user: Introduce host_interrupt_signal

Attaching to the gdbstub of a running process requires stopping its
threads. For threads that run on a CPU, cpu_exit() is enough, but the
only way to grab attention of a thread that is stuck in a long-running
syscall is to interrupt it with a signal.

Reserve a host realtime signal for this, just like it's already done
for TARGET_SIGABRT on Linux. This may reduce the number of available
guest realtime signals by one, but this is acceptable, since there are
quite a lot of them, and it's unlikely that there are apps that need
them all.

Set signal_pending for the safe_sycall machinery to prevent invoking
the syscall. This is a lie, since we don't queue a guest signal, but
process_pending_signals() can handle the absence of pending signals.
The syscall returns with QEMU_ERESTARTSYS errno, which arranges for
the automatic restart. This is important, because it helps avoiding
disturbing poorly written guests.

Reviewed-by: Warner Losh <imp@bsdimp.com>
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Message-Id: <20250117001542.8290-5-iii@linux.ibm.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20250207153112.3939799-14-alex.bennee@linaro.org>

1 files changed, 12 insertions, 0 deletions

diff --git a/bsd-user/signal.c b/bsd-user/signal.c
index 8c51f6ce65..ff2ccbbf60 100644
--- a/bsd-user/signal.c
+++ b/bsd-user/signal.c
@@ -51,6 +51,8 @@ static inline int sas_ss_flags(TaskState *ts, unsigned long sp)
         on_sig_stack(ts, sp) ? SS_ONSTACK : 0;
 }
 
+int host_interrupt_signal = SIGRTMAX;
+
 /*
  * The BSD ABIs use the same signal numbers across all the CPU architectures, so
  * (unlike Linux) these functions are just the identity mapping. This might not
@@ -491,6 +493,12 @@ static void host_signal_handler(int host_sig, siginfo_t *info, void *puc)
     uintptr_t pc = 0;
     bool sync_sig = false;
 
+    if (host_sig == host_interrupt_signal) {
+        ts->signal_pending = 1;
+        cpu_exit(thread_cpu);
+        return;
+    }
+
     /*
      * Non-spoofed SIGSEGV and SIGBUS are synchronous, and need special
      * handling wrt signal blocking and unwinding.
@@ -854,6 +862,9 @@ void signal_init(void)
 
     for (i = 1; i <= TARGET_NSIG; i++) {
         host_sig = target_to_host_signal(i);
+        if (host_sig == host_interrupt_signal) {
+            continue;
+        }
         sigaction(host_sig, NULL, &oact);
         if (oact.sa_sigaction == (void *)SIG_IGN) {
             sigact_table[i - 1]._sa_handler = TARGET_SIG_IGN;
@@ -872,6 +883,7 @@ void signal_init(void)
             sigaction(host_sig, &act, NULL);
         }
     }
+    sigaction(host_interrupt_signal, &act, NULL);
 }
 
 static void handle_pending_signal(CPUArchState *env, int sig,