From a30cd7a5b95675adf31b6092235b6c1f42c59520 Mon Sep 17 00:00:00 2001 From: Jon Turney Date: Thu, 27 Aug 2020 16:12:47 +0100 Subject: [PATCH] Cygwin: Remove waitloop argument from try_to_debug() Currently, when using CYGWIN's error_start facility, the faulting process isn't stopped while the error_start process is started when the fault is caused by an exception. (it even seems possible in theory that the faulting process could have exited before the error_start process attaches). This leads to e.g. the core dump written by CYGWIN='error_start=dumper' in response to an exception being non-deterministic. Remove the waitloop argument from try_to_debug(), only used in the exception case, so the faulting process busy-waits until the error_start process attaches. Code archaeology to determine why the code is this way didn't really turn up any answers, but this seems a low-risk change, as this only changes the behaviour when: - a debugger isn't already attached - an error_start is specified in CYGWIN env var - an exception has occurred which will be translated to a signal If error_start invokes something which doesn't attach using DebugActiveProcess(), we will spin indefinitely, but that will also currently occur for any of the existing other uses of try_to_debug(), which default to waitloop=TRUE. --- winsup/cygwin/exceptions.cc | 14 ++++++++------ winsup/cygwin/winsup.h | 2 +- 2 files changed, 9 insertions(+), 7 deletions(-) diff --git a/winsup/cygwin/exceptions.cc b/winsup/cygwin/exceptions.cc index bb7704f94..c98b92d30 100644 --- a/winsup/cygwin/exceptions.cc +++ b/winsup/cygwin/exceptions.cc @@ -461,10 +461,8 @@ cygwin_stackdump () exc.dumpstack (); } -#define TIME_TO_WAIT_FOR_DEBUGGER 10000 - extern "C" int -try_to_debug (bool waitloop) +try_to_debug () { if (!debugger_command) return 0; @@ -532,13 +530,17 @@ try_to_debug (bool waitloop) &si, &pi); + /* We want to stop here and wait until the error_start process attaches. But + we can't wait here for the error_start process to exit, as if it's a + debugger, it might want to continue this thread. So we busy wait until a + debugger attaches, which stops this process, after which it can decide if + we continue or not. */ + *dbg_end = L'\0'; if (!dbg) system_printf ("Failed to start debugger, %E"); else { - if (!waitloop) - return dbg; SetThreadPriority (GetCurrentThread (), THREAD_PRIORITY_IDLE); while (!being_debugged ()) Sleep (1); @@ -812,7 +814,7 @@ exception::handle (EXCEPTION_RECORD *e, exception_list *frame, CONTEXT *in, if (exit_state >= ES_SIGNAL_EXIT && (NTSTATUS) e->ExceptionCode != STATUS_CONTROL_C_EXIT) api_fatal ("Exception during process exit"); - else if (!try_to_debug (0)) + else if (!try_to_debug ()) rtl_unwind (frame, e); else { diff --git a/winsup/cygwin/winsup.h b/winsup/cygwin/winsup.h index 79844cb87..0ffd8c5af 100644 --- a/winsup/cygwin/winsup.h +++ b/winsup/cygwin/winsup.h @@ -190,7 +190,7 @@ void close_all_files (bool = false); /* debug_on_trap support. see exceptions.cc:try_to_debug() */ extern "C" void error_start_init (const char*); -extern "C" int try_to_debug (bool waitloop = 1); +extern "C" int try_to_debug (); void ld_preload (); void fixup_hooks_after_fork ();