From a9deee17d33235eeba401eebed55c85a742e3358 Mon Sep 17 00:00:00 2001 From: Pedro Alves Date: Mon, 27 Jun 2022 20:41:50 +0100 Subject: gdb+gdbserver/Linux: avoid reading registers while going through shell For every stop, Linux GDB and GDBserver save the stopped thread's PC, in lwp->stop_pc. This is done in save_stop_reason, in both gdb/linux-nat.c and gdbserver/linux-low.cc. However, while we're going through the shell after "run", in startup_inferior, we shouldn't be reading registers, as we haven't yet determined the target's architecture -- the shell's architecture may not even be the same as the final inferior's. In gdb/linux-nat.c, lwp->stop_pc is only needed when the thread has stopped for a breakpoint, and since when going through the shell, no breakpoint is going to hit, we could simply teach save_stop_reason to only record the stop pc when the thread stopped for a breakpoint. However, in gdbserver/linux-low.cc, lwp->stop_pc is used in more cases than breakpoint hits (e.g., it's used in tracepoints & the "while-stepping" feature). So to avoid GDB vs GDBserver divergence, we apply the same approach to both implementations. We set a flag in the inferior (process in GDBserver) whenever it is being nursed through the shell, and when that flag is set, save_stop_reason bails out early. While going through the shell, we'll only ever get process exits (normal or signalled), random signals, and exec events, so nothing is lost. Change-Id: If0f01831514d3a74d17efd102875de7d2c6401ad --- gdbserver/fork-child.cc | 5 +++++ gdbserver/inferiors.h | 7 +++++++ gdbserver/linux-low.cc | 16 ++++++++++++---- 3 files changed, 24 insertions(+), 4 deletions(-) (limited to 'gdbserver') diff --git a/gdbserver/fork-child.cc b/gdbserver/fork-child.cc index 96dd4d0..7ea66f2 100644 --- a/gdbserver/fork-child.cc +++ b/gdbserver/fork-child.cc @@ -18,6 +18,7 @@ #include "server.h" #include "gdbsupport/job-control.h" +#include "gdbsupport/scoped_restore.h" #include "nat/fork-inferior.h" #ifdef HAVE_SIGNAL_H #include @@ -103,6 +104,10 @@ post_fork_inferior (int pid, const char *program) atexit (restore_old_foreground_pgrp); #endif + process_info *proc = find_process_pid (pid); + scoped_restore save_starting_up + = make_scoped_restore (&proc->starting_up, true); + startup_inferior (the_target, pid, START_INFERIOR_TRAPS_EXPECTED, &cs.last_status, &cs.last_ptid); diff --git a/gdbserver/inferiors.h b/gdbserver/inferiors.h index f3ba4d8..6de746c 100644 --- a/gdbserver/inferiors.h +++ b/gdbserver/inferiors.h @@ -75,6 +75,13 @@ struct process_info /* Flag to mark that the DLL list has changed. */ bool dlls_changed = false; + + /* True if the inferior is starting up (inside startup_inferior), + and we're nursing it along (through the shell) until it is ready + to execute its first instruction. Until that is done, we must + not access inferior memory or registers, as we haven't determined + the target architecture/description. */ + bool starting_up = false; }; /* Get the pid of PROC. */ diff --git a/gdbserver/linux-low.cc b/gdbserver/linux-low.cc index 8b8614f..2f71360 100644 --- a/gdbserver/linux-low.cc +++ b/gdbserver/linux-low.cc @@ -747,8 +747,8 @@ linux_process_target::handle_extended_wait (lwp_info **orig_event_lwp, CORE_ADDR linux_process_target::get_pc (lwp_info *lwp) { - struct regcache *regcache; - CORE_ADDR pc; + process_info *proc = get_thread_process (get_lwp_thread (lwp)); + gdb_assert (!proc->starting_up); if (!low_supports_breakpoints ()) return 0; @@ -756,8 +756,8 @@ linux_process_target::get_pc (lwp_info *lwp) scoped_restore_current_thread restore_thread; switch_to_thread (get_lwp_thread (lwp)); - regcache = get_thread_regcache (current_thread, 1); - pc = low_get_pc (regcache); + struct regcache *regcache = get_thread_regcache (current_thread, 1); + CORE_ADDR pc = low_get_pc (regcache); threads_debug_printf ("pc is 0x%lx", (long) pc); @@ -797,6 +797,14 @@ linux_process_target::save_stop_reason (lwp_info *lwp) if (!low_supports_breakpoints ()) return false; + process_info *proc = get_thread_process (get_lwp_thread (lwp)); + if (proc->starting_up) + { + /* Claim we have the stop PC so that the caller doesn't try to + fetch it itself. */ + return true; + } + pc = get_pc (lwp); sw_breakpoint_pc = pc - low_decr_pc_after_break (); -- cgit v1.1