CODEWITHSUNDEEP
linuxld-preload
GrassInWind
GrassInWind

Reputation: 1

LD_PRELOAD didn't load my library and no errors or warnings

I want to use LD_PRELOAD to hook my library before target running in Oracle Linux 7.9. LD_PRELOAD load my library failed in first scenario, but success in second scenario and third scenario. I have no idea to make LD_PRELOAD work in first scenario.

1
[test@k8s-node2 hook]$ ls -l /usr/lib64/libtest.so 
-rwsr-sr-x 1 test test2 501328 Jan 31 07:04 /usr/lib64/libtest.so
`[test@k8s-node2 hook]$ ls -l child`
-rwsr-sr-x 1 test test2 12848 Jan 31 16:08 child
[test@k8s-node2 hook]$ LD_PRELOAD=/usr/lib64/libtest.so ./child &
[test@k8s-node2 hook]$ pmap -X `pidof child` | grep libtest
[test@k8s-node2 hook]$

2
[test@k8s-node2 hook]$ ls -l /usr/lib64/libtest.so 
-rwsr-sr-x 1 test test 501328 Jan 31 07:04 /usr/lib64/libtest.so
`[test@k8s-node2 hook]$ ls -l child`
-rwsr-sr-x 1 test test 12848 Jan 31 16:08 child
[test@k8s-node2 hook]$ LD_PRELOAD=/usr/lib64/libtest.so ./child &
[test@k8s-node2 hook]$  pmap -X `pidof child` | grep libtest
7fded2929000 r-xp 00000000  fd:02 40593442     4    4   2          4         0    0      0 libtest.so
7fded292a000 ---p 00001000  fd:02 40593442  2044    0   0          0         0    0      0 libtest.so
7fded2b29000 r--p 00000000  fd:02 40593442     4    4   4          4         4    0      0 libtest.so
7fded2b2a000 rw-p 00001000  fd:02 40593442     4    4   4          4         4    0      0 libtest.so

3
bash-4.2# chmod u-s,g-s child; chmod u-s,g-s /usr/lib64/libtest.so 
bash-4.2# ls -l child 
-rwxr-xr-x 1 test test2 8544 Feb  1 05:51 child
bash-4.2# ls -l /usr/lib64/libtest.so 
-rwxr-xr-x 1 test test2 8048 Feb  1 05:55 /usr/lib64/libtest.so
bash-4.2# su - test
[test@k8s-node2 hook]$ LD_PRELOAD=/usr/lib64/libtest.so ./child &
[test@k8s-node2 hook]$ pmap -X `pidof child` | grep libtest
7f6df60c0000 r-xp 00000000  fd:02 40593442     4    4   2          4         0    0      0 libtest.so
7f6df60c1000 ---p 00001000  fd:02 40593442  2044    0   0          0         0    0      0 libtest.so
7f6df62c0000 r--p 00000000  fd:02 40593442     4    4   4          4         4    0      0 libtest.so
7f6df62c1000 rw-p 00001000  fd:02 40593442     4    4   4          4         4    0      0 libtest.so

child.cpp

#include <stdio.h>
#include <unistd.h>

int main(int argc, char* argv[], char* envp[]) {
    for(int i=0;i<argc;i++) {
        printf("child argv%d:%s\n",i, argv[i]);
    }
    int i = 0;
    while(envp[i] != NULL) {
        printf("child envp%d: %s\n", i, envp[i]);
        i++;
    }

    while(1) {
        sleep(1);
    }

    return 0;
} 

g++ child.cpp -o child

test.cpp

#include <stdio.h>

void myhello(void) {
    printf("hello world!");
}

g++ -fPIC -shared test.cpp -o libtest.so -ldl

I try success in 2 and 3, but failed in 1.

Upvotes: 0

Views: 66

Answers (1)

Employed Russian
Employed Russian

Reputation: 213879

I have no idea to make LD_PRELOAD work in first scenario

You can't: LD_PRELOAD is ignored for setuid and setgid programs for obvious reasons.

See "secure execution mode" in man ld.so.

Upvotes: 0

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