From af7fc7ff9e4c7081cc7b163111c3db2d4b9f46b8 Mon Sep 17 00:00:00 2001 From: Pedro Alves Date: Tue, 7 May 2024 20:41:37 +0100 Subject: Windows gdb: Avoid writing debug registers if watchpoint hit pending 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 --- gdb/nat/windows-nat.h | 7 ++++ gdb/windows-nat.c | 108 +++++++++++++++++++++++++++++++++++++------------- 2 files changed, 88 insertions(+), 27 deletions(-) (limited to 'gdb') diff --git a/gdb/nat/windows-nat.h b/gdb/nat/windows-nat.h index 6283ff0..2efb54e 100644 --- a/gdb/nat/windows-nat.h +++ b/gdb/nat/windows-nat.h @@ -205,6 +205,13 @@ struct windows_process_info virtual bool handle_access_violation (const EXCEPTION_RECORD *rec) = 0; + /* Fill in the thread's CONTEXT/WOW64_CONTEXT, if it wasn't filled + in yet. + + This function must be supplied by the embedding application. */ + + virtual void fill_thread_context (windows_thread_info *th) = 0; + handle_exception_result handle_exception (DEBUG_EVENT ¤t_event, struct target_waitstatus *ourstatus, bool debug_exceptions); diff --git a/gdb/windows-nat.c b/gdb/windows-nat.c index bdabe67..e11cc95 100644 --- a/gdb/windows-nat.c +++ b/gdb/windows-nat.c @@ -114,6 +114,7 @@ struct windows_per_inferior : public windows_process_info bool handle_access_violation (const EXCEPTION_RECORD *rec) override; void invalidate_context (windows_thread_info *th); + void fill_thread_context (windows_thread_info *th) override; void continue_one_thread (windows_thread_info *th, windows_continue_flags cont_flags); @@ -747,17 +748,10 @@ windows_fetch_one_register (struct regcache *regcache, } void -windows_nat_target::fetch_registers (struct regcache *regcache, int r) +windows_per_inferior::fill_thread_context (windows_thread_info *th) { - windows_thread_info *th = windows_process.find_thread (regcache->ptid ()); - - /* Check if TH exists. Windows sometimes uses a non-existent - thread id in its events. */ - if (th == NULL) - return; - #ifdef __x86_64__ - if (windows_process.wow64_process) + if (wow64_process) { if (th->wow64_context.ContextFlags == 0) { @@ -774,6 +768,19 @@ windows_nat_target::fetch_registers (struct regcache *regcache, int r) CHECK (GetThreadContext (th->h, &th->context)); } } +} + +void +windows_nat_target::fetch_registers (struct regcache *regcache, int r) +{ + windows_thread_info *th = windows_process.find_thread (regcache->ptid ()); + + /* Check if TH exists. Windows sometimes uses a non-existent + thread id in its events. */ + if (th == nullptr) + return; + + windows_process.fill_thread_context (th); if (r < 0) for (r = 0; r < gdbarch_num_regs (regcache->arch()); r++) @@ -1292,36 +1299,83 @@ windows_per_inferior::continue_one_thread (windows_thread_info *th, DWORD &context_flags_ref = (wow64_process ? th->wow64_context.ContextFlags : th->context.ContextFlags); + const DWORD64 dr6 = (wow64_process + ? th->wow64_context.Dr6 + : th->context.Dr6); #else DWORD &context_flags_ref = th->context.ContextFlags; + const DWORD dr6 = th->context.Dr6; #endif if (th->debug_registers_changed) { - context_flags_ref |= CONTEXT_DEBUG_REGISTERS; -#ifdef __x86_64__ - if (wow64_process) + windows_process.fill_thread_context (th); + + gdb_assert ((context_flags_ref & CONTEXT_DEBUG_REGISTERS) != 0); + + /* Check whether the thread has Dr6 set indicating a + watchpoint hit, and we haven't seen the watchpoint event + yet (reported as + EXCEPTION_SINGLE_STEP/STATUS_WX86_SINGLE_STEP). In that + case, don't change the debug registers. Changing debug + registers, even if to the same values, makes the kernel + clear Dr6. The result would be we would lose the + unreported watchpoint hit. */ + if ((dr6 & ~DR6_CLEAR_VALUE) != 0) { - th->wow64_context.Dr0 = state->dr_mirror[0]; - th->wow64_context.Dr1 = state->dr_mirror[1]; - th->wow64_context.Dr2 = state->dr_mirror[2]; - th->wow64_context.Dr3 = state->dr_mirror[3]; - th->wow64_context.Dr6 = DR6_CLEAR_VALUE; - th->wow64_context.Dr7 = state->dr_control_mirror; + if (th->last_event.dwDebugEventCode == EXCEPTION_DEBUG_EVENT + && (th->last_event.u.Exception.ExceptionRecord.ExceptionCode + == EXCEPTION_SINGLE_STEP + || (th->last_event.u.Exception.ExceptionRecord.ExceptionCode + == STATUS_WX86_SINGLE_STEP))) + { + DEBUG_EVENTS ("0x%x already reported watchpoint", th->tid); + } + else + { + DEBUG_EVENTS ("0x%x last reported something else (0x%x)", + th->tid, + th->last_event.dwDebugEventCode); + + /* Don't touch debug registers. Let the pending + watchpoint event be reported instead. We will + update the debug registers later when the thread + is re-resumed by the core after the watchpoint + event. */ + context_flags_ref &= ~CONTEXT_DEBUG_REGISTERS; + } } else -#endif + DEBUG_EVENTS ("0x%x has no dr6 set", th->tid); + + if ((context_flags_ref & CONTEXT_DEBUG_REGISTERS) != 0) { - th->context.Dr0 = state->dr_mirror[0]; - th->context.Dr1 = state->dr_mirror[1]; - th->context.Dr2 = state->dr_mirror[2]; - th->context.Dr3 = state->dr_mirror[3]; - th->context.Dr6 = DR6_CLEAR_VALUE; - th->context.Dr7 = state->dr_control_mirror; - } + DEBUG_EVENTS ("0x%x changing dregs", th->tid); +#ifdef __x86_64__ + if (wow64_process) + { + th->wow64_context.Dr0 = state->dr_mirror[0]; + th->wow64_context.Dr1 = state->dr_mirror[1]; + th->wow64_context.Dr2 = state->dr_mirror[2]; + th->wow64_context.Dr3 = state->dr_mirror[3]; + th->wow64_context.Dr6 = DR6_CLEAR_VALUE; + th->wow64_context.Dr7 = state->dr_control_mirror; + } + else +#endif + { + th->context.Dr0 = state->dr_mirror[0]; + th->context.Dr1 = state->dr_mirror[1]; + th->context.Dr2 = state->dr_mirror[2]; + th->context.Dr3 = state->dr_mirror[3]; + th->context.Dr6 = DR6_CLEAR_VALUE; + th->context.Dr7 = state->dr_control_mirror; + } - th->debug_registers_changed = false; + th->debug_registers_changed = false; + } } + if (context_flags_ref != 0) { DWORD ec = 0; -- cgit v1.1