forked from Imagelibrary/binutils-gdb
af7fc7ff9e
Several watchpoint-related testcases, such as gdb.threads/watchthreads.exp for example, when tested with the backend in non-stop mode, exposed an interesting detail of the Windows debug API that wasn't considered before. The symptom observed is spurious SIGTRAPs, like: Thread 1 "watchthreads" received signal SIGTRAP, Trace/breakpoint trap. 0x00000001004010b1 in main () at .../src/gdb/testsuite/gdb.threads/watchthreads.c:48 48 args[i] = 1; usleep (1); /* Init value. */ After a good amount of staring at logs and headscratching, I realized the problem: #0 - It all starts in the fact that multiple threads can hit an event at the same time. Say, a watchpoint for thread A, and a breakpoint for thread B. #1 - Say, WaitForDebugEvent reports the breakpoint hit for thread B first, then GDB for some reason decides to update debug registers, and continue. Updating debug registers means writing the debug registers to _all_ threads, with SetThreadContext. #2 - WaitForDebugEvent reports the watchpoint hit for thread A. Watchpoint hits are reported as EXCEPTION_SINGLE_STEP. #3 - windows-nat checks the Dr6 debug register to check if the step was a watchpoint or hardware breakpoint stop, and finds that Dr6 is completely cleared. So windows-nat reports a plain SIGTRAP (given EXCEPTION_SINGLE_STEP) to the core. #4 - Thread A was not supposed to be stepping, so infrun reports the SIGTRAP to the user as a random signal. The strange part is #3 above. Why was Dr6 cleared? Turns out what (at least in Windows 10 & 11), writing to _any_ debug register has the side effect of clearing Dr6, even if you write the same values the registers already had, back to the registers. I confirmed it clearly by adding this hack to GDB: if (th->context.ContextFlags == 0) { th->context.ContextFlags = CONTEXT_DEBUGGER_DR; /* Get current values of debug registers. */ CHECK (GetThreadContext (th->h, &th->context)); DEBUG_EVENTS ("For 0x%x (once), Dr6=0x%llx", th->tid, th->context.Dr6); /* Write debug registers back to thread, same values, and re-read them. */ CHECK (SetThreadContext (th->h, &th->context)); CHECK (GetThreadContext (th->h, &th->context)); DEBUG_EVENTS ("For 0x%x (twice), Dr6=0x%llx", th->tid, th->context.Dr6); } Which showed Dr6=0 after the write + re-read: [windows events] fill_thread_context: For 0x6a0 (once), Dr6=0xffff0ff1 [windows events] fill_thread_context: For 0x6a0 (twice), Dr6=0x0 This commit fixes the issue by detecting that a thread has a pending watchpoint hit to report (Dr6 has interesting bits set), and if so, avoid mofiying any debug register. Instead, let the pending watchpoint hit be reported by WaitForDebugEvent. If infrun did want to modify watchpoints, it will still be done when the thread is eventually re-resumed after the pending watchpoint hit is reported. (infrun knows how to gracefully handle the case of a watchpoint hit for a watchpoint that has since been deleted.) Change-Id: I21a3daa9e34eecfa054f0fea706e5ab40aabe70a
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.