[Bug gdb/28616] New: GDB doesn't rollback displaced stepping state after a failure to insert hardware watchpoints

[Bug gdb/28616] New: GDB doesn't rollback displaced stepping state after a failure to insert hardware watchpoints

[luis.machado at linaro dot org]

https://sourceware.org/bugzilla/show_bug.cgi?id=28616

            Bug ID: 28616
           Summary: GDB doesn't rollback displaced stepping state after a
                    failure to insert hardware watchpoints
           Product: gdb
           Version: HEAD
            Status: NEW
          Severity: critical
          Priority: P2
         Component: gdb
          Assignee: unassigned at sourceware dot org
          Reporter: luis.machado at linaro dot org
                CC: luis.machado at linaro dot org
  Target Milestone: 12.1
              Host: x86_64-linux-gnu
            Target: x86_64-linux-gnu
             Build: x86_64-linux-gnu

I first noticed this one on aarch64-linux-gnu, but it also reproduces on
x86_64-linux-gnu. It is very likely this will reproduce on any native targets
that implement/override the low_prepare_to_resume method and throw an error
from it.

Follows a reproducer. The binary can be any dummy binary with a main function
(I used a hello world).

---

./gdb/gdb /tmp/hello -ex "start" -ex "b" -ex "watch *0xffffffffdeadbeef"
GNU gdb (GDB) 12.0.50.20211119-git
Copyright (C) 2021 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.
Type "show copying" and "show warranty" for details.
This GDB was configured as "x86_64-pc-linux-gnu".
Type "show configuration" for configuration details.
For bug reporting instructions, please see:
<https://www.gnu.org/software/gdb/bugs/>.
Find the GDB manual and other documentation resources online at:
    <http://www.gnu.org/software/gdb/documentation/>.

For help, type "help".
Type "apropos word" to search for commands related to "word"...
Reading symbols from /tmp/hello...
Temporary breakpoint 1 at 0x115c: file /tmp/hello.c, line 5.
Starting program: /tmp/hello 

Temporary breakpoint 1, main (argc=1, argv=0x7fffffffe558) at /tmp/hello.c:5
5         printf ("Hello world!\n");
Breakpoint 2 at 0x55555555515c: file /tmp/hello.c, line 5.
Hardware watchpoint 3: *0xffffffffdeadbeef
(gdb) c
Continuing.
Couldn't write debug register: Invalid argument.
(gdb) bt
#0  0x0000555555555062 in _start ()
(gdb) 

---

So there are a few things going on:

1 - We are stopped on top of a breakpoint.
2 - Moving the program means we need to step over such a breakpoint.
3 - The target supports displaced stepping, and so stepping over the breakpoint
means doing a displaced stepping.
4 - We have an impossible hardware watchpoint that will trigger an error when
we attempt to move the program and thus write the proper debug register
information.
5 - The code in infrun.c:keep_going_pass_signal, more specifically when it
calls "resume", is not guarded against faults. "resume_1" is guarded, but
doesn't handle this situation properly like infrun.c:keep_going_pass_signal
does when calling "insert_breakpoints".

As shown above, GDB gets an unexpected error from the low_prepare_to_resume
method, goes through infrun.c:resume_1's catch block and then re-throws the
exception. So GDB never attempts to rollback the state of displaced stepping,
leaving the PC pointing at the scratch pad location.

If we disable displaced stepping, we have the following:

---
./gdb/gdb /tmp/hello -ex "set displaced-stepping off" -ex "start" -ex "b" -ex
"watch *0xffffffffdeadbeef"
GNU gdb (GDB) 12.0.50.20211119-git
Copyright (C) 2021 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.
Type "show copying" and "show warranty" for details.
This GDB was configured as "x86_64-pc-linux-gnu".
Type "show configuration" for configuration details.
For bug reporting instructions, please see:
<https://www.gnu.org/software/gdb/bugs/>.
Find the GDB manual and other documentation resources online at:
    <http://www.gnu.org/software/gdb/documentation/>.

For help, type "help".
Type "apropos word" to search for commands related to "word"...
Reading symbols from /tmp/hello...
Temporary breakpoint 1 at 0x115c: file /tmp/hello.c, line 5.
Starting program: /tmp/hello 

Temporary breakpoint 1, main (argc=1, argv=0x7fffffffe558) at /tmp/hello.c:5
5         printf ("Hello world!\n");
Breakpoint 2 at 0x55555555515c: file /tmp/hello.c, line 5.
Hardware watchpoint 3: *0xffffffffdeadbeef
(gdb) c
Continuing.
Couldn't write debug register: Invalid argument.
(gdb) bt
#0  main (argc=1, argv=0x7fffffffe558) at /tmp/hello.c:5
(gdb) 
---

I thought about this a little, and adding a displaced_step_finish call in one
of the catch blocks of either resume/resume_1 seems to work. But I haven't done
further testing with it. It didn't seem to have any regressions.

The documentation of low_prepare_to_resume doesn't mention anything about
implementations being forbidden to throw, so I guess this is an oversight.

-- 
You are receiving this mail because:
You are on the CC list for the bug.

[Bug gdb/28616] GDB doesn't rollback displaced stepping state after a failure to insert hardware watchpoints

[brobecker at gnat dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=28616

Joel Brobecker <brobecker at gnat dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |brobecker at gnat dot com

--- Comment #1 from Joel Brobecker <brobecker at gnat dot com> ---
Hi Luis,

IIUC, I think you set the target milestone for this PR to 12.1. As a
side-effect, this is taken as indicating this bug needs to be fixed before we
can release GDB 12. Was that your intention, and if yes, can you provide more
details why?

-- 
You are receiving this mail because:
You are on the CC list for the bug.

[Bug gdb/28616] GDB doesn't rollback displaced stepping state after a failure to insert hardware watchpoints

[brobecker at gnat dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=28616

Joel Brobecker <brobecker at gnat dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|12.1                        |---

--- Comment #2 from Joel Brobecker <brobecker at gnat dot com> ---
As discussed on gdb-patches: Removing target milestone date, to reflect the
fact that this is not deemed release-blocking.

-- 
You are receiving this mail because:
You are on the CC list for the bug.

[Bug gdb/28616] GDB doesn't rollback displaced stepping state after a failure to insert hardware watchpoints

[luis.machado at arm dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=28616

--- Comment #3 from Luis Machado <luis.machado at arm dot com> ---
I dug this up and noticed I did send a patch to the ML, but unfortunately I
received no feedback on it and it went to patch limbo. :-)

I'll pursue it again.

https://sourceware.org/pipermail/gdb-patches/2021-June/179691.html

-- 
You are receiving this mail because:
You are on the CC list for the bug.

[Bug gdb/28616] GDB doesn't rollback displaced stepping state after a failure to insert hardware watchpoints

[luis.machado at arm dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=28616

--- Comment #4 from Luis Machado <luis.machado at arm dot com> ---
Ah, nevermind. It was a similar issue. The one I posted was an earlier
displaced-stepping-related bug as well.

-- 
You are receiving this mail because:
You are on the CC list for the bug.

[Bug gdb/28616] GDB doesn't rollback displaced stepping state after a failure to insert hardware watchpoints

[luis.machado at arm dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=28616

Luis Machado <luis.machado at arm dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Assignee|unassigned at sourceware dot org   |luis.machado at arm dot com

-- 
You are receiving this mail because:
You are on the CC list for the bug.

[Bug gdb/28616] GDB doesn't rollback displaced stepping state after a failure to insert hardware watchpoints

[luis.machado at arm dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=28616

Luis Machado <luis.machado at arm dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Severity|critical                    |normal

-- 
You are receiving this mail because:
You are on the CC list for the bug.