dlsym 如何成功从剥离的二进制库导入函数？
奇怪的是 dlsym 可以从剥离的二进制文件中导入函数。
谁能告诉我为什么/如何？
=== FILE: a.c ===
int a1() { return 1; }
int a2() { return 2; }
=== end of a.c ===
=== FILE: b.c ===
#include <stdio.h>
#include <dlfcn.h>
#include <stdlib.h>
typedef int (*fint)();
fint dlsym_fint(void *handle, char *name)
{
fint x = (fint)dlsym(handle, name);
char *err = NULL;
if ((err = dlerror()) != NULL) {
printf("dlsym: %s\n", err);
exit(1);
}
return x;
}
int main()
{
void *dl = dlopen("a.so", RTLD_NOW);
fint a = NULL;
a = dlsym_fint(dl, "a1");
printf("%p: %d\n", a, a());
a = dlsym_fint(dl, "a2");
printf("%p: %d\n", a, a());
return 0;
}
=== end of b.c ===
$ gcc -shared -fPIC -o a.so a.c
$ nm a.so
...
00000000000004ec T a1
00000000000004f7 T a2
...
$ strip a.so
$ nm a.so
nm: a.so: no symbols
$ gcc -o b b.c -ldl
$ ./b
0x2aaaaaac74ec: 1
0x2aaaaaac74f7: 2
It's weird that dlsym can import functions from stripped binaries.
Can anyone tell me why/how?
=== FILE: a.c ===
int a1() { return 1; }
int a2() { return 2; }
=== end of a.c ===
=== FILE: b.c ===
#include <stdio.h>
#include <dlfcn.h>
#include <stdlib.h>
typedef int (*fint)();
fint dlsym_fint(void *handle, char *name)
{
fint x = (fint)dlsym(handle, name);
char *err = NULL;
if ((err = dlerror()) != NULL) {
printf("dlsym: %s\n", err);
exit(1);
}
return x;
}
int main()
{
void *dl = dlopen("a.so", RTLD_NOW);
fint a = NULL;
a = dlsym_fint(dl, "a1");
printf("%p: %d\n", a, a());
a = dlsym_fint(dl, "a2");
printf("%p: %d\n", a, a());
return 0;
}
=== end of b.c ===
$ gcc -shared -fPIC -o a.so a.c
$ nm a.so
...
00000000000004ec T a1
00000000000004f7 T a2
...
$ strip a.so
$ nm a.so
nm: a.so: no symbols
$ gcc -o b b.c -ldl
$ ./b
0x2aaaaaac74ec: 1
0x2aaaaaac74f7: 2
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尝试 readelf -s a.so 。动态符号在
条带之后仍然存在。（或者直接切换到 nm -D a.so。）
Try
readelf -s a.so. The dynamic symbols are still there after thatstrip.(Or just switch to
nm -D a.so.)strip删除调试符号表，而不是动态链接器使用的动态符号表。要删除这些符号，请使用-fvisibility=hidden和 符号可见性函数/变量属性来选择要公开的函数。stripremoves debugging symbol tables, not the dynamic symbol tables used by the dynamic linker. To remove those symbols as well, use-fvisibility=hidden, and the symbol visibility function/variable attributes to select which functions you want to expose.