Avoid use after free with logging and debug redirect.

This patch addresses PR gdb/27133. Before it, the following succession
of commands would cause gdb to crash:

	set logging redirect on
	set logging debugredirect on
	set logging on

The problem eventually comes down to a use after free. The function
cli_interp_base::set_logging is called with a unique_ptr argument that
holds a pointer to the redirection file. In the problematic use case,
no-one ever took ownership of that pointer (as far as unique_ptr is
concerned), so the call to its dtor at the end of the function causes
the file object to be deleted. Any later use of the pointer to the
redirection file is therefore an error.

This patch ensures that the unique_ptr is released  when required (so it
does not assume ownership anymore). The internal logic of
cli_interp_base::set_logging takes care of freeing the ui_file when it
is not necessary anymore using the saved_output.file_to_delete field.

gdb/ChangeLog:

	PR gdb/27133
	* cli/cli-interp.c (cli_interp_base::set_logging): Ensure the
	unique_ptr is released when the wrapped pointer is kept for later
	use.

gdb/testsuite/ChangeLog:

	PR gdb/27133
	* gdb.base/ui-redirect.exp: Add test case that ensures that
	redirecting both logging and debug does not cause gdb to crash.

1 files changed, 8 insertions, 0 deletions

diff --git a/gdb/cli/cli-interp.c b/gdb/cli/cli-interp.c
index 424264a..882384e 100644
--- a/gdb/cli/cli-interp.c
+++ b/gdb/cli/cli-interp.c
@@ -430,6 +430,14 @@ cli_interp_base::set_logging (ui_file_up logfile, bool logging_redirect,
 	  saved_output.file_to_delete = tee;
 	}
 
+      /* Make sure that the call to logfile's dtor does not delete the
+         underlying pointer if we still keep a reference to it.  If
+         logfile_p is not referenced as the file_to_delete, then either
+         the logfile is not used (no redirection) and it should be
+         deleted, or a tee took ownership of the pointer. */
+      if (logfile_p != nullptr && saved_output.file_to_delete == logfile_p)
+	logfile.release ();
+
       gdb_stdout = logging_redirect ? logfile_p : tee;
       gdb_stdlog = debug_redirect ? logfile_p : tee;
       gdb_stderr = logging_redirect ? logfile_p : tee;