) ? new_len : len);
    my_munmap((void *)(user_ptr - diff), len);
    return new_mem;
}


void *calloc(size_t nmemb, size_t len)
{
    size_t x = nmemb * len;
    if (unlikely(nmemb != 0 && x / nmemb != len)) {
        errno = EOVERFLOW;
        return NULL;
    }
    return malloc(x);
}

// #include <stdio.h>
// int main(void)
// {
//  char *test = malloc(1);

//  for (size_t i = 2; i <= (1024 * 1024); i++) {
//      test = realloc(test, i);
//      memset(test, 'q', i);
//  }

//  free(test);
// }

使用`-Wall -Wextra -ggdb3` 和`strace` 输出重新编译

ammarfaizi2@integral:~$ 
ammarfaizi2@integral:~$ gcc -Wall -Wextra -ggdb3 -shared mem.c -O3 -o my_mem.so
ammarfaizi2@integral:~$ strace -tf /usr/bin/env LD_PRELOAD=$(pwd)/my_mem.so ls
12:59:15 execve("/usr/bin/env", ["/usr/bin/env", "LD_PRELOAD=/home/ammarfaizi2/my_"..., "ls"], 0x7ffd2d6ee188 /* 34 vars */) = 0
12:59:15 brk(NULL)                      = 0x565552193000
12:59:15 arch_prctl(0x3001 /* ARCH_??? */, 0x7ffd13017120) = -1 EINVAL (Invalid argument)
12:59:15 access("/etc/ld.so.nohwcap", F_OK) = -1 ENOENT (No such file or directory)
12:59:15 access("/etc/ld.so.preload", R_OK) = -1 ENOENT (No such file or directory)
12:59:15 openat(AT_FDCWD, "/etc/ld.so.cache", O_RDONLY|O_CLOEXEC) = 3
12:59:15 newfstatat(3, "", {st_mode=S_IFREG|0644, st_size=74118, ...}, AT_EMPTY_PATH) = 0
12:59:15 mmap(NULL, 74118, PROT_READ, MAP_PRIVATE, 3, 0) = 0x7facba2ba000
12:59:15 close(3)                       = 0
12:59:15 access("/etc/ld.so.nohwcap", F_OK) = -1 ENOENT (No such file or directory)
12:59:15 openat(AT_FDCWD, "/lib/x86_64-linux-gnu/libc.so.6", O_RDONLY|O_CLOEXEC) = 3
12:59:15 read(3, "\177ELF\2\1\1\3\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0\240\206\2\0\0\0\0\0"..., 832) = 832
12:59:15 pread64(3, "\6\0\0\0\4\0\0\0@\0\0\0\0\0\0\0@\0\0\0\0\0\0\0@\0\0\0\0\0\0\0"..., 784, 64) = 784
12:59:15 pread64(3, "\4\0\0\0 \0\0\0\5\0\0\0GNU\0\2\0\0\300\4\0\0\0\3\0\0\0\0\0\0\0"..., 48, 848) = 48
12:59:15 pread64(3, "\4\0\0\0\24\0\0\0\3\0\0\0GNU\0\30\355\366\266\203\242\371v\214\300\356\234\306J\346\373"..., 68, 896) = 68
12:59:15 newfstatat(3, "", {st_mode=S_IFREG|0755, st_size=1983576, ...}, AT_EMPTY_PATH) = 0
12:59:15 mmap(NULL, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7facba2b8000
12:59:15 pread64(3, "\6\0\0\0\4\0\0\0@\0\0\0\0\0\0\0@\0\0\0\0\0\0\0@\0\0\0\0\0\0\0"..., 784, 64) = 784
12:59:15 mmap(NULL, 2012056, PROT_READ, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0x7facba0cc000
12:59:15 mmap(0x7facba0f2000, 1486848, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x26000) = 0x7facba0f2000
12:59:15 mmap(0x7facba25d000, 311296, PROT_READ, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x191000) = 0x7facba25d000
12:59:15 mmap(0x7facba2a9000, 24576, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x1dc000) = 0x7facba2a9000
12:59:15 mmap(0x7facba2af000, 33688, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0x7facba2af000
12:59:15 close(3)                       = 0
12:59:15 mmap(NULL, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7facba0ca000
12:59:15 arch_prctl(ARCH_SET_FS, 0x7facba2b95c0) = 0
12:59:15 mprotect(0x7facba2a9000, 12288, PROT_READ) = 0
12:59:15 mprotect(0x565550cb5000, 4096, PROT_READ) = 0
12:59:15 mprotect(0x7facba2ff000, 8192, PROT_READ) = 0
12:59:15 munmap(0x7facba2ba000, 74118)  = 0
12:59:15 brk(NULL)                      = 0x565552193000
12:59:15 brk(0x5655521b4000)            = 0x5655521b4000
12:59:15 openat(AT_FDCWD, "/usr/lib/locale/locale-archive", O_RDONLY|O_CLOEXEC) = 3
12:59:15 newfstatat(3, "", {st_mode=S_IFREG|0644, st_size=3041456, ...}, AT_EMPTY_PATH) = 0
12:59:15 mmap(NULL, 3041456, PROT_READ, MAP_PRIVATE, 3, 0) = 0x7facb9de3000
12:59:15 close(3)                       = 0
12:59:15 execve("/home/ammarfaizi2/.local/bin/ls", ["ls"], 0x565552194550 /* 35 vars */) = -1 ENOENT (No such file or directory)
12:59:15 execve("/usr/local/sbin/ls", ["ls"], 0x565552194550 /* 35 vars */) = -1 ENOENT (No such file or directory)
12:59:15 execve("/usr/local/bin/ls", ["ls"], 0x565552194550 /* 35 vars */) = -1 ENOENT (No such file or directory)
12:59:15 execve("/usr/sbin/ls", ["ls"], 0x565552194550 /* 35 vars */) = -1 ENOENT (No such file or directory)
12:59:15 execve("/usr/bin/ls", ["ls"], 0x565552194550 /* 35 vars */) = 0
12:59:15 brk(NULL)                      = 0x557f8624b000
12:59:15 arch_prctl(0x3001 /* ARCH_??? */, 0x7fff39dc1a30) = -1 EINVAL (Invalid argument)
12:59:15 access("/etc/ld.so.nohwcap", F_OK) = -1 ENOENT (No such file or directory)
12:59:15 openat(AT_FDCWD, "/home/ammarfaizi2/my_mem.so", O_RDONLY|O_CLOEXEC) = 3
12:59:15 read(3, "\177ELF\2\1\1\0\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0\240\20\0\0\0\0\0\0"..., 832) = 832
12:59:15 newfstatat(3, "", {st_mode=S_IFREG|0700, st_size=58768, ...}, AT_EMPTY_PATH) = 0
12:59:15 mmap(NULL, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7fa7daa68000
12:59:15 mmap(NULL, 16448, PROT_READ, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0x7fa7daa63000
12:59:15 mmap(0x7fa7daa64000, 4096, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x1000) = 0x7fa7daa64000
12:59:15 mmap(0x7fa7daa65000, 4096, PROT_READ, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x2000) = 0x7fa7daa65000
12:59:15 mmap(0x7fa7daa66000, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x2000) = 0x7fa7daa66000
12:59:15 close(3)                       = 0
12:59:15 access("/etc/ld.so.preload", R_OK) = -1 ENOENT (No such file or directory)
12:59:15 openat(AT_FDCWD, "/etc/ld.so.cache", O_RDONLY|O_CLOEXEC) = 3
12:59:15 newfstatat(3, "", {st_mode=S_IFREG|0644, st_size=74118, ...}, AT_EMPTY_PATH) = 0
12:59:15 mmap(NULL, 74118, PROT_READ, MAP_PRIVATE, 3, 0) = 0x7fa7daa50000
12:59:15 close(3)                       = 0
12:59:15 access("/etc/ld.so.nohwcap", F_OK) = -1 ENOENT (No such file or directory)
12:59:15 openat(AT_FDCWD, "/lib/x86_64-linux-gnu/libselinux.so.1", O_RDONLY|O_CLOEXEC) = 3
12:59:15 read(3, "\177ELF\2\1\1\0\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0 p\0\0\0\0\0\0"..., 832) = 832
12:59:15 newfstatat(3, "", {st_mode=S_IFREG|0644, st_size=167352, ...}, AT_EMPTY_PATH) = 0
12:59:15 mmap(NULL, 178664, PROT_READ, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0x7fa7daa24000
12:59:15 mmap(0x7fa7daa2a000, 106496, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x6000) = 0x7fa7daa2a000
12:59:15 mmap(0x7fa7daa44000, 32768, PROT_READ, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x20000) = 0x7fa7daa44000
12:59:15 mmap(0x7fa7daa4c000, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x27000) = 0x7fa7daa4c000
12:59:15 mmap(0x7fa7daa4e000, 6632, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0x7fa7daa4e000
12:59:15 close(3)                       = 0
12:59:15 access("/etc/ld.so.nohwcap", F_OK) = -1 ENOENT (No such file or directory)
12:59:15 openat(AT_FDCWD, "/lib/x86_64-linux-gnu/libc.so.6", O_RDONLY|O_CLOEXEC) = 3
12:59:15 read(3, "\177ELF\2\1\1\3\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0\240\206\2\0\0\0\0\0"..., 832) = 832
12:59:15 pread64(3, "\6\0\0\0\4\0\0\0@\0\0\0\0\0\0\0@\0\0\0\0\0\0\0@\0\0\0\0\0\0\0"..., 784, 64) = 784
12:59:15 pread64(3, "\4\0\0\0 \0\0\0\5\0\0\0GNU\0\2\0\0\300\4\0\0\0\3\0\0\0\0\0\0\0"..., 48, 848) = 48
12:59:15 pread64(3, "\4\0\0\0\24\0\0\0\3\0\0\0GNU\0\30\355\366\266\203\242\371v\214\300\356\234\306J\346\373"..., 68, 896) = 68
12:59:15 newfstatat(3, "", {st_mode=S_IFREG|0755, st_size=1983576, ...}, AT_EMPTY_PATH) = 0
12:59:15 pread64(3, "\6\0\0\0\4\0\0\0@\0\0\0\0\0\0\0@\0\0\0\0\0\0\0@\0\0\0\0\0\0\0"..., 784, 64) = 784
12:59:15 mmap(NULL, 2012056, PROT_READ, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0x7fa7da838000
12:59:15 mmap(0x7fa7da85e000, 1486848, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x26000) = 0x7fa7da85e000
12:59:15 mmap(0x7fa7da9c9000, 311296, PROT_READ, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x191000) = 0x7fa7da9c9000
12:59:15 mmap(0x7fa7daa15000, 24576, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x1dc000) = 0x7fa7daa15000
12:59:15 mmap(0x7fa7daa1b000, 33688, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0x7fa7daa1b000
12:59:15 close(3)                       = 0
12:59:15 access("/etc/ld.so.nohwcap", F_OK) = -1 ENOENT (No such file or directory)
12:59:15 openat(AT_FDCWD, "/lib/x86_64-linux-gnu/libpcre2-8.so.0", O_RDONLY|O_CLOEXEC) = 3
12:59:15 read(3, "\177ELF\2\1\1\0\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0\340\"\0\0\0\0\0\0"..., 832) = 832
12:59:15 newfstatat(3, "", {st_mode=S_IFREG|0644, st_size=617160, ...}, AT_EMPTY_PATH) = 0
12:59:15 mmap(NULL, 619304, PROT_READ, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0x7fa7da7a0000
12:59:15 mmap(0x7fa7da7a2000, 438272, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x2000) = 0x7fa7da7a2000
12:59:15 mmap(0x7fa7da80d000, 167936, PROT_READ, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x6d000) = 0x7fa7da80d000
12:59:15 mmap(0x7fa7da836000, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x95000) = 0x7fa7da836000
12:59:15 close(3)                       = 0
12:59:15 access("/etc/ld.so.nohwcap", F_OK) = -1 ENOENT (No such file or directory)
12:59:15 openat(AT_FDCWD, "/lib/x86_64-linux-gnu/libdl.so.2", O_RDONLY|O_CLOEXEC) = 3
12:59:15 read(3, "\177ELF\2\1\1\0\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0 \"\0\0\0\0\0\0"..., 832) = 832
12:59:15 newfstatat(3, "", {st_mode=S_IFREG|0644, st_size=22912, ...}, AT_EMPTY_PATH) = 0
12:59:15 mmap(NULL, 24848, PROT_READ, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0x7fa7da799000
12:59:15 mmap(0x7fa7da79b000, 8192, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x2000) = 0x7fa7da79b000
12:59:15 mmap(0x7fa7da79d000, 4096, PROT_READ, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x4000) = 0x7fa7da79d000
12:59:15 mmap(0x7fa7da79e000, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x4000) = 0x7fa7da79e000
12:59:15 close(3)                       = 0
12:59:15 access("/etc/ld.so.nohwcap", F_OK) = -1 ENOENT (No such file or directory)
12:59:15 openat(AT_FDCWD, "/lib/x86_64-linux-gnu/libpthread.so.0", O_RDONLY|O_CLOEXEC) = 3
12:59:15 read(3, "\177ELF\2\1\1\0\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0\300\200\0\0\0\0\0\0"..., 832) = 832
12:59:15 pread64(3, "\4\0\0\0 \0\0\0\5\0\0\0GNU\0\2\0\0\300\4\0\0\0\3\0\0\0\0\0\0\0"..., 48, 792) = 48
12:59:15 pread64(3, "\4\0\0\0\24\0\0\0\3\0\0\0GNU\0a7\363\352;k|\2228\244\6\253\346\2569\312"..., 68, 840) = 68
12:59:15 newfstatat(3, "", {st_mode=S_IFREG|0755, st_size=150456, ...}, AT_EMPTY_PATH) = 0
12:59:15 mmap(NULL, 136208, PROT_READ, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0x7fa7da777000
12:59:15 mmap(0x7fa7da77e000, 65536, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x7000) = 0x7fa7da77e000
12:59:15 mmap(0x7fa7da78e000, 20480, PROT_READ, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x17000) = 0x7fa7da78e000
12:59:15 mmap(0x7fa7da793000, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x1b000) = 0x7fa7da793000
12:59:15 mmap(0x7fa7da795000, 13328, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0x7fa7da795000
12:59:15 close(3)                       = 0
12:59:15 mmap(NULL, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7fa7da775000
12:59:15 mmap(NULL, 12288, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7fa7da772000
12:59:15 arch_prctl(ARCH_SET_FS, 0x7fa7da772800) = 0
12:59:15 mprotect(0x7fa7daa15000, 12288, PROT_READ) = 0
12:59:15 mprotect(0x7fa7da793000, 4096, PROT_READ) = 0
12:59:15 mprotect(0x7fa7da79e000, 4096, PROT_READ) = 0
12:59:15 mprotect(0x7fa7da836000, 4096, PROT_READ) = 0
12:59:15 mprotect(0x7fa7daa4c000, 4096, PROT_READ) = 0
12:59:15 mprotect(0x7fa7daa66000, 4096, PROT_READ) = 0
12:59:15 mprotect(0x557f842f9000, 8192, PROT_READ) = 0
12:59:15 mprotect(0x7fa7daa9c000, 8192, PROT_READ) = 0
12:59:15 munmap(0x7fa7daa50000, 74118)  = 0
12:59:15 set_tid_address(0x7fa7da772ad0) = 1639769
12:59:15 set_robust_list(0x7fa7da772ae0, 24) = 0
12:59:15 rt_sigaction(SIGRTMIN, {sa_handler=0x7fa7da77eb70, sa_mask=[], sa_flags=SA_RESTORER|SA_SIGINFO, sa_restorer=0x7fa7da78b160}, NULL, 8) = 0
12:59:15 rt_sigaction(SIGRT_1, {sa_handler=0x7fa7da77ec10, sa_mask=[], sa_flags=SA_RESTORER|SA_RESTART|SA_SIGINFO, sa_restorer=0x7fa7da78b160}, NULL, 8) = 0
12:59:15 rt_sigprocmask(SIG_UNBLOCK, [RTMIN RT_1], NULL, 8) = 0
12:59:15 prlimit64(0, RLIMIT_STACK, NULL, {rlim_cur=8192*1024, rlim_max=RLIM64_INFINITY}) = 0
12:59:15 statfs("/sys/fs/selinux", 0x7fff39dc1a00) = -1 ENOENT (No such file or directory)
12:59:15 statfs("/selinux", 0x7fff39dc1a00) = -1 ENOENT (No such file or directory)
12:59:15 mmap(NULL, 512, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7fa7daa9b000
12:59:15 openat(AT_FDCWD, "/proc/filesystems", O_RDONLY|O_CLOEXEC) = 3
12:59:15 mmap(NULL, 160, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7fa7daa62000
12:59:15 newfstatat(3, "", {st_mode=S_IFREG|0444, st_size=0, ...}, AT_EMPTY_PATH) = 0
12:59:15 mmap(NULL, 1064, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7fa7daa61000
12:59:15 read(3, "nodev\tsysfs\nnodev\ttmpfs\nnodev\tbd"..., 1024) = 410
12:59:15 read(3, "", 1024)              = 0
12:59:15 munmap(0x7fa7daa62000, 120)    = 0
12:59:15 close(3)                       = 0
12:59:15 munmap(0x7fa7daa61000, 1024)   = 0
12:59:15 munmap(0x7fa7daa9b000, 472)    = 0
12:59:15 --- SIGSEGV {si_signo=SIGSEGV, si_code=SEGV_MAPERR, si_addr=0xfffffffffffffff7} ---
12:59:16 +++ killed by SIGSEGV (core dumped) +++
Segmentation fault (core dumped)
ammarfaizi2@integral:~$

使用/usr/bin/env 后的 Valgrind 输出

ammarfaizi2@integral:~$ valgrind --track-origins=yes --leak-check=full --show-leak-kinds=all /usr/bin/env LD_PRELOAD=$(pwd)/my_mem.so ls
==1640100== Memcheck, a memory error detector
==1640100== Copyright (C) 2002-2017, and GNU GPL'd, by Julian Seward et al.
==1640100== Using Valgrind-3.17.0 and LibVEX; rerun with -h for copyright info
==1640100== Command: /usr/bin/env LD_PRELOAD=/home/ammarfaizi2/my_mem.so ls
==1640100== 
Segmentation fault (core dumped)
ammarfaizi2@integral:~$

最佳答案

我已经调试了核心文件并修复了崩溃，对于free函数，您需要检查参数是否为nullptr，对于realloc我们需要处理__user_ptr 也是 nullptr。

void free(void *__user_ptr) {
  if (!__user_ptr) return;
  // ...
}

void *realloc(void *__user_ptr, size_t new_len) {
  void *new_mem;
  size_t len;
  uint8_t diff;
  uintptr_t user_ptr = (uintptr_t)__user_ptr;

  new_mem = malloc(new_len);
  if (!__user_ptr) return new_mem;
  // ....
}

我在编写内存分配器库方面有一些经验。在调试过程中，我发现一些旧的 c 程序将 realloc 用作 malloc 并带有 nullptr 参数，这很奇怪但完全有效，请引用手册页

The realloc() function changes the size of the memory block pointed toby ptr to size bytes. The contents will be unchanged in the range fromthe start of the region up to the minimum of the old and new sizes. Ifthe new size is larger than the old size, the added memory will not beinitialized. If ptr is NULL, then the call is equivalent tomalloc(size), for all values of size; if size is equal to zero, andptr is not NULL, then the call is equivalent to free(ptr). Unless ptris NULL, it must have been returned by an earlier call to malloc(),calloc() or realloc(). If the area pointed to was moved, a free(ptr)is done.

顺便说一句，我看到你试图包装 mmap 和 munmap 的 syscall，我建议我们用 https://github.com/linux-on-ibm-z/linux-syscall-support 替换它们，这是一个生产级包装器库，被广泛使用。我认为我们应该编写尽可能少的代码以减少潜在的错误。

关于c - 通过 `malloc` 替换 `calloc` 、 `realloc` 、 `free` 和 `LD_PRELOAD` 时出现段错误，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/66898984/

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