[signals] use an alternate signal stack #2424

src/lib/direct.c

@@ -9,6 +9,7 @@
 #include "visual-details.h"
 #include "notcurses/direct.h"
 #include "internal.h"
+#include "unixsig.h"
 
 // conform to the foreground and background channels of 'channels'
 static int

src/lib/in.c

@@ -3,6 +3,7 @@
 #include <unistd.h>
 #include "automaton.h"
 #include "internal.h"
+#include "unixsig.h"
 #include "render.h"
 #include "in.h"
 
@@ -2161,6 +2162,7 @@ read_inputs_nblock(inputctx* ictx){
 
 static void*
 input_thread(void* vmarshall){
+  setup_alt_sig_stack();
   inputctx* ictx = vmarshall;
   if(prep_all_keys(ictx) || build_cflow_automaton(ictx)){
     ictx->failed = true;

src/lib/internal.h

@@ -1510,14 +1510,6 @@ int get_tty_fd(FILE* ttyfp);
 // If you only want to check n < 4 channels, just duplicate one.
 bool check_gradient_args(uint64_t ul, uint64_t ur, uint64_t bl, uint64_t br);
 
-int setup_signals(void* nc, bool no_quit_sigs, bool no_winch_sig,
-                  int(*handler)(void*));
-int drop_signals(void* nc);
-
-// block a few signals for the duration of a write to the terminal.
-int block_signals(sigset_t* old_blocked_signals);
-int unblock_signals(const sigset_t* old_blocked_signals);
-
 // takes a signed starting coordinate (where -1 indicates the cursor's
 // position), and an unsigned vector (where 0 indicates "everything
 // remaining", i.e. to the right and below). returns 0 iff everything

src/lib/notcurses.c

@@ -16,6 +16,7 @@
 #include <inttypes.h>
 #include <notcurses/direct.h>
 #include "compat/compat.h"
+#include "unixsig.h"
 #include "banner.h"
 
 #define ESC "\x1b"

src/lib/render.c

@@ -3,6 +3,7 @@
 #include <unistd.h>
 #include <notcurses/direct.h>
 #include "internal.h"
+#include "unixsig.h"
 
 sig_atomic_t sigcont_seen_for_render = 0;
 

src/lib/unixsig.c

@@ -50,6 +50,9 @@ static pthread_mutex_t lock = PTHREAD_MUTEX_INITIALIZER;
 static bool handling_winch;
 static bool handling_fatals;
 
+// alternate signal stack (per-thread; call setup_alt_sig_stack() to use)
+static stack_t alt_signal_stack;
+
 // saved signal actions, restored in drop_signals() FIXME make an array
 static struct sigaction old_winch;
 static struct sigaction old_cont;
@@ -104,6 +107,15 @@ int drop_signals(void* nc){
       sigaction(SIGTERM, &old_term, NULL);
       handling_fatals = false;
     }
+    if(alt_signal_stack.ss_sp){
+      alt_signal_stack.ss_flags = SS_DISABLE;
+      if(sigaltstack(&alt_signal_stack, NULL)){
+        if(errno != EPERM){
+          fprintf(stderr, "couldn't remove alternate signal stack (%s)\n", strerror(errno));
+        }
+      }
+      free(alt_signal_stack.ss_sp);
+    }
     ret = !atomic_compare_exchange_strong(&signal_nc, &expected, NULL);
   }
   pthread_mutex_unlock(&lock);
@@ -145,6 +157,16 @@ fatal_handler(int signo, siginfo_t* siginfo, void* v){
   }
 }
 
+// the alternate signal stack is a thread property; any other threads we
+// create ought go ahead and install the same alternate signal stack.
+void setup_alt_sig_stack(void){
+  pthread_mutex_lock(&lock);
+  if(alt_signal_stack.ss_sp){
+    sigaltstack(&alt_signal_stack, NULL);
+  }
+  pthread_mutex_unlock(&lock);
+}
+
 // this both sets up our signal handlers (unless that behavior has been
 // inhibited), and ensures that only one notcurses/ncdirect context is active
 // at any given time.
@@ -171,7 +193,7 @@ int setup_signals(void* vnc, bool no_quit_sigs, bool no_winch_sigs,
     if(ret){
       atomic_store(&signal_nc, NULL);
       pthread_mutex_unlock(&lock);
-      fprintf(stderr, "Error installing term signal handler (%s)\n", strerror(errno));
+      fprintf(stderr, "error installing term signal handler (%s)\n", strerror(errno));
       return -1;
     }
     // we're not going to be restoring the old mask at exit, as who knows,
@@ -180,6 +202,17 @@ int setup_signals(void* vnc, bool no_quit_sigs, bool no_winch_sigs,
     handling_winch = true;
   }
   if(!no_quit_sigs){
+    alt_signal_stack.ss_size = SIGSTKSZ * 4;
+    alt_signal_stack.ss_sp = malloc(alt_signal_stack.ss_size);
+    if(alt_signal_stack.ss_sp == NULL){
+      fprintf(stderr, "warning: couldn't create alternate signal stack (%s)\n", strerror(errno));
+    }else{
+      if(sigaltstack(&alt_signal_stack, NULL)){
+        fprintf(stderr, "warning: couldn't set up alternate signal stack (%s)\n", strerror(errno));
+        free(alt_signal_stack.ss_sp);
+        alt_signal_stack.ss_sp = NULL;
+      }
+    }
     memset(&sa, 0, sizeof(sa));
     fatal_callback = handler;
     sa.sa_sigaction = fatal_handler;
@@ -191,7 +224,8 @@ int setup_signals(void* vnc, bool no_quit_sigs, bool no_winch_sigs,
     sigaddset(&sa.sa_mask, SIGQUIT);
     sigaddset(&sa.sa_mask, SIGSEGV);
     sigaddset(&sa.sa_mask, SIGTERM);
-    sa.sa_flags = SA_SIGINFO | SA_RESETHAND; // don't try fatal signals twice
+    // don't try to handle fatal signals twice, and use our alternative stack
+    sa.sa_flags = SA_ONSTACK | SA_SIGINFO | SA_RESETHAND;
     int ret = 0;
     ret |= sigaction(SIGABRT, &sa, &old_abrt);
     ret |= sigaction(SIGBUS, &sa, &old_bus);
@@ -210,7 +244,7 @@ int setup_signals(void* vnc, bool no_quit_sigs, bool no_winch_sigs,
     handling_fatals = true;
   }
   // we don't really want to go blocking SIGSEGV etc while we write, but
-  // we *do* temporarily block user-inititated signals.
+  // we *do* temporarily block user-initiated signals.
   sigaddset(&wblock_signals, SIGINT);
   sigaddset(&wblock_signals, SIGTERM);
   sigaddset(&wblock_signals, SIGQUIT);

src/lib/unixsig.h

@@ -0,0 +1,26 @@
+#ifndef NOTCURSES_UNIXSIG
+#define NOTCURSES_UNIXSIG
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+#include <signal.h>
+
+int setup_signals(void* nc, bool no_quit_sigs, bool no_winch_sig,
+                  int(*handler)(void*));
+int drop_signals(void* nc);
+
+// block a few signals for the duration of a write to the terminal.
+int block_signals(sigset_t* old_blocked_signals);
+int unblock_signals(const sigset_t* old_blocked_signals);
+
+// the alternate signal stack is a thread property; any other threads we
+// create ought go ahead and install the same alternate signal stack.
+void setup_alt_sig_stack(void);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif