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

[Luis Machado <luis.machado@arm.com>]

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>