[rfc] PR 20569, segv in follow_exec

[Sandra Loosemore <sandra at codesourcery dot com>]

As I noted in PR20569, several exec-related tests cause GDB to segv when 
sysroot translation fails on the executable pathname reported by 
gdbserver.  The immediate cause of the segv is that follow_exec is 
passing a NULL argument (the result of exec_file_find) to strlen, but as 
I looked at the code in more detail it seemed like follow_exec simply 
isn't prepared for the case where sysroot translation fails to locate 
the new executable, and there is no obvious recovery strategy.

I thought I could copy logic from the other caller of exec_file_find, 
exec_file_locate_attach, but I think it's doing the wrong thing there as 
well.  Plus, from reading the code I found other bugs in both callers of 
exec_file_find due to confusion between host and target pathnames.

The attached patch attempts to fix all the bugs.  In terms of the 
testcases that were formerly segv'ing, GDB now prints a warning but 
continues execution of the new program, so that the tests now mostly 
FAIL instead.  You could argue the FAILs are due to a legitimate problem 
with the test environment setting up the sysroot translation 
incorrectly, but I'm not sure continuing execution rather than leaving 
the target stopped is the most user-friendly fallback behavior, either. 
 E.g. the GDB manual suggests that you can set a breakpoint on main and 
GDB will stop on main of the newly exec'ed program, too, but it can't do 
that if it can't find the symbol information, and there's no way for the 
user to specify the executable file to GDB explicitly before it resumes 
execution.  But, seemed better not to complicate this 
already-too-complicated patch further by trying to address that in the 
first cut.

Comments?  Suggestions?  Etc.

-Sandra

2016-10-06  Sandra Loosemore  <sandra@codesourcery.com>

	PR gdb/20569
	gdb/
	* exceptions.c (exception_print_same): Moved here from exec.c.
	Fixed message comparison.
	* exceptions.h (exception_print_same): Declare.
	* exec_file_locate_attach (exception_print_same): Delete copy here.
	(exec_file_locate_attach): Rename exec_file and full_exec_path
	variables to avoid confusion between target and host pathnames.  
	Move pathname processing logic to exec_file_find.  Do not return 
	early if pathname lookup fails;	guard symbol_file_add_main call 
	instead.
	* infrun.c (follow_exec): Split and rename execd_pathname variable
	to avoid confusion between target and host pathnames.  Replace 
	brokenpathname copy with cleanup to free malloc'ed string.  Warn 
	if pathname lookup fails.  Pass target pathname to 
	target_follow_exec, not hostpathname.  Borrow exception-handling 
	logic from exec_file_locate_attach.
	* solib.c (exec_file_find): Incorporate fallback logic for relative
	pathnames formerly in exec_file_locate_attach.

diff --git a/gdb/exceptions.c b/gdb/exceptions.c
index 9a10f66..402a5af 100644
--- a/gdb/exceptions.c
+++ b/gdb/exceptions.c
@@ -256,3 +256,16 @@ catch_errors (catch_errors_ftype *func, void *func_args, char *errstring,
     return 0;
   return val;
 }
+
+/* Returns non-zero if exceptions E1 and E2 are equal.  Returns zero
+   otherwise.  */
+
+int
+exception_print_same (struct gdb_exception e1, struct gdb_exception e2)
+{
+  return (e1.reason == e2.reason
+	  && e1.error == e2.error
+	  && (e1.message == e2.message
+	      || (e1.message && e2.message
+		  && strcmp (e1.message, e2.message) == 0)));
+}
diff --git a/gdb/exceptions.h b/gdb/exceptions.h
index 5711c1a..a2d39d7 100644
--- a/gdb/exceptions.h
+++ b/gdb/exceptions.h
@@ -88,4 +88,7 @@ extern int catch_exceptions_with_msg (struct ui_out *uiout,
 typedef int (catch_errors_ftype) (void *);
 extern int catch_errors (catch_errors_ftype *, void *, char *, return_mask);
 
+/* Compare two exception objects for print equality.  */
+extern int exception_print_same (struct gdb_exception e1,
+				 struct gdb_exception e2);
 #endif
diff --git a/gdb/exec.c b/gdb/exec.c
index d858e99..b418ef3 100644
--- a/gdb/exec.c
+++ b/gdb/exec.c
@@ -139,39 +139,23 @@ exec_file_clear (int from_tty)
 /* Returns non-zero if exceptions E1 and E2 are equal.  Returns zero
    otherwise.  */
 
-static int
-exception_print_same (struct gdb_exception e1, struct gdb_exception e2)
-{
-  const char *msg1 = e1.message;
-  const char *msg2 = e2.message;
-
-  if (msg1 == NULL)
-    msg1 = "";
-  if (msg2 == NULL)
-    msg2 = "";
-
-  return (e1.reason == e2.reason
-	  && e1.error == e2.error
-	  && strcmp (e1.message, e2.message) == 0);
-}
-
 /* See gdbcore.h.  */
 
 void
 exec_file_locate_attach (int pid, int defer_bp_reset, int from_tty)
 {
-  char *exec_file, *full_exec_path = NULL;
+  char *exec_file_target, *exec_file_host = NULL;
   struct cleanup *old_chain;
   struct gdb_exception prev_err = exception_none;
 
   /* Do nothing if we already have an executable filename.  */
-  exec_file = (char *) get_exec_file (0);
-  if (exec_file != NULL)
+  exec_file_target = (char *) get_exec_file (0);
+  if (exec_file_target != NULL)
     return;
 
   /* Try to determine a filename from the process itself.  */
-  exec_file = target_pid_to_exec_file (pid);
-  if (exec_file == NULL)
+  exec_file_target = target_pid_to_exec_file (pid);
+  if (exec_file_target == NULL)
     {
       warning (_("No executable has been specified and target does not "
 		 "support\n"
@@ -180,28 +164,8 @@ exec_file_locate_attach (int pid, int defer_bp_reset, int from_tty)
       return;
     }
 
-  /* If gdb_sysroot is not empty and the discovered filename
-     is absolute then prefix the filename with gdb_sysroot.  */
-  if (*gdb_sysroot != '\0' && IS_ABSOLUTE_PATH (exec_file))
-    {
-      full_exec_path = exec_file_find (exec_file, NULL);
-      if (full_exec_path == NULL)
-	return;
-    }
-  else
-    {
-      /* It's possible we don't have a full path, but rather just a
-	 filename.  Some targets, such as HP-UX, don't provide the
-	 full path, sigh.
-
-	 Attempt to qualify the filename against the source path.
-	 (If that fails, we'll just fall back on the original
-	 filename.  Not much more we can do...)  */
-      if (!source_full_path_of (exec_file, &full_exec_path))
-	full_exec_path = xstrdup (exec_file);
-    }
-
-  old_chain = make_cleanup (xfree, full_exec_path);
+  exec_file_host = exec_file_find (exec_file_target, NULL);
+  old_chain = make_cleanup (xfree, exec_file_target);
   make_cleanup (free_current_contents, &prev_err.message);
 
   /* exec_file_attach and symbol_file_add_main may throw an error if the file
@@ -217,7 +181,9 @@ exec_file_locate_attach (int pid, int defer_bp_reset, int from_tty)
      errors/exceptions in the following code.  */
   TRY
     {
-      exec_file_attach (full_exec_path, from_tty);
+      /* We must do this step even if exec_file_host is NULL, so that
+	 exec_file_attach will clear state.  */
+      exec_file_attach (exec_file_host, from_tty);
     }
   CATCH (err, RETURN_MASK_ERROR)
     {
@@ -231,19 +197,22 @@ exec_file_locate_attach (int pid, int defer_bp_reset, int from_tty)
     }
   END_CATCH
 
-  TRY
-    {
-      if (defer_bp_reset)
-	current_inferior ()->symfile_flags |= SYMFILE_DEFER_BP_RESET;
-      symbol_file_add_main (full_exec_path, from_tty);
-    }
-  CATCH (err, RETURN_MASK_ERROR)
+  if (exec_file_host)
     {
-      if (!exception_print_same (prev_err, err))
-	warning ("%s", err.message);
-    }
-  END_CATCH
-  current_inferior ()->symfile_flags &= ~SYMFILE_DEFER_BP_RESET;
+      TRY
+	{
+	  if (defer_bp_reset)
+	    current_inferior ()->symfile_flags |= SYMFILE_DEFER_BP_RESET;
+	  symbol_file_add_main (exec_file_host, from_tty);
+	}
+      CATCH (err, RETURN_MASK_ERROR)
+	{
+	  if (!exception_print_same (prev_err, err))
+	    warning ("%s", err.message);
+	}
+      END_CATCH
+    current_inferior ()->symfile_flags &= ~SYMFILE_DEFER_BP_RESET;
+  }
 
   do_cleanups (old_chain);
 }
diff --git a/gdb/infrun.c b/gdb/infrun.c
index 2636a19..a365a6b 100644
--- a/gdb/infrun.c
+++ b/gdb/infrun.c
@@ -1077,15 +1077,18 @@ show_follow_exec_mode_string (struct ui_file *file, int from_tty,
   fprintf_filtered (file, _("Follow exec mode is \"%s\".\n"),  value);
 }
 
-/* EXECD_PATHNAME is assumed to be non-NULL.  */
+/* EXEC_FILE_TARGET is assumed to be non-NULL.  */
 
 static void
-follow_exec (ptid_t ptid, char *execd_pathname)
+follow_exec (ptid_t ptid, char *exec_file_target)
 {
   struct thread_info *th, *tmp;
   struct inferior *inf = current_inferior ();
   int pid = ptid_get_pid (ptid);
   ptid_t process_ptid;
+  char *exec_file_host = NULL;
+  struct cleanup *old_chain;
+  struct gdb_exception prev_err = exception_none;
 
   /* This is an exec event that we actually wish to pay attention to.
      Refresh our symbol table to the newly exec'd program, remove any
@@ -1155,7 +1158,7 @@ follow_exec (ptid_t ptid, char *execd_pathname)
   process_ptid = pid_to_ptid (pid);
   printf_unfiltered (_("%s is executing new program: %s\n"),
 		     target_pid_to_str (process_ptid),
-		     execd_pathname);
+		     exec_file_target);
 
   /* We've followed the inferior through an exec.  Therefore, the
      inferior has essentially been killed & reborn.  */
@@ -1164,14 +1167,18 @@ follow_exec (ptid_t ptid, char *execd_pathname)
 
   breakpoint_init_inferior (inf_execd);
 
-  if (*gdb_sysroot != '\0')
-    {
-      char *name = exec_file_find (execd_pathname, NULL);
+  exec_file_host = exec_file_find (exec_file_target, NULL);
+  old_chain = make_cleanup (xfree, exec_file_host);
+  make_cleanup (free_current_contents, &prev_err.message);
 
-      execd_pathname = (char *) alloca (strlen (name) + 1);
-      strcpy (execd_pathname, name);
-      xfree (name);
-    }
+  /* If we were unable to map the executable target pathname onto a host
+     pathname, tell the user that.  Otherwise GDB's subsequent behavior
+     is confusing.  Maybe it would even be better to stop at this point
+     so that the user can specify a file manually before continuing.  */
+  if (!exec_file_host)
+    warning (_("Could not load symbols for executable %s.\n"
+	       "Do you need \"set sysroot\"?"),
+	     exec_file_target);
 
   /* Reset the shared library package.  This ensures that we get a
      shlib event when the child reaches "_start", at which point the
@@ -1193,7 +1200,7 @@ follow_exec (ptid_t ptid, char *execd_pathname)
 
       inf = add_inferior_with_spaces ();
       inf->pid = pid;
-      target_follow_exec (inf, execd_pathname);
+      target_follow_exec (inf, exec_file_target);
 
       set_current_inferior (inf);
       set_current_program_space (inf->pspace);
@@ -1212,22 +1219,62 @@ follow_exec (ptid_t ptid, char *execd_pathname)
 
   gdb_assert (current_program_space == inf->pspace);
 
-  /* That a.out is now the one to use.  */
-  exec_file_attach (execd_pathname, 0);
+  /* exec_file_attach and symbol_file_add_main may throw an error if the file
+     cannot be opened either locally or remotely.
+
+     This happens for example, when the file is first found in the local
+     sysroot (above), and then disappears (a TOCTOU race), or when it doesn't
+     exist in the target filesystem, or when the file does exist, but
+     is not readable.
+
+     Even without a symbol file, the remote-based debugging session should
+     continue normally instead of ending abruptly.  Hence we catch thrown
+     errors/exceptions in the following code.  */
+  TRY
+    {
+      /* We must do this step even if exec_file_host is NULL, so that
+	 exec_file_attach will clear state.  */
+      exec_file_attach (exec_file_host, 0);
+    }
+  CATCH (err, RETURN_MASK_ERROR)
+    {
+      if (err.message != NULL)
+	warning ("%s", err.message);
+
+      prev_err = err;
+
+      /* Save message so it doesn't get trashed by the catch below.  */
+      prev_err.message = xstrdup (err.message);
+    }
+  END_CATCH
+
+  if (exec_file_host)
+    {
+      TRY
+	{
+	  symbol_file_add (exec_file_host,
+			   (inf->symfile_flags
+			    | SYMFILE_MAINLINE | SYMFILE_DEFER_BP_RESET),
+			   NULL, 0);
+	  
+	  if ((inf->symfile_flags & SYMFILE_NO_READ) == 0)
+	    set_initial_language ();
+	}
+      CATCH (err, RETURN_MASK_ERROR)
+	{
+	  if (!exception_print_same (prev_err, err))
+	    warning ("%s", err.message);
+	}
+      END_CATCH
+    }
+
+  do_cleanups (old_chain);
 
   /* SYMFILE_DEFER_BP_RESET is used as the proper displacement for PIE
      (Position Independent Executable) main symbol file will get applied by
      solib_create_inferior_hook below.  breakpoint_re_set would fail to insert
      the breakpoints with the zero displacement.  */
 
-  symbol_file_add (execd_pathname,
-		   (inf->symfile_flags
-		    | SYMFILE_MAINLINE | SYMFILE_DEFER_BP_RESET),
-		   NULL, 0);
-
-  if ((inf->symfile_flags & SYMFILE_NO_READ) == 0)
-    set_initial_language ();
-
   /* If the target can specify a description, read it.  Must do this
      after flipping to the new executable (because the target supplied
      description must be compatible with the executable's
diff --git a/gdb/solib.c b/gdb/solib.c
index 2235505..cfcbd5d 100644
--- a/gdb/solib.c
+++ b/gdb/solib.c
@@ -388,13 +388,17 @@ solib_find_1 (char *in_pathname, int *fd, int is_solib)
 char *
 exec_file_find (char *in_pathname, int *fd)
 {
-  char *result = solib_find_1 (in_pathname, fd, 0);
+  char *result;
+  const char *fskind = effective_target_file_system_kind ();
+
+  if (!in_pathname)
+    return NULL;
 
-  if (result == NULL)
+  if (*gdb_sysroot != '\0' && IS_TARGET_ABSOLUTE_PATH (fskind, in_pathname))
     {
-      const char *fskind = effective_target_file_system_kind ();
+      result = solib_find_1 (in_pathname, fd, 0);
 
-      if (fskind == file_system_kind_dos_based)
+      if (result == NULL && fskind == file_system_kind_dos_based)
 	{
 	  char *new_pathname;
 
@@ -405,6 +409,21 @@ exec_file_find (char *in_pathname, int *fd)
 	  result = solib_find_1 (new_pathname, fd, 0);
 	}
     }
+  else
+    {
+      /* It's possible we don't have a full path, but rather just a
+	 filename.  Some targets, such as HP-UX, don't provide the
+	 full path, sigh.
+
+	 Attempt to qualify the filename against the source path.
+	 (If that fails, we'll just fall back on the original
+	 filename.  Not much more we can do...)  */
+
+      if (!source_full_path_of (in_pathname, &result))
+	result = xstrdup (in_pathname);
+      if (fd != NULL)
+	*fd = -1;
+    }
 
   return result;
 }