Re: [PATCH] [gdb/tdep] Fix gdb.base/watchpoint-running on {arm, ppc64le}-linux

[Tom de Vries <tdevries@suse.de>]

On 6/7/24 14:05, Tom de Vries wrote:
> On 6/7/24 12:18, Luis Machado wrote:
>> On 6/7/24 07:35, Tom de Vries wrote:
>>> When running test-case gdb.base/watchpoint-running on ppc64le-linux, 
>>> we get:
>>> ...
>>> (gdb) watch global_var^M
>>> warning: Error when detecting the debug register interface. \
>>>    Debug registers will be unavailable.^M
>>> Watchpoint 2: global_var^M
>>> (gdb) FAIL: $exp: all-stop: hardware: watch global_var
>>> FAIL: $exp: all-stop: hardware: watchpoint hit (timeout)
>>> ...
>>>
>>> The problem is that ppc_linux_dreg_interface::detect fails to detect the
>>> hardware watchpoint interface, because the calls to ptrace return 
>>> with errno
>>> set to ESRCH.
>>>
>>> This is a feature of ptrace: if a call is done while the tracee is not
>>> ptrace-stopped, it returns ESRCH.
>>>
>>> Indeed, in the test-case "watch global_var" is executed while the 
>>> inferior is
>>> running, and that triggers the first call to 
>>> ppc_linux_dreg_interface::detect.
>>>
>>> And because the detection failure is cached, subsequent attempts at 
>>> setting
>>> hardware watchpoints will also fail, even if the tracee is 
>>> ptrace-stopped.
>>>
>>> Fix this by calling target_can_use_hardware_watchpoint from
>>> linux_init_ptrace_procfs, which is called from both:
>>> - linux_nat_target::post_attach, and
>>> - linux_nat_target::post_startup_inferior.
>>>
>>> By fixing this here, we also fix the same problem for arm-linux.
>>>
>>> Tested on ppc64le-linux and arm-linux.
>>>
>>> PR tdep/31834
>>> Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=31834
>>> PR tdep/31705
>>> Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=31705
>>> ---
>>>   gdb/linux-nat.c | 12 ++++++++++++
>>>   1 file changed, 12 insertions(+)
>>>
>>> diff --git a/gdb/linux-nat.c b/gdb/linux-nat.c
>>> index c95d420d416..d8b5a99269b 100644
>>> --- a/gdb/linux-nat.c
>>> +++ b/gdb/linux-nat.c
>>> @@ -454,6 +454,18 @@ linux_init_ptrace_procfs (pid_t pid, int attached)
>>>     linux_ptrace_init_warnings ();
>>>     linux_proc_init_warnings ();
>>>     proc_mem_file_is_writable ();
>>> +
>>> +  /* Some targets (for instance ppc and arm) may call ptrace to 
>>> answer a
>>> +     target_can_use_hardware_watchpoint query, and cache the 
>>> result.  However,
>>> +     the ptrace call will fail with errno ESRCH if the tracee is not
>>> +     ptrace-stopped, making the query fail.  And if the caching 
>>> mechanism does
>>> +     not disregard an ESRCH result, all subsequent queries will also 
>>> fail.
>>> +     Call it now, where we known the tracee is ptrace-stopped.
>>> +
>>> +     Other targets (for instance aarch64) do the relevant ptrace 
>>> call and
>>> +     caching in their implementation of post_attach and 
>>> post_startup_inferior,
>>> +     in which case this call is expected to have no effect.  */
>>> +  target_can_use_hardware_watchpoint (bp_hardware_watchpoint, 1, 0);
>>>   }
>>>   linux_nat_target::~linux_nat_target ()
>>>
>>> base-commit: f9478936896ada7786e8d68622f6e6ff78b97b0d
>>
>> Looks good from arm-linux's side. Thanks!
>>
>> Reviewed-By: Luis Machado <luis.machado@arm.com>
>> Tested-By: Luis Machado <luis.machado@arm.com>
> 
> Luis, thanks for the review and the testing.
> 
> Pedro, since you reviewed a target-specific patch for the arm PR, any 
> comments?

And just to mention this, I'd like to backport this fix to the 
gdb-15-branch.

Thanks,
- Tom