Re: x86 watchpoints bug (Re: ping: Re: PATCH : allow to set length of hw watchpoints (e.g. for Valgrind gdbserver))

Pedro Alves
2011-05-31T23:04:50Z

On Tuesday 31 May 2011 23:15:13, Philippe Waroquiers wrote:
>
> > +gdb_test_no_output "set breakpoint always-inserted on"
>
> I confirm that the bug of the DR register kept busy is also present
> on a native GDB debugging.

Thanks. I'm trying a different fix, but I don't think
I'll be able to finish it tonight.

>
> Note the bug is slightly more difficult to trigger without
> the "set breakpoint always-inserted on"
> as it seems to depend on the order in which watchpoints are inserted.
>
> If you just do:
> break s.c:24
> run
> watch s1
> watch s2
> watch s4
> watch s3
> c
> you do not trigger the busy bug as gdb inserts them
> in the order : s3
> s4
> s1
> s2
> and then there is no busy register remaining.
>
> I was able to trigger the bug with the following sequence: s1 s2 s4 s16 (that gdb inserts
> in the order s4 s1 s2 s16. See below the full trace of the bug with this order.
>
> Note that it would be less user confusing if gdb would always insert the watchpoints in the
> order the user entered them. No idea if this is easy to do.
> Of course, the confusion only happens with limited hw watchpoint (with Valgrind gdbserver,
> there will be no confusion :).
> E.g. this is what you can obtain on amd64 with the Valgrind gdbserver (and the patched gdb
> allowing unlimited length):
> (gdb) info watch
> Num Type Disp Enb Address What
> 1 hw watchpoint keep y s1
> 2 hw watchpoint keep y s2
> 3 hw watchpoint keep y s3
> 4 hw watchpoint keep y s4
> 5 hw watchpoint keep y s5
> 6 hw watchpoint keep y s6
> 7 hw watchpoint keep y s7
> 8 hw watchpoint keep y s8
> 9 hw watchpoint keep y s16
> 10 hw watchpoint keep y s32
> 11 hw watchpoint keep y s64
> 12 hw watchpoint keep y s128
> 13 hw watchpoint keep y s1000
> breakpoint already hit 1 time
> (gdb)
> (the above is an advertisement for the Valgrind gdbserver + patched gdb to allow unlimited length :)
>
> Philippe
>
>
> ########################## bug without always-inserted, using s1 s2 s4 s16
> ...
> stopped_data_addr:
> CONTROL (DR7): 0000000000000000 STATUS (DR6): 00000000ffff4ff0
> DR0: addr=0x0000000000000000, ref.count=0 DR1: addr=0x0000000000000000, ref.count=0
> DR2: addr=0x0000000000000000, ref.count=0 DR3: addr=0x0000000000000000, ref.count=0
>
> Breakpoint 1, main () at s.c:24
> 24 p[i] = 1;
> (gdb) watch s1
> Hardware watchpoint 2: s1
> (gdb) watch s2
> Hardware watchpoint 3: s2
> (gdb) watch s4
> Hardware watchpoint 4: s4
> (gdb) watch s16
> Hardware watchpoint 5: s16
> (gdb) c
> Continuing.
> stopped_data_addr:
> CONTROL (DR7): 0000000000000000 STATUS (DR6): 00000000ffff4ff0
> DR0: addr=0x0000000000000000, ref.count=0 DR1: addr=0x0000000000000000, ref.count=0
> DR2: addr=0x0000000000000000, ref.count=0 DR3: addr=0x0000000000000000, ref.count=0
> insert_watchpoint (addr=606ac0, len=4, type=data-write):
> CONTROL (DR7): 00000000000d0101 STATUS (DR6): 00000000ffff4ff0
> DR0: addr=0x0000000000606ac0, ref.count=1 DR1: addr=0x0000000000000000, ref.count=0
> DR2: addr=0x0000000000000000, ref.count=0 DR3: addr=0x0000000000000000, ref.count=0
> insert_watchpoint (addr=609248, len=1, type=data-write):
> CONTROL (DR7): 00000000001d0105 STATUS (DR6): 00000000ffff4ff0
> DR0: addr=0x0000000000606ac0, ref.count=1 DR1: addr=0x0000000000609248, ref.count=1
> DR2: addr=0x0000000000000000, ref.count=0 DR3: addr=0x0000000000000000, ref.count=0
> insert_watchpoint (addr=60d128, len=2, type=data-write):
> CONTROL (DR7): 00000000051d0115 STATUS (DR6): 00000000ffff4ff0
> DR0: addr=0x0000000000606ac0, ref.count=1 DR1: addr=0x0000000000609248, ref.count=1
> DR2: addr=0x000000000060d128, ref.count=1 DR3: addr=0x0000000000000000, ref.count=0
> insert_watchpoint (addr=60d1c0, len=16, type=data-write):
> CONTROL (DR7): 00000000951d0155 STATUS (DR6): 00000000ffff4ff0
> DR0: addr=0x0000000000606ac0, ref.count=1 DR1: addr=0x0000000000609248, ref.count=1
> DR2: addr=0x000000000060d128, ref.count=1 DR3: addr=0x000000000060d1c0, ref.count=1
> Warning:
> Could not insert hardware watchpoint 5.
> Could not insert hardware breakpoints:
> You may have requested too many hardware breakpoints/watchpoints.
>
> remove_watchpoint (addr=606ac0, len=4, type=data-write):
> CONTROL (DR7): 00000000951d0154 STATUS (DR6): 00000000ffff4ff0
> DR0: addr=0x0000000000000000, ref.count=0 DR1: addr=0x0000000000609248, ref.count=1
> DR2: addr=0x000000000060d128, ref.count=1 DR3: addr=0x000000000060d1c0, ref.count=1
> remove_watchpoint (addr=609248, len=1, type=data-write):
> CONTROL (DR7): 00000000951d0150 STATUS (DR6): 00000000ffff4ff0
> DR0: addr=0x0000000000000000, ref.count=0 DR1: addr=0x0000000000000000, ref.count=0
> DR2: addr=0x000000000060d128, ref.count=1 DR3: addr=0x000000000060d1c0, ref.count=1
> remove_watchpoint (addr=60d128, len=2, type=data-write):
> CONTROL (DR7): 00000000951d0140 STATUS (DR6): 00000000ffff4ff0
> DR0: addr=0x0000000000000000, ref.count=0 DR1: addr=0x0000000000000000, ref.count=0
> DR2: addr=0x0000000000000000, ref.count=0 DR3: addr=0x000000000060d1c0, ref.count=1
> 0x0000000000400494 in main () at s.c:24
> 24 p[i] = 1;
> (gdb)
>
>

--
Pedro Alves