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

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

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?

Thanks,
- Tom