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

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

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
-- 
2.35.3