I have this small testcode atfork_demo.c:
#include <stdio.h>
#include <pthread.h>
void hello_from_fork_prepare() {
printf("Hello from atfork prepare.\n");
fflush(stdout);
}
void register_hello_from_fork_prepare() {
pthread_atfork(&hello_from_fork_prepare, 0, 0);
}
Now, I compile it in two different ways:
gcc -shared -fPIC atfork_demo.c -o atfork_demo1.so
gcc -shared -fPIC atfork_demo.c -o atfork_demo2.so -lpthread
My demo main atfork_demo_main.c is this:
#include <dlfcn.h>
#include <stdio.h>
#include <unistd.h>
int main(int argc, const char** argv) {
if(argc <= 1) {
printf("usage: ... lib.so\n");
return 1;
}
void* plib = dlopen("libpthread.so.0", RTLD_NOW|RTLD_GLOBAL);
if(!plib) {
printf("cannot load pthread, error %s\n", dlerror());
return 1;
}
void* lib = dlopen(argv[1], RTLD_LAZY);
if(!lib) {
printf("cannot load %s, error %s\n", argv[1], dlerror());
return 1;
}
void (*reg)();
reg = dlsym(lib, "register_hello_from_fork_prepare");
if(!reg) {
printf("did not found func, error %s\n", dlerror());
return 1;
}
reg();
fork();
}
Which I compile like this:
gcc atfork_demo_main.c -o atfork_demo_main.exec -ldl
Now, I have another small demo atfork_patch.c where I want to override pthread_atfork:
#include <stdio.h>
int pthread_atfork(void (*prepare)(void), void (*parent)(void), void (*child)(void)) {
printf("Ignoring pthread_atfork call!\n");
fflush(stdout);
return 0;
}
Which I compile like this:
gcc -shared -O2 -fPIC patch_atfork.c -o patch_atfork.so
And then I set LD_PRELOAD=./atfork_patch.so, and do these two calls:
./atfork_demo_main.exec ./atfork_demo1.so
./atfork_demo_main.exec ./atfork_demo2.so
In the first case, the LD_PRELOAD-override of pthread_atfork worked and in the second, it did not. I get the output:
Ignoring pthread_atfork call!
Hello from atfork prepare.
So, now to the question(s):
- Why did it not work in the second case?
- How can I make it work also in the second case, i.e. also override it?
In my real use case,
atfork_demois some library which I cannot change. I also cannot changeatfork_demo_mainbut I can make it load any other code. I would prefer if I can just do it with some change inatfork_patch.
You get some more debug output if you also use LD_DEBUG=all. Maybe interesting is this bit, for the second case:
841: symbol=__register_atfork; lookup in file=./atfork_demo_main.exec [0]
841: symbol=__register_atfork; lookup in file=./atfork_patch_extended.so [0]
841: symbol=__register_atfork; lookup in file=/lib/x86_64-linux-gnu/libdl.so.2 [0]
841: symbol=__register_atfork; lookup in file=/lib/x86_64-linux-gnu/libc.so.6 [0]
841: binding file ./atfork_demo2.so [0] to /lib/x86_64-linux-gnu/libc.so.6 [0]: normal symbol `__register_atfork' [GLIBC_2.3.2]
So, it searches for the symbol __register_atfork. I added that to atfork_patch_extended.so but it doesn't find it and uses it from libc instead. How can I make it find and use my __register_atfork?
As a side note, my main goal is to ignore the atfork handlers when fork() is called, but this is not the question here, but actually here. One solution to that, which seems to work, is to override fork() itself by this:
pid_t fork(void) {
return syscall(SYS_clone, SIGCHLD, 0);
}
