Wrong stack pointer in signal handler

Consider this code:

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

static void handler (int signum)
{
  printf ("handler sp = %p\n", __builtin_frame_address (0));
}

int main ()
{
  printf ("sp = %p\n", __builtin_frame_address (0));
  signal (SIGUSR1, handler);
  kill (getpid (), SIGUSR1);
  return 0;
}

When running on bare x86_64 GNU/Linux, I see:

sp = 0x7fff5c116230
handler sp = 0x7fff5c115b70

However, under rr, I get:

$ rr record /tmp/a.out
rr: Saving execution to trace directory `/home/ludo/.local/share/rr/a.out-4'.
sp = 0x7ffd59dc9c60
handler sp = 0x681ff770

The "weird" %sp is problematic when debugging applications using the BDW garbage collector (libgc). For stop-the-world garbage collections, libgc sends a signal to the process and stores the receiving thread's stack pointer and later, from another thread, scans that stack starting from that thread's recorded stack pointer in GC_thread.stop_info.stack_ptr.

What would you suggest to work around this issue?

Thanks,
Ludo'.

Reproduce the discrepancy with the supplied C program under bare Linux and rr record, comparing the signal-handler frame address with the normal process value. Read the linked BDWGC pthread_stop_world.c locations around lines 333 and 697 to understand the required stack-pointer behavior. Done means a documented, reliable workaround for libgc or a confirmed rr fix path.