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binutils-gdb/gdb/linux-nat.h
Pedro Alves 6652277f92 Step over clone syscall w/ breakpoint, TARGET_WAITKIND_THREAD_CLONED 
(A good chunk of the problem statement in the commit log below is
Andrew's, adjusted for a different solution, and for covering
displaced stepping too.)

This commit addresses bugs gdb/19675 and gdb/27830, which are about
stepping over a breakpoint set at a clone syscall instruction, one is
about displaced stepping, and the other about in-line stepping.

Currently, when a new thread is created through a clone syscall, GDB
sets the new thread running.  With 'continue' this makes sense
(assuming no schedlock):

 - all-stop mode, user issues 'continue', all threads are set running,
   a newly created thread should also be set running.

 - non-stop mode, user issues 'continue', other pre-existing threads
   are not effected, but as the new thread is (sort-of) a child of the
   thread the user asked to run, it makes sense that the new threads
   should be created in the running state.

Similarly, if we are stopped at the clone syscall, and there's no
software breakpoint at this address, then the current behaviour is
fine:

 - all-stop mode, user issues 'stepi', stepping will be done in place
   (as there's no breakpoint to step over).  While stepping the thread
   of interest all the other threads will be allowed to continue.  A
   newly created thread will be set running, and then stopped once the
   thread of interest has completed its step.

 - non-stop mode, user issues 'stepi', stepping will be done in place
   (as there's no breakpoint to step over).  Other threads might be
   running or stopped, but as with the continue case above, the new
   thread will be created running.  The only possible issue here is
   that the new thread will be left running after the initial thread
   has completed its stepi.  The user would need to manually select
   the thread and interrupt it, this might not be what the user
   expects.  However, this is not something this commit tries to
   change.

The problem then is what happens when we try to step over a clone
syscall if there is a breakpoint at the syscall address.

- For both all-stop and non-stop modes, with in-line stepping:

   + user issues 'stepi',
   + [non-stop mode only] GDB stops all threads.  In all-stop mode all
     threads are already stopped.
   + GDB removes s/w breakpoint at syscall address,
   + GDB single steps just the thread of interest, all other threads
     are left stopped,
   + New thread is created running,
   + Initial thread completes its step,
   + [non-stop mode only] GDB resumes all threads that it previously
     stopped.

There are two problems in the in-line stepping scenario above:

  1. The new thread might pass through the same code that the initial
     thread is in (i.e. the clone syscall code), in which case it will
     fail to hit the breakpoint in clone as this was removed so the
     first thread can single step,

  2. The new thread might trigger some other stop event before the
     initial thread reports its step completion.  If this happens we
     end up triggering an assertion as GDB assumes that only the
     thread being stepped should stop.  The assert looks like this:

     infrun.c:5899: internal-error: int finish_step_over(execution_control_state*): Assertion `ecs->event_thread->control.trap_expected' failed.

- For both all-stop and non-stop modes, with displaced stepping:

   + user issues 'stepi',
   + GDB starts the displaced step, moves thread's PC to the
     out-of-line scratch pad, maybe adjusts registers,
   + GDB single steps the thread of interest, [non-stop mode only] all
     other threads are left as they were, either running or stopped.
     In all-stop, all other threads are left stopped.
   + New thread is created running,
   + Initial thread completes its step, GDB re-adjusts its PC,
     restores/releases scratchpad,
   + [non-stop mode only] GDB resumes the thread, now past its
     breakpoint.
   + [all-stop mode only] GDB resumes all threads.

There is one problem with the displaced stepping scenario above:

  3. When the parent thread completed its step, GDB adjusted its PC,
     but did not adjust the child's PC, thus that new child thread
     will continue execution in the scratch pad, invoking undefined
     behavior.  If you're lucky, you see a crash.  If unlucky, the
     inferior gets silently corrupted.

What is needed is for GDB to have more control over whether the new
thread is created running or not.  Issue #1 above requires that the
new thread not be allowed to run until the breakpoint has been
reinserted.  The only way to guarantee this is if the new thread is
held in a stopped state until the single step has completed.  Issue #3
above requires that GDB is informed of when a thread clones itself,
and of what is the child's ptid, so that GDB can fixup both the parent
and the child.

When looking for solutions to this problem I considered how GDB
handles fork/vfork as these have some of the same issues.  The main
difference between fork/vfork and clone is that the clone events are
not reported back to core GDB.  Instead, the clone event is handled
automatically in the target code and the child thread is immediately
set running.

Note we have support for requesting thread creation events out of the
target (TARGET_WAITKIND_THREAD_CREATED).  However, those are reported
for the new/child thread.  That would be sufficient to address in-line
stepping (issue #1), but not for displaced-stepping (issue #3).  To
handle displaced-stepping, we need an event that is reported to the
_parent_ of the clone, as the information about the displaced step is
associated with the clone parent.  TARGET_WAITKIND_THREAD_CREATED
includes no indication of which thread is the parent that spawned the
new child.  In fact, for some targets, like e.g., Windows, it would be
impossible to know which thread that was, as thread creation there
doesn't work by "cloning".

The solution implemented here is to model clone on fork/vfork, and
introduce a new TARGET_WAITKIND_THREAD_CLONED event.  This event is
similar to TARGET_WAITKIND_FORKED and TARGET_WAITKIND_VFORKED, except
that we end up with a new thread in the same process, instead of a new
thread of a new process.  Like FORKED and VFORKED, THREAD_CLONED
waitstatuses have a child_ptid property, and the child is help stopped
until GDB explicitly resumes it.  This addresses the in-line stepping
case (issues #1 and #2).

The infrun code that handles displaced stepping fixup for the child
after a fork/vfork event is thus reused for THREAD_CLONE, with some
minimal conditions added, addressing the displaced stepping case
(issue #3).

The native Linux backend is adjusted to unconditionally report
TARGET_WAITKIND_THREAD_CLONED events to the core.

Following the follow_fork model in core GDB, we introduce a
target_follow_clone target method, which is responsible for making the
new clone child visible to the rest of GDB.

Subsequent patches will add clone events support to the remote
protocol and gdbserver.

A testcase will be added by a later patch.

displaced_step_in_progress_thread is removed in this patch, but it is
added back again in a subsequent patch.  We need to do this because
the function is static, and with no callers, the compiler would warn,
(error with -Werror), breaking the build.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=19675
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=27830

Change-Id: I474e9a7015dd3d33469e322a5764ae83f8a32787
2022-07-18 17:33:30 +01:00

340 lines
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/* Native debugging support for GNU/Linux (LWP layer).
Copyright (C) 2000-2022 Free Software Foundation, Inc.
This file is part of GDB.
This program is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation; either version 3 of the License, or
(at your option) any later version.
This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU General Public License for more details.
You should have received a copy of the GNU General Public License
along with this program. If not, see <http://www.gnu.org/licenses/>. */
#ifndef LINUX_NAT_H
#define LINUX_NAT_H
#include "nat/linux-nat.h"
#include "inf-ptrace.h"
#include "target.h"
#include <signal.h>
/* A prototype generic GNU/Linux target. A concrete instance should
override it with local methods. */
class linux_nat_target : public inf_ptrace_target
{
public:
linux_nat_target ();
~linux_nat_target () override = 0;
thread_control_capabilities get_thread_control_capabilities () override
{ return tc_schedlock; }
void create_inferior (const char *, const std::string &,
char **, int) override;
void attach (const char *, int) override;
void detach (inferior *, int) override;
void resume (ptid_t, int, enum gdb_signal) override;
ptid_t wait (ptid_t, struct target_waitstatus *, target_wait_flags) override;
void pass_signals (gdb::array_view<const unsigned char>) override;
enum target_xfer_status xfer_partial (enum target_object object,
const char *annex,
gdb_byte *readbuf,
const gdb_byte *writebuf,
ULONGEST offset, ULONGEST len,
ULONGEST *xfered_len) override;
void kill () override;
void mourn_inferior () override;
bool thread_alive (ptid_t ptid) override;
void update_thread_list () override;
std::string pid_to_str (ptid_t) override;
const char *thread_name (struct thread_info *) override;
struct address_space *thread_address_space (ptid_t) override;
bool stopped_by_watchpoint () override;
bool stopped_data_address (CORE_ADDR *) override;
bool stopped_by_sw_breakpoint () override;
bool supports_stopped_by_sw_breakpoint () override;
bool stopped_by_hw_breakpoint () override;
bool supports_stopped_by_hw_breakpoint () override;
void thread_events (int) override;
bool can_async_p () override;
bool supports_non_stop () override;
bool always_non_stop_p () override;
void async (int) override;
void stop (ptid_t) override;
bool supports_multi_process () override;
bool supports_disable_randomization () override;
int core_of_thread (ptid_t ptid) override;
bool filesystem_is_local () override;
int fileio_open (struct inferior *inf, const char *filename,
int flags, int mode, int warn_if_slow,
int *target_errno) override;
gdb::optional<std::string>
fileio_readlink (struct inferior *inf,
const char *filename,
int *target_errno) override;
int fileio_unlink (struct inferior *inf,
const char *filename,
int *target_errno) override;
int insert_fork_catchpoint (int) override;
int remove_fork_catchpoint (int) override;
int insert_vfork_catchpoint (int) override;
int remove_vfork_catchpoint (int) override;
int insert_exec_catchpoint (int) override;
int remove_exec_catchpoint (int) override;
int set_syscall_catchpoint (int pid, bool needed, int any_count,
gdb::array_view<const int> syscall_counts) override;
const char *pid_to_exec_file (int pid) override;
void post_attach (int) override;
void follow_fork (inferior *, ptid_t, target_waitkind, bool, bool) override;
void follow_clone (ptid_t) override;
std::vector<static_tracepoint_marker>
static_tracepoint_markers_by_strid (const char *id) override;
/* Methods that are meant to overridden by the concrete
arch-specific target instance. */
virtual void low_resume (ptid_t ptid, int step, enum gdb_signal sig)
{ inf_ptrace_target::resume (ptid, step, sig); }
virtual bool low_stopped_by_watchpoint ()
{ return false; }
virtual bool low_stopped_data_address (CORE_ADDR *addr_p)
{ return false; }
/* The method to call, if any, when a new thread is attached. */
virtual void low_new_thread (struct lwp_info *)
{}
/* The method to call, if any, when a thread is destroyed. */
virtual void low_delete_thread (struct arch_lwp_info *lp)
{
gdb_assert (lp == NULL);
}
/* The method to call, if any, when a new fork is attached. */
virtual void low_new_fork (struct lwp_info *parent, pid_t child_pid)
{}
/* The method to call, if any, when a new clone event is detected. */
virtual void low_new_clone (struct lwp_info *parent, pid_t child_lwp)
{}
/* The method to call, if any, when a process is no longer
attached. */
virtual void low_forget_process (pid_t pid)
{}
/* Hook to call prior to resuming a thread. */
virtual void low_prepare_to_resume (struct lwp_info *)
{}
/* Convert a ptrace/host siginfo object, into/from the siginfo in
the layout of the inferiors' architecture. Returns true if any
conversion was done; false otherwise, in which case the caller
does a straight memcpy. If DIRECTION is 1, then copy from INF to
PTRACE. If DIRECTION is 0, copy from PTRACE to INF. */
virtual bool low_siginfo_fixup (siginfo_t *ptrace, gdb_byte *inf,
int direction)
{ return false; }
/* SIGTRAP-like breakpoint status events recognizer. The default
recognizes SIGTRAP only. */
virtual bool low_status_is_event (int status);
protected:
void post_startup_inferior (ptid_t) override;
};
/* The final/concrete instance. */
extern linux_nat_target *linux_target;
struct arch_lwp_info;
/* Structure describing an LWP. */
struct lwp_info : intrusive_list_node<lwp_info>
{
lwp_info (ptid_t ptid)
: ptid (ptid)
{}
~lwp_info ();
DISABLE_COPY_AND_ASSIGN (lwp_info);
/* The process id of the LWP. This is a combination of the LWP id
and overall process id. */
ptid_t ptid = null_ptid;
/* If this flag is set, we need to set the event request flags the
next time we see this LWP stop. */
int must_set_ptrace_flags = 0;
/* Non-zero if we sent this LWP a SIGSTOP (but the LWP didn't report
it back yet). */
int signalled = 0;
/* Non-zero if this LWP is stopped. */
int stopped = 0;
/* Non-zero if this LWP will be/has been resumed. Note that an LWP
can be marked both as stopped and resumed at the same time. This
happens if we try to resume an LWP that has a wait status
pending. We shouldn't let the LWP run until that wait status has
been processed, but we should not report that wait status if GDB
didn't try to let the LWP run. */
int resumed = 0;
/* The last resume GDB requested on this thread. */
resume_kind last_resume_kind = resume_continue;
/* If non-zero, a pending wait status. */
int status = 0;
/* When 'stopped' is set, this is where the lwp last stopped, with
decr_pc_after_break already accounted for. If the LWP is
running and stepping, this is the address at which the lwp was
resumed (that is, it's the previous stop PC). If the LWP is
running and not stepping, this is 0. */
CORE_ADDR stop_pc = 0;
/* Non-zero if we were stepping this LWP. */
int step = 0;
/* The reason the LWP last stopped, if we need to track it
(breakpoint, watchpoint, etc.). */
target_stop_reason stop_reason = TARGET_STOPPED_BY_NO_REASON;
/* On architectures where it is possible to know the data address of
a triggered watchpoint, STOPPED_DATA_ADDRESS_P is non-zero, and
STOPPED_DATA_ADDRESS contains such data address. Otherwise,
STOPPED_DATA_ADDRESS_P is false, and STOPPED_DATA_ADDRESS is
undefined. Only valid if STOPPED_BY_WATCHPOINT is true. */
int stopped_data_address_p = 0;
CORE_ADDR stopped_data_address = 0;
/* Non-zero if we expect a duplicated SIGINT. */
int ignore_sigint = 0;
/* If WAITSTATUS->KIND != TARGET_WAITKIND_SPURIOUS, the waitstatus
for this LWP's last event. This may correspond to STATUS above,
or to a local variable in lin_lwp_wait. */
struct target_waitstatus waitstatus;
/* Signal whether we are in a SYSCALL_ENTRY or
in a SYSCALL_RETURN event.
Values:
- TARGET_WAITKIND_SYSCALL_ENTRY
- TARGET_WAITKIND_SYSCALL_RETURN */
enum target_waitkind syscall_state;
/* The processor core this LWP was last seen on. */
int core = -1;
/* Arch-specific additions. */
struct arch_lwp_info *arch_private = nullptr;
};
/* lwp_info iterator and range types. */
using lwp_info_iterator
= reference_to_pointer_iterator<intrusive_list<lwp_info>::iterator>;
using lwp_info_range = iterator_range<lwp_info_iterator>;
using lwp_info_safe_range = basic_safe_range<lwp_info_range>;
/* Get an iterable range over all lwps. */
lwp_info_range all_lwps ();
/* Same as the above, but safe against deletion while iterating. */
lwp_info_safe_range all_lwps_safe ();
/* Does the current host support PTRACE_GETREGSET? */
extern enum tribool have_ptrace_getregset;
/* Called from the LWP layer to inform the thread_db layer that PARENT
spawned CHILD. Both LWPs are currently stopped. This function
does whatever is required to have the child LWP under the
thread_db's control --- e.g., enabling event reporting. Returns
true on success, false if the process isn't using libpthread. */
extern int thread_db_notice_clone (ptid_t parent, ptid_t child);
/* Return the number of signals used by the threads library. */
extern unsigned int lin_thread_get_thread_signal_num (void);
/* Return the i-th signal used by the threads library. */
extern int lin_thread_get_thread_signal (unsigned int i);
/* Find process PID's pending signal set from /proc/pid/status. */
void linux_proc_pending_signals (int pid, sigset_t *pending,
sigset_t *blocked, sigset_t *ignored);
/* For linux_stop_lwp see nat/linux-nat.h. */
/* Stop all LWPs, synchronously. (Any events that trigger while LWPs
are being stopped are left pending.) */
extern void linux_stop_and_wait_all_lwps (void);
/* Set resumed LWPs running again, as they were before being stopped
with linux_stop_and_wait_all_lwps. (LWPS with pending events are
left stopped.) */
extern void linux_unstop_all_lwps (void);
/* Update linux-nat internal state when changing from one fork
to another. */
void linux_nat_switch_fork (ptid_t new_ptid);
/* Store the saved siginfo associated with PTID in *SIGINFO.
Return 1 if it was retrieved successfully, 0 otherwise (*SIGINFO is
uninitialized in such case). */
int linux_nat_get_siginfo (ptid_t ptid, siginfo_t *siginfo);
#endif /* LINUX_NAT_H */
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