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GDB that can debug Mach-Os on Linux
12696c1090
In http://sourceware.org/ml/gdb-patches/2013-08/msg00174.html , the issue of child signal handling around ptrace option support discovery being different between GDB and GDBserver came up. I recalled adding these block_child_signals calls, and the "We don't want those ptrace calls to be interrupted" comment, but not exactly why. So I looked into it. My first guess is that I got confused. The patch that added this <http://sourceware.org/ml/gdb-patches/2009-04/msg00125.html> rewrote the linux native async support completely, and the old async support code had the SIGCHLD handler itself do waitpid, so in places that we'd want a blocking waitpid, we'd have to have the signal handler blocked. That was probably the mindset I had at the time. Anyway, whatever the case, looks like I was wrong on the need for this blocking. Given GDBserver doesn't block like this, I investigated why this is currently needed on GDB but not on GDBserver. I removed the block_child_signals (and restore) calls, and hacked linux-nat.c to call linux_test_for_tracefork in a loop, like: @@ -534,7 +534,10 @@ static int linux_supports_tracefork (int pid) { if (linux_supports_tracefork_flag == -1) - linux_test_for_tracefork (pid); + { + while (1) + linux_test_for_tracefork (pid); + } return linux_supports_tracefork_flag; } Running the resulting GDB, I then saw bad things happening. Specifically, I'd end up with a bunch of zombies, and eventually, the machine would refuse to spawn new processes, claming insufficient resources. The issue is that linux_test_for_tracefork test forks, and has the child fork again. If we don't block SIGCHLD on entry to the function, the children will inherit SIGCHLD's action/disposition (meaning, SIGCHLD will be unblocked in the child). When the first child forks again a second child, and that child exits, the first child gets a SIGCHLD. Now, when we try to wrap up for the whole options test, we kill the first child, and collect the waitstatus. Here, when SIGCHLD isn't blocked, GDB will first see the child reporting a stop with SIGCHLD. gdbserver's ptrace options test does a PTRACE_KILL loop at the end, which catches the SIGCHLD, and retries the kill. The GDB version did not do that. So the GDB version would proceed, leaving the child zombie (until GDB exists), as nothing collected its final waitstatus. So this patch makes the GDB version of linux_test_for_tracefork do the exact same as the GDBserver version, removes all this unnecessary blocking throughout, and adds a couple comments at places that do need it -- namely: places where we'll use sleep with sigsuspend; and linux_async_pipe, as that destroys the pipe the signal handler touches. Tested on x86_64 Fedora 17, sync and async. gdb/ 2013-08-19 Pedro Alves <palves@redhat.com> * linux-nat.c (linux_test_for_tracefork) (linux_test_for_tracesysgood, linux_child_follow_fork) (lin_lwp_attach_lwp, linux_nat_resume): Don't block child signals. (linux_nat_wait_1): Extend comment. (linux_async_pipe): Add comment. |
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| gas | ||
| gdb | ||
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| include | ||
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| MAINTAINERS | ||
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README for GNU development tools This directory contains various GNU compilers, assemblers, linkers, debuggers, etc., plus their support routines, definitions, and documentation. If you are receiving this as part of a GDB release, see the file gdb/README. If with a binutils release, see binutils/README; if with a libg++ release, see libg++/README, etc. That'll give you info about this package -- supported targets, how to use it, how to report bugs, etc. It is now possible to automatically configure and build a variety of tools with one command. To build all of the tools contained herein, run the ``configure'' script here, e.g.: ./configure make To install them (by default in /usr/local/bin, /usr/local/lib, etc), then do: make install (If the configure script can't determine your type of computer, give it the name as an argument, for instance ``./configure sun4''. You can use the script ``config.sub'' to test whether a name is recognized; if it is, config.sub translates it to a triplet specifying CPU, vendor, and OS.) If you have more than one compiler on your system, it is often best to explicitly set CC in the environment before running configure, and to also set CC when running make. For example (assuming sh/bash/ksh): CC=gcc ./configure make A similar example using csh: setenv CC gcc ./configure make Much of the code and documentation enclosed is copyright by the Free Software Foundation, Inc. See the file COPYING or COPYING.LIB in the various directories, for a description of the GNU General Public License terms under which you can copy the files. REPORTING BUGS: Again, see gdb/README, binutils/README, etc., for info on where and how to report problems.