Re: [PATCH 3/7] Force to insert software single step breakpoint

Yao Qi
2016-04-20T07:50:02Z

Pedro Alves <palves@redhat.com> writes:

> Sounds like we may need to look at the single-step breakpoint's thread
> id, and only insert it if it is for the thread that is going to be
> doing the step-over? We may need to record that in step_over_info and
> pass more info to stepping_past_instruction_at.

The patch below does this, however, I don't pass more info to
stepping_past_instruction_at, instead I add a new function
thread_is_being_stepped_over_p.

>
>> --- a/gdb/gdbarch.sh
>> +++ b/gdb/gdbarch.sh
>> @@ -609,7 +609,10 @@ m:CORE_ADDR:addr_bits_remove:CORE_ADDR
>> addr:addr::core_addr_identity::0
>> # target can single step. If not, then implement single step using
>> breakpoints.
>> #
>> # A return value of 1 means that the software_single_step breakpoints
>> -# were inserted; 0 means they were not.
>> +# were inserted; 0 means they were not. Multiple breakpoints may be
>> +# inserted for some instructions such as conditional branch. However,
>> +# each implementation must always evaluate the condition and only put
>> +# the breakpoint at the branch destination if the condition is true.
>
> I'd add:
>
> (...) condition is true, so that we ensure forward progress when
> stepping past a conditional branch to self.
>
> This will help porters evaluate whether that's really necessary
> for their ports.

Done.

--
Yao (齐尧)

Subject: [PATCH] Force to insert software single step breakpoint

GDB doesn't insert software single step breakpoint if the instruction
branches to itself, so that the program can't stop after command "si".

(gdb) b 32
Breakpoint 2 at 0x8680: file git/gdb/testsuite/gdb.base/branch-to-self.c, line 32.
(gdb) c
Continuing.

Breakpoint 2, main () at gdb/git/gdb/testsuite/gdb.base/branch-to-self.c:32
32 asm (".Lhere: " BRANCH_INSN " .Lhere"); /* loop-line */
(gdb) si
infrun: clear_proceed_status_thread (Thread 3991.3991)
infrun: proceed (addr=0xffffffff, signal=GDB_SIGNAL_DEFAULT)
infrun: step-over queue now empty
infrun: resuming [Thread 3991.3991] for step-over
infrun: skipping breakpoint: stepping past insn at: 0x8680
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Sending packet: $Z0,8678,4#f3...Packet received: OK
infrun: skipping breakpoint: stepping past insn at: 0x8680
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Sending packet: $Z0,b6fe86c8,4#82...Packet received: OK
infrun: resume (step=1, signal=GDB_SIGNAL_0), trap_expected=1, current thread [Thread 3991.3991] at 0x868

breakpoint.c:should_be_inserted thinks the breakpoint shouldn't be
inserted, which is wrong. This patch restrict the condition that
only skip the non-single-step breakpoints if they are inserted at
the place we are stepping over, however we don't want to skip
single-step breakpoint if its thread is the thread we are stepping
over, so in this patch, I add a thread num in 'struct step_over_info'
to record the thread we're stepping over.

gdb:

2016-04-20 Yao Qi <yao.qi@linaro.org>

* breakpoint.c (should_be_inserted): Return 0 if the location's
owner is not single step breakpoint or single step brekapoint's
owner isn't the thread we are stepping over.
* gdbarch.sh (software_single_step): Update comments.
* gdbarch.h: Regenerated.
* infrun.c (struct step_over_info) <thread>: New field.
(set_step_over_info): New argument 'thread'. Callers updated.
(clear_step_over_info): Set field thread to -1.
(thread_is_being_stepped_over_p): New function.
* infrun.h (thread_is_being_stepped_over_p): Declaration.

diff --git a/gdb/breakpoint.c b/gdb/breakpoint.c
index f99a7ab..64e97c6 100644
--- a/gdb/breakpoint.c
+++ b/gdb/breakpoint.c
@@ -2219,11 +2219,22 @@ should_be_inserted (struct bp_location *bl)
return 0;

/* Don't insert a breakpoint if we're trying to step past its
- location. */
+ location except that the breakpoint is single-step breakpoint
+ and the single-step breakpoint's owner is the thread we're
+ stepping over. */
if ((bl->loc_type == bp_loc_software_breakpoint
|| bl->loc_type == bp_loc_hardware_breakpoint)
&& stepping_past_instruction_at (bl->pspace->aspace,
- bl->address))
+ bl->address)
+ /* The single-step breakpoint may be inserted at the location
+ we're trying to step if the instruction branches to itself.
+ However, the instruction won't be executed at all and it may
+ break the semantics of the instruction, for example, the
+ instruction is a conditional branch or updates some flags.
+ We can't fix it unless GDB is able to emulate the instruction
+ or switch to displaced stepping. */
+ && !(bl->owner->type == bp_single_step
+ && thread_is_being_stepped_over_p (bl->owner->thread)))
{
if (debug_infrun)
{
diff --git a/gdb/gdbarch.h b/gdb/gdbarch.h
index 252fc4b..859ba85 100644
--- a/gdb/gdbarch.h
+++ b/gdb/gdbarch.h
@@ -650,7 +650,12 @@ extern void set_gdbarch_addr_bits_remove (struct gdbarch *gdbarch, gdbarch_addr_
target can single step. If not, then implement single step using breakpoints.

A return value of 1 means that the software_single_step breakpoints
- were inserted; 0 means they were not. */
+ were inserted; 0 means they were not. Multiple breakpoints may be
+ inserted for some instructions such as conditional branch. However,
+ each implementation must always evaluate the condition and only put
+ the breakpoint at the branch destination if the condition is true, so
+ that we ensure forward progress when stepping past a conditional
+ branch to self. */

extern int gdbarch_software_single_step_p (struct gdbarch *gdbarch);

diff --git a/gdb/gdbarch.sh b/gdb/gdbarch.sh
index 37f59b7..c8787c2 100755
--- a/gdb/gdbarch.sh
+++ b/gdb/gdbarch.sh
@@ -609,7 +609,12 @@ m:CORE_ADDR:addr_bits_remove:CORE_ADDR addr:addr::core_addr_identity::0
# target can single step. If not, then implement single step using breakpoints.
#
# A return value of 1 means that the software_single_step breakpoints
-# were inserted; 0 means they were not.
+# were inserted; 0 means they were not. Multiple breakpoints may be
+# inserted for some instructions such as conditional branch. However,
+# each implementation must always evaluate the condition and only put
+# the breakpoint at the branch destination if the condition is true, so
+# that we ensure forward progress when stepping past a conditional
+# branch to self.
F:int:software_single_step:struct frame_info *frame:frame

# Return non-zero if the processor is executing a delay slot and a
diff --git a/gdb/infrun.c b/gdb/infrun.c
index 696105d..1ccd648 100644
--- a/gdb/infrun.c
+++ b/gdb/infrun.c
@@ -1296,6 +1296,9 @@ struct step_over_info
/* The instruction being stepped over triggers a nonsteppable
watchpoint. If true, we'll skip inserting watchpoints. */
int nonsteppable_watchpoint_p;
+
+ /* The thread's global number. */
+ int thread;
};

/* The step-over info of the location that is being stepped over.
@@ -1329,11 +1332,13 @@ static struct step_over_info step_over_info;

static void
set_step_over_info (struct address_space *aspace, CORE_ADDR address,
- int nonsteppable_watchpoint_p)
+ int nonsteppable_watchpoint_p,
+ int thread)
{
step_over_info.aspace = aspace;
step_over_info.address = address;
step_over_info.nonsteppable_watchpoint_p = nonsteppable_watchpoint_p;
+ step_over_info.thread = thread;
}

/* Called when we're not longer stepping over a breakpoint / an
@@ -1348,6 +1353,7 @@ clear_step_over_info (void)
step_over_info.aspace = NULL;
step_over_info.address = 0;
step_over_info.nonsteppable_watchpoint_p = 0;
+ step_over_info.thread = -1;
}

/* See infrun.h. */
@@ -1365,6 +1371,15 @@ stepping_past_instruction_at (struct address_space *aspace,
/* See infrun.h. */

int
+thread_is_being_stepped_over_p (int thread)
+{
+ return (step_over_info.aspace != NULL
+ && thread == step_over_info.thread);
+}
+
+/* See infrun.h. */
+
+int
stepping_past_nonsteppable_watchpoint (void)
{
return step_over_info.nonsteppable_watchpoint_p;
@@ -2579,7 +2594,7 @@ resume (enum gdb_signal sig)
stop_all_threads ();

set_step_over_info (get_regcache_aspace (regcache),
- regcache_read_pc (regcache), 0);
+ regcache_read_pc (regcache), 0, tp->global_num);

step = maybe_software_singlestep (gdbarch, pc);

@@ -7750,10 +7765,11 @@ keep_going_pass_signal (struct execution_control_state *ecs)
&& (remove_wps || !use_displaced_stepping (ecs->event_thread)))
{
set_step_over_info (get_regcache_aspace (regcache),
- regcache_read_pc (regcache), remove_wps);
+ regcache_read_pc (regcache), remove_wps,
+ ecs->event_thread->global_num);
}
else if (remove_wps)
- set_step_over_info (NULL, 0, remove_wps);
+ set_step_over_info (NULL, 0, remove_wps, -1);

/* If we now need to do an in-line step-over, we need to stop
all other threads. Note this must be done before
diff --git a/gdb/infrun.h b/gdb/infrun.h
index 61d3b20..5f98414 100644
--- a/gdb/infrun.h
+++ b/gdb/infrun.h
@@ -133,6 +133,10 @@ extern void insert_step_resume_breakpoint_at_sal (struct gdbarch *,
extern int stepping_past_instruction_at (struct address_space *aspace,
CORE_ADDR address);

+/* Returns true if thread whose thread number is THREAD is being
+ stepped over. */
+extern int thread_is_being_stepped_over_p (int thread);
+
/* Returns true if we're trying to step past an instruction that
triggers a non-steppable watchpoint. */
extern int stepping_past_nonsteppable_watchpoint (void);