ggml : Fix missing backtrace on Linux (ggml/1228)

* Modern Linux defaults /proc/sys/kernel/yama/ptrace_scope to 1
* Fixed lldb attach
* Simplify by having the child do ggml_print_backtrace_symbols

ggml/src/ggml.c

@@ -70,6 +70,9 @@ float ggml_table_f32_f16[1 << 16];
 #include <sys/types.h>
 #include <sys/stat.h>
 #include <sys/wait.h>
+#if defined(__linux__)
+#include <sys/prctl.h>
+#endif
 
 #if defined(__ANDROID__)
 #include <unwind.h>
@@ -133,10 +136,36 @@ static void ggml_print_backtrace(void) {
     if (GGML_NO_BACKTRACE) {
         return;
     }
-    char attach[32];
-    snprintf(attach, sizeof(attach), "attach %d", getpid());
-    int pid = fork();
-    if (pid == 0) {
+#if defined(__linux__)
+    FILE * f = fopen("/proc/self/status", "r");
+    size_t size = 0;
+    char * line = NULL;
+    ssize_t length = 0;
+    while ((length = getline(&line, &size, f)) > 0) {
+        if (!strncmp(line, "TracerPid:", sizeof("TracerPid:") - 1) &&
+            (length != sizeof("TracerPid:\t0\n") - 1 || line[length - 2] != '0')) {
+            // Already being debugged, and the breakpoint is the later abort()
+            free(line);
+            fclose(f);
+            return;
+        }
+    }
+    free(line);
+    fclose(f);
+    int lock[2] = { -1, -1 };
+    (void) !pipe(lock); // Don't start gdb until after PR_SET_PTRACER
+#endif
+    const int parent_pid = getpid();
+    const int child_pid = fork();
+    if (child_pid < 0) { // error
+        return;
+    } else if (child_pid == 0) { // child
+        char attach[32];
+        snprintf(attach, sizeof(attach), "attach %d", parent_pid);
+#if defined(__linux__)
+        close(lock[1]);
+        (void) !read(lock[0], lock, 1);
+#endif
         // try gdb
         execlp("gdb", "gdb", "--batch",
             "-ex", "set style enabled on",
@@ -149,18 +178,18 @@ static void ggml_print_backtrace(void) {
         execlp("lldb", "lldb", "--batch",
             "-o", "bt",
             "-o", "quit",
-            "-p", attach,
+            "-p", &attach[sizeof("attach ") - 1],
             (char *) NULL);
-        exit(EXIT_FAILURE);
-    } else {
-        int wstatus;
-        waitpid(pid, &wstatus, 0);
-        if (WIFEXITED(wstatus)) {
-            if (WEXITSTATUS(wstatus) == EXIT_FAILURE) {
-                // gdb failed, fallback to backtrace_symbols
-                ggml_print_backtrace_symbols();
-            }
-        }
+        // gdb failed, fallback to backtrace_symbols
+        ggml_print_backtrace_symbols();
+        _Exit(0);
+    } else { // parent
+#if defined(__linux__)
+        prctl(PR_SET_PTRACER, child_pid);
+        close(lock[1]);
+        close(lock[0]);
+#endif
+        waitpid(child_pid, NULL, 0);
     }
 }
 #else