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Question

For some research related project of mine, I'm trying to setup a second TLS after a new thread is created.

I think I was able to find the code responsible of setting up the TLS in the pthread library. For instance one function that interests me is: _dl_allocate_tls () from here .

So ideally I would like to write something along these lines:

#include <elf/dl-tls.c>
#include <sys/syscall.h>

int main(int argc, char** argv) {
  void* my_new_tls = _dl_allocate_tls();
  syscall(SYS_arch_prctl, ARCH_SET_FS, my_new_tls);
}

Installing libc6-dev on Ubuntu won't give me access to the above files/headers/code. Any ideas on how could I call the above functions?

Also, any advice on how a custom TLS can be installed in less hacky, more proper way, is more than welcome!

Answer 1

If you want to use a second TLS space on x86-64, you need to use TLS with a %gs segment base, not %fs like the rest of the system. If you do that, you can manage the thread-local memory just like any other memory, and you do not need to coordinate with glibc. It's what Wine does internally.

Answer 2

I actually found that I could piggyback (or piggy-bug) on libc's loader to do the job for me. Here is what I did for future reference:

// This is an amazing hack to directly call libc's loader in: glibc/elf/dl-tls.c
void *_dl_allocate_tls(void *mem);

// Include related structs from glibc/sysdeps/generic/dl-dtv.h and 
// glibc/sysdeps/x86_64/nptl/tls.h

void *new_tls() {
  tcbhead_t *n_tls = (tcbhead_t *)_dl_allocate_tls(NULL);

  // For the following see: glibc/nptl/pthread_create.c

  /* Reference to the TCB itself.  */
  // pd->header.self = pd;
  n_tls->self = n_tls;

  /* Self-reference for TLS.  */
  // pd->header.tcb = pd;
  n_tls->tcb = n_tls;

  /* Copy the stack guard canary.  */
  // THREAD_COPY_STACK_GUARD (pd);
  n_tls->stack_guard = current_tls->stack_guard;

  /* Copy the pointer guard value.  */
  // THREAD_COPY_POINTER_GUARD (pd);
  n_tls->pointer_guard = current_tls->pointer_guard;

  /* Setup tcbhead.  */
  // tls_setup_tcbhead (pd);
  n_tls->feature_1 = current_tls->feature_1;

  // End of: glibc/nptl/pthread_create.c

  return n_tls;
}