[PATCH v4 05/11] Introduce alternate_signal_stack RAII class

Tom Tromey
2019-10-01T20:12:45Z

This introduces a new RAII class that temporarily installs an
alternate signal stack (on systems that have sigaltstack); then
changes the one gdb use of sigaltstack to use this class instead.

This will be used in a later patch, when creating new threads that may
want to handle SIGSEGV.

gdb/ChangeLog
2019-10-01 Tom Tromey <tom@tromey.com>

* main.c (setup_alternate_signal_stack): Remove.
(captured_main_1): Use gdb::alternate_signal_stack.
* gdbsupport/alt-stack.h: New file.
---
gdb/ChangeLog | 6 ++++
gdb/gdbsupport/alt-stack.h | 70 ++++++++++++++++++++++++++++++++++++++
gdb/main.c | 26 ++------------
3 files changed, 78 insertions(+), 24 deletions(-)
create mode 100644 gdb/gdbsupport/alt-stack.h

diff --git a/gdb/gdbsupport/alt-stack.h b/gdb/gdbsupport/alt-stack.h
new file mode 100644
index 00000000000..1708fb4ce92
--- /dev/null
+++ b/gdb/gdbsupport/alt-stack.h
@@ -0,0 +1,70 @@
+/* Temporarily install an alternate signal stack
+
+ Copyright (C) 2019 Free Software Foundation, Inc.
+
+ This file is part of GDB.
+
+ This program is free software; you can redistribute it and/or modify
+ it under the terms of the GNU General Public License as published by
+ the Free Software Foundation; either version 3 of the License, or
+ (at your option) any later version.
+
+ This program is distributed in the hope that it will be useful,
+ but WITHOUT ANY WARRANTY; without even the implied warranty of
+ MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ GNU General Public License for more details.
+
+ You should have received a copy of the GNU General Public License
+ along with this program. If not, see <http://www.gnu.org/licenses/>. */
+
+#ifndef GDBSUPPORT_ALT_STACK_H
+#define GDBSUPPORT_ALT_STACK_H
+
+#include <signal.h>
+
+namespace gdb
+{
+
+/* Try to set up an alternate signal stack for SIGSEGV handlers.
+ This allows us to handle SIGSEGV signals generated when the
+ normal process stack is exhausted. If this stack is not set
+ up (sigaltstack is unavailable or fails) and a SIGSEGV is
+ generated when the normal stack is exhausted then the program
+ will behave as though no SIGSEGV handler was installed. */
+class alternate_signal_stack
+{
+public:
+ alternate_signal_stack ()
+ {
+#ifdef HAVE_SIGALTSTACK
+ m_stack.reset ((char *) xmalloc (SIGSTKSZ));
+
+ stack_t stack;
+ stack.ss_sp = m_stack.get ();
+ stack.ss_size = SIGSTKSZ;
+ stack.ss_flags = 0;
+
+ sigaltstack (&stack, &m_old_stack);
+#endif
+ }
+
+ ~alternate_signal_stack ()
+ {
+#ifdef HAVE_SIGALTSTACK
+ sigaltstack (&m_old_stack, nullptr);
+#endif
+ }
+
+ DISABLE_COPY_AND_ASSIGN (alternate_signal_stack);
+
+private:
+
+#ifdef HAVE_SIGALTSTACK
+ gdb::unique_xmalloc_ptr<char> m_stack;
+ stack_t m_old_stack;
+#endif
+};
+
+}
+
+#endif /* GDBSUPPORT_ALT_STACK_H */
diff --git a/gdb/main.c b/gdb/main.c
index 14d9e796538..5f6e8bec86e 100644
--- a/gdb/main.c
+++ b/gdb/main.c
@@ -48,6 +48,7 @@
#include <vector>
#include "gdbsupport/pathstuff.h"
#include "cli/cli-style.h"
+#include "gdbsupport/alt-stack.h"

/* The selected interpreter. This will be used as a set command
variable, so it should always be malloc'ed - since
@@ -289,29 +290,6 @@ get_init_files (std::string *system_gdbinit,
*local_gdbinit = localinit;
}

-/* Try to set up an alternate signal stack for SIGSEGV handlers.
- This allows us to handle SIGSEGV signals generated when the
- normal process stack is exhausted. If this stack is not set
- up (sigaltstack is unavailable or fails) and a SIGSEGV is
- generated when the normal stack is exhausted then the program
- will behave as though no SIGSEGV handler was installed. */
-
-static void
-setup_alternate_signal_stack (void)
-{
-#ifdef HAVE_SIGALTSTACK
- stack_t ss;
-
- /* FreeBSD versions older than 11.0 use char * for ss_sp instead of
- void *. This cast works with both types. */
- ss.ss_sp = (char *) xmalloc (SIGSTKSZ);
- ss.ss_size = SIGSTKSZ;
- ss.ss_flags = 0;
-
- sigaltstack(&ss, NULL);
-#endif
-}
-
/* Call command_loop. */

/* Prevent inlining this function for the benefit of GDB's selftests
@@ -857,7 +835,7 @@ captured_main_1 (struct captured_main_args *context)
save_original_signals_state (quiet);

/* Try to set up an alternate signal stack for SIGSEGV handlers. */
- setup_alternate_signal_stack ();
+ gdb::alternate_signal_stack signal_stack;

/* Initialize all files. */
gdb_init (gdb_program_name);
--
2.17.2