LLGS: fix tracking execve on linux

[labath]

Summary

Due to a logic error, lldb-server ended up asserting/crashing every time
the debugged process attempted an execve(). This fixes the error, and
extends TestExec to work on other platforms too. The "extension"
consists of avoiding non-standard posix_spawn extensions and using the
classic execve() call, which should be available on any platform that
actually supports re-execing. I change the test decorator from
@skipUnlessDarwin to @skipIfWindows.

[clayborg]

Is this not going to be racy on other platforms? Or do all platforms stop at entry point once a program has been exec'ed? I am worried we might miss the breakpoint.

[jasonmolenda]

LGTM. I don't think the use of the posix_spawn API are important for the test case to exercise. If posix_spawn had a problem on darwin systems, lldb/debugserver's process launching would turn it up pretty quick.

[labath]

In D65207#1600100, @clayborg wrote:

Is this not going to be racy on other platforms? Or do all platforms stop at entry point once a program has been exec'ed? I am worried we might miss the breakpoint.

stopping on exec is a pretty standard feature of all ptrace-ish implementations, so I think all platforms should be able to implement that. Whether they actually implement it is a different question, and I wouldn't be surprised if this test ends up failing on some BSDs for example.

lldb/trunk/packages/Python/lldbsuite/test/functionalities/exec/TestExec.py

 
 
 class ExecTestCase(TestBase):
 
     NO_DEBUG_INFO_TESTCASE = True
 
     mydir = TestBase.compute_mydir(__file__)
 
-    @skipUnlessDarwin
     @expectedFailureAll(archs=['i386'], bugnumber="rdar://28656532")
     @expectedFailureAll(oslist=["ios", "tvos", "watchos", "bridgeos"], bugnumber="rdar://problem/34559552") # this exec test has problems on ios systems
     @skipIfSanitized # rdar://problem/43756823
+    @skipIfWindows
     def test_hitting_exec (self):
         self.do_test(False)
 
-    @skipUnlessDarwin
     @expectedFailureAll(archs=['i386'], bugnumber="rdar://28656532")
     @expectedFailureAll(oslist=["ios", "tvos", "watchos", "bridgeos"], bugnumber="rdar://problem/34559552") # this exec test has problems on ios systems
     @skipIfSanitized # rdar://problem/43756823
+    @skipIfWindows
     def test_skipping_exec (self):
         self.do_test(True)
 
     def do_test(self, skip_exec):
         self.build()
         exe = self.getBuildArtifact("a.out")
         secondprog = self.getBuildArtifact("secondprog")
 

lldb/trunk/packages/Python/lldbsuite/test/functionalities/exec/main.cpp

-#include <errno.h>
-#include <mach/mach.h>
-#include <signal.h>
-#include <stdio.h>
-#include <stdint.h>
-#include <stdlib.h>
-#include <spawn.h>
-#include <unistd.h>
+#include <cstdio>
+#include <cstdlib>
+#include <cstring>
 #include <libgen.h>
 #include <string>
+#include <unistd.h>
 
-static void
-exit_with_errno (int err, const char *prefix)
-{
-    if (err)
-    {
-        fprintf (stderr,
-                 "%s%s",
-                 prefix ? prefix : "",
-                 strerror(err));
-        exit (err);
-    }
-}
-
-static pid_t
-spawn_process (const char *progname,
-               const char **argv,
-               const char **envp,
-               int &err)
-{
-    pid_t pid = 0;
-
-    const posix_spawn_file_actions_t *file_actions = NULL;
-    posix_spawnattr_t attr;
-    err = posix_spawnattr_init (&attr);
-    if (err)
-        return pid;
-
-    short flags = POSIX_SPAWN_SETEXEC | POSIX_SPAWN_SETSIGDEF | POSIX_SPAWN_SETSIGMASK;
-    err = posix_spawnattr_setflags (&attr, flags);
-    if (err == 0)
-    {
-        // Use the default signal masks
-        sigset_t no_signals;
-        sigset_t all_signals;
-        sigemptyset (&no_signals);
-        sigfillset (&all_signals);
-        posix_spawnattr_setsigmask(&attr, &no_signals);
-        posix_spawnattr_setsigdefault(&attr, &all_signals);
-
-        err = posix_spawn (&pid,
-                           progname,
-                           file_actions,
-                           &attr,
-                           (char * const *)argv,
-                           (char * const *)envp);
-        
-        posix_spawnattr_destroy(&attr);
-    }
-    return pid;
-}
-
-int 
-main (int argc, char const **argv)
-{
-    char *buf = (char*) malloc (strlen (argv[0]) + 12);
-    strlcpy (buf, argv[0], strlen (argv[0]) + 1);
-    std::string directory_name (::dirname (buf));
+int main(int argc, char const **argv) {
+  char *buf = strdup(argv[0]); // Set breakpoint 1 here
+  std::string directory_name(::dirname(buf));
 
-    std::string other_program = directory_name + "/secondprog";
-    int err = 0;    // Set breakpoint 1 here
-    spawn_process (other_program.c_str(), argv, NULL, err);
-    if (err)
-        exit_with_errno (err, "posix_spawn x86_64 error");
-    return 0;
+  execve(other_program.c_str(), const_cast<char *const *>(argv), nullptr);
+  perror("execve");
+  abort();
 }

lldb/trunk/source/Plugins/Process/Linux/NativeProcessLinux.cpp

case (SIGTRAP | (PTRACE_EVENT_EXEC << 8)): {
 
     // Exec clears any pending notifications.
     m_pending_notification_tid = LLDB_INVALID_THREAD_ID;
 
     // Remove all but the main thread here.  Linux fork creates a new process
     // which only copies the main thread.
     LLDB_LOG(log, "exec received, stop tracking all but main thread");
 
-    for (auto i = m_threads.begin(); i != m_threads.end();) {
-      if ((*i)->GetID() == GetID())
-        i = m_threads.erase(i);
-      else
-        ++i;
-    }
+    llvm::erase_if(m_threads, [&](std::unique_ptr<NativeThreadProtocol> &t) {
+      return t->GetID() != GetID();
+    });
     assert(m_threads.size() == 1);
     auto *main_thread = static_cast<NativeThreadLinux *>(m_threads[0].get());
 
     SetCurrentThreadID(main_thread->GetID());
     main_thread->SetStoppedByExec();
 
     // Tell coordinator about about the "new" (since exec) stopped main thread.
     ThreadWasCreated(*main_thread);