ircd: Add interface to backtrace(3).

include/ircd/backtrace.h

@@ -0,0 +1,45 @@
+// Matrix Construct
+//
+// Copyright (C) Matrix Construct Developers, Authors & Contributors
+// Copyright (C) 2016-2019 Jason Volk <jason@zemos.net>
+//
+// Permission to use, copy, modify, and/or distribute this software for any
+// purpose with or without fee is hereby granted, provided that the above
+// copyright notice and this permission notice is present in all copies. The
+// full license for this software is available in the LICENSE file.
+
+#pragma once
+#define HAVE_IRCD_BACKTRACE_H
+
+namespace ircd
+{
+	struct backtrace;
+}
+
+struct ircd::backtrace
+{
+	void **array;
+	size_t count;
+
+  public:
+	void *operator[](const size_t &) const;
+	const size_t &size() const;
+
+	backtrace(void **const &, const size_t &);
+	backtrace(const mutable_buffer &);
+	backtrace();
+};
+
+inline void *
+ircd::backtrace::operator[](const size_t &i)
+const
+{
+	return i < count? array[i] : nullptr;
+}
+
+inline const size_t &
+ircd::backtrace::size()
+const
+{
+	return count;
+}

include/ircd/ircd.h

@@ -24,6 +24,7 @@
 #include "exception.h"
 #include "run.h"
 #include "demangle.h"
+#include "backtrace.h"
 #include "localee.h"
 #include "timedate.h"
 #include "logger.h"

ircd/Makefile.am

@@ -122,6 +122,7 @@ libircd_la_SOURCES += vg.cc
 libircd_la_SOURCES += exception.cc
 libircd_la_SOURCES += util.cc
 libircd_la_SOURCES += demangle.cc
+libircd_la_SOURCES += backtrace.cc
 libircd_la_SOURCES += locale.cc
 libircd_la_SOURCES += timedate.cc
 libircd_la_SOURCES += lex_cast.cc

ircd/backtrace.cc

@@ -0,0 +1,108 @@
+// Matrix Construct
+//
+// Copyright (C) Matrix Construct Developers, Authors & Contributors
+// Copyright (C) 2016-2019 Jason Volk <jason@zemos.net>
+//
+// Permission to use, copy, modify, and/or distribute this software for any
+// purpose with or without fee is hereby granted, provided that the above
+// copyright notice and this permission notice is present in all copies. The
+// full license for this software is available in the LICENSE file.
+
+#include <RB_INC_EXECINFO_H
+#include <RB_INC_SYS_MMAN_H
+
+#if defined(__GNUC__) && defined(HAVE_EXECINFO_H)
+	#define IRCD_BACKTRACE_SUPPORT
+#endif
+
+#if defined(IRCD_BACKTRACE_SUPPORT) && defined(HAVE_SYS_MMAN_H)
+	#define IRCD_BACKTRACE_GLIBC_WORKAROUND
+#endif
+
+#if defined(IRCD_BACKTRACE_GLIBC_WORKAROUND)
+// The problem here is that backtrace(3) performs checks against this value
+// which is not correct for our own allocated `ircd::ctx` stacks. To workaround
+// this we neutralize the value before calling backtrace(3) and restore it
+// afterward. While this symbol is extern accessible, the page its on is
+// write-protected at some point, so we have to re-allow writing to it here.
+//
+// sysdeps/i386/backtrace.c | elf/dl-support.c
+// > This is a global variable set at program start time.  It marks the
+// > highest used stack address.
+//
+extern void *
+__libc_stack_end;
+
+void
+__attribute__((constructor))
+ircd_backtrace_allow_libc_fix()
+{
+	static const auto &prot
+	{
+		PROT_READ | PROT_WRITE
+	};
+
+	const auto &addr
+	{
+		uintptr_t(&__libc_stack_end) & ~(getpagesize() - 1UL)
+	};
+
+	ircd::syscall(::mprotect, (void *)addr, sizeof(__libc_stack_end), prot);
+}
+#endif defined(IRCD_BACKTRACE_GLIBC_WORKAROUND)
+
+//
+// backtrace::backtrace
+//
+
+namespace ircd
+{
+	thread_local std::array<void *, 512> backtrace_buffer;
+}
+
+ircd::backtrace::backtrace()
+:backtrace
+{
+	backtrace_buffer.data(),
+	backtrace_buffer.size()
+}
+{
+}
+
+ircd::backtrace::backtrace(const mutable_buffer &buf)
+:backtrace
+{
+	reinterpret_cast<void **>
+	(
+		const_cast<char **>
+		(
+			std::addressof(data(buf))
+		)
+	),
+	buffer::size(buf) / sizeof(void *)
+}
+{
+}
+
+ircd::backtrace::backtrace(void **const &array,
+                           const size_t &count)
+:array
+{
+	array
+}
+,count
+{
+	0UL
+}
+{
+	#if defined(IRCD_BACKTRACE_GLIBC_WORKAROUND)
+	const scope_restore stack_check_workaround
+	{
+		__libc_stack_end, reinterpret_cast<void *>(uintptr_t(-1UL))
+	};
+	#endif
+
+	#if defined(IRCD_BACKTRACE_SUPPORT)
+	this->count = ::backtrace(this->array, count);
+	#endif
+}

modules/console.cc

@@ -511,6 +511,21 @@ console_cmd__demangle(opt &out, const string_view &line)
 	return true;
 }
 
+bool
+console_cmd__bt(opt &out, const string_view &line)
+{
+	const ircd::backtrace bt;
+	for(size_t i(0); i < bt.size(); ++i)
+	{
+		out
+		<< std::dec << std::setw(3) << i << ':'
+		<< ' ' << std::hex << '[' << uintptr_t(bt[i]) << ']'
+		<< std::endl;
+	}
+
+	return true;
+}
+
 //
 // main
 //