From 351031f22adc77a63663cb2ac729864867d149c1 Mon Sep 17 00:00:00 2001 From: Andrew Burgess Date: Tue, 10 Aug 2021 14:41:30 +0100 Subject: [PATCH] gdb: make thread_suspend_state::stop_pc optional Currently the stop_pc field of thread_suspect_state is a CORE_ADDR and when we want to indicate that there is no stop_pc available we set this field back to a special value. There are actually two special values used, in post_create_inferior the stop_pc is set to 0. This is a little unfortunate, there are plenty of embedded targets where 0 is a valid pc value. The more common special value for stop_pc though, is set in thread_info::set_executing, where the value (~(CORE_ADDR) 0) is used. This commit changes things so that the stop_pc is instead a gdb::optional. We can now explicitly reset the field to an uninitialised state, we also have asserts that we don't read the stop_pc when its in an uninitialised state (both in gdbsupport/gdb_optional.h, when compiling with _GLIBCXX_DEBUG defined, and in thread_info::stop_pc). One situation where a thread will not have a stop_pc value is when the thread is stopped as a consequence of GDB being in all stop mode, and some other thread stopped at an interesting event. When GDB brings all the other threads to a stop those other threads will not have a stop_pc set (thus avoiding an unnecessary read of the pc register). Previously, when GDB passed through handle_one (in infrun.c) the threads executing flag was set to false and the stop_pc field was left unchanged, i.e. it would (previous) have been left as ~0. Now, handle_one leaves the stop_pc with no value. This caused a problem when we later try to set these threads running again, in proceed() we compare the current pc with the cached stop_pc. If the thread was stopped via handle_one then the stop_pc would have been left as ~0, and the compare (in proceed) would (likely) fail. Now however, this compare tries to read the stop_pc when it has no value and this would trigger an assert. To resolve this I've added thread_info::stop_pc_p() which returns true if the thread has a cached stop_pc. We should only ever call thread_info::stop_pc() if we know that there is a cached stop_pc, however, this doesn't mean that every call to thread_info::stop_pc() needs to be guarded with a call to thread_info::stop_pc_p(), in most cases we know that the thread we are looking at stopped due to some interesting event in that thread, and so, we know that the stop_pc is valid. After running the testsuite I've seen no other situations where stop_pc is read uninitialised. There should be no user visible changes after this commit. --- gdb/gdbthread.h | 32 +++++++++++++++++++++++++++----- gdb/infcmd.c | 2 +- gdb/infrun.c | 3 ++- gdb/thread.c | 2 +- 4 files changed, 31 insertions(+), 8 deletions(-) diff --git a/gdb/gdbthread.h b/gdb/gdbthread.h index e6f383cca61..4b271943d50 100644 --- a/gdb/gdbthread.h +++ b/gdb/gdbthread.h @@ -197,9 +197,12 @@ struct thread_suspend_state stop_reason: if the thread's PC has changed since the thread last stopped, a pending breakpoint waitstatus is discarded. - - If the thread is running, this is set to -1, to avoid leaving - it with a stale value, to make it easier to catch bugs. */ - CORE_ADDR stop_pc = 0; + - If the thread is running, then this field has its value removed by + calling stop_pc.reset() (see thread_info::set_executing()). + Attempting to read a gdb::optional with no value is undefined + behaviour and will trigger an assertion error when _GLIBCXX_DEBUG is + defined, which should make error easier to track down. */ + gdb::optional stop_pc; }; /* Base class for target-specific thread data. */ @@ -327,11 +330,15 @@ public: m_suspend = suspend; } - /* Return this thread's stop PC. */ + /* Return this thread's stop PC. This should only be called when it is + known that stop_pc has a value. If this function is being used in a + situation where a thread may not have had a stop_pc assigned, then + stop_pc_p() can be used to check if the stop_pc is defined. */ CORE_ADDR stop_pc () const { - return m_suspend.stop_pc; + gdb_assert (m_suspend.stop_pc.has_value ()); + return *m_suspend.stop_pc; } /* Set this thread's stop PC. */ @@ -341,6 +348,21 @@ public: m_suspend.stop_pc = stop_pc; } + /* Remove the stop_pc stored on this thread. */ + + void clear_stop_pc () + { + m_suspend.stop_pc.reset (); + } + + /* Return true if this thread has a cached stop pc value, otherwise + return false. */ + + bool stop_pc_p () const + { + return m_suspend.stop_pc.has_value (); + } + /* Return true if this thread has a pending wait status. */ bool has_pending_waitstatus () const diff --git a/gdb/infcmd.c b/gdb/infcmd.c index e6ee49bc43d..d948f4bafc5 100644 --- a/gdb/infcmd.c +++ b/gdb/infcmd.c @@ -249,7 +249,7 @@ post_create_inferior (int from_tty) missing registers info), ignore it. */ thread_info *thr = inferior_thread (); - thr->set_stop_pc (0); + thr->clear_stop_pc (); try { regcache *rc = get_thread_regcache (thr); diff --git a/gdb/infrun.c b/gdb/infrun.c index 8e778576eab..d1ac9b4cbbb 100644 --- a/gdb/infrun.c +++ b/gdb/infrun.c @@ -3051,7 +3051,8 @@ proceed (CORE_ADDR addr, enum gdb_signal siggnal) if (addr == (CORE_ADDR) -1) { - if (pc == cur_thr->stop_pc () + if (cur_thr->stop_pc_p () + && pc == cur_thr->stop_pc () && breakpoint_here_p (aspace, pc) == ordinary_breakpoint_here && execution_direction != EXEC_REVERSE) /* There is a breakpoint at the address we will resume at, diff --git a/gdb/thread.c b/gdb/thread.c index c95a9186681..10c3dcd6991 100644 --- a/gdb/thread.c +++ b/gdb/thread.c @@ -324,7 +324,7 @@ thread_info::set_executing (bool executing) { m_executing = executing; if (executing) - this->set_stop_pc (~(CORE_ADDR) 0); + this->clear_stop_pc (); } /* See gdbthread.h. */ -- 2.30.2