[gdb/tdep] Fix gdb.base/watch-bitfields.exp on aarch64

On aarch64-linux, with test-case gdb.base/watch-bitfields.exp I run into:
...
(gdb) continue^M
Continuing.^M
^M
Hardware watchpoint 2: -location q.a^M
^M
Old value = 1^M
New value = 0^M
main () at watch-bitfields.c:42^M
42        q.h--;^M
(gdb) FAIL: $exp: -location watch against bitfields: q.e: 0->5: continue
...

In a minimal form, if we step past line 37 which sets q.e, and we have a
watchpoint set on q.e, it triggers:
...
$ gdb -q -batch watch-bitfields -ex "b 37" -ex run -ex "watch q.e" -ex step
Breakpoint 1 at 0x410204: file watch-bitfields.c, line 37.

Breakpoint 1, main () at watch-bitfields.c:37
37        q.e = 5;
Hardware watchpoint 2: q.e

Hardware watchpoint 2: q.e

Old value = 0
New value = 5
main () at /home/vries/gdb/src/gdb/testsuite/gdb.base/watch-bitfields.c:38
38        q.f = 6;
...

However, if we set in addition a watchpoint on q.a, the watchpoint on q.e
doesn't trigger.

How does this happen?

Bitfield q.a is just bit 0 of byte 0, and bitfield q.e is bit 4..7 of byte 1
and bit 1 of byte 2.  So, watch q.a should watch byte 0, and watch q.e should
watch bytes 1 and 2.

Using "maint set show-debug-regs on" (and some more detailed debug prints) we
get:
...
WP2: addr=0x440028 (orig=0x440029), ctrl=0x000000d5, ref.count=1
  ctrl: enabled=1, offset=1, len=2
WP3: addr=0x440028 (orig=0x440028), ctrl=0x00000035, ref.count=1
  ctrl: enabled=1, offset=0, len=1
...
which matches that.

When executing line 37, a hardware watchpoint trap triggers and we hit
aarch64_stopped_data_address with addr_trap == 0x440028:
...
(gdb) p /x addr_trap
$1 = 0x440028
....
and since the loop in aarch64_stopped_data_address walks backward, we check
WP3 first, which matches, and consequently target_stopped_by_watchpoint
returns true in watchpoints_triggered.

Likewise for target_stopped_data_address, which also returns addr == 0x440028.
Watchpoints_triggered matches watchpoint q.a to that address, and sets
watch_triggered_yes.

However, subsequently the value of q.a is checked, and it's the same value as
before (becase the insn in line 37 didn't change q.a), so the watchpoint
hardware trap is not reported to the user.

The problem originates from that fact that aarch64_stopped_data_address picked
WP3 instead of WP2.

There's something we can do about this.  In the example above, both
target_stopped_by_watchpoint and target_stopped_data_address returned true.
Instead we can return true in target_stopped_by_watchpoint but false in
target_stopped_data_address.  This lets watchpoints_triggered known that a
watchpoint was triggered, but we don't know where, and both watchpoints
get set to watch_triggered_unknown.

Subsequently, the values of both q.a and q.e are checked, and since q.e is not
the same value as before, the watchpoint hardware trap is reported to the user.

Note that this works well for regular (write) watchpoints (watch command), but
not for read watchpoints (rwatch command), because for those no value is
checked.  Likewise for access watchpoints (awatch command).

So, fix this by:
- passing a nullptr in aarch64_fbsd_nat_target::stopped_by_watchpoint and
  aarch64_linux_nat_target::stopped_by_watchpoint to make clear we're not
  interested in the stop address,
- introducing a two-phase approach in aarch64_stopped_data_address, where:
  - phase one handles access and read watchpoints, as before, and
  - phase two handles write watchpoints, where multiple matches cause:
    - return true if addr_p == null, and
    - return false if addr_p != null.

Tested on aarch64-linux.

Approved-By: Luis Machado <luis.machado@arm.com>

PR tdep/31214
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=31214

2 files changed, 27 insertions, 0 deletions

diff --git a/gdb/nat/aarch64-hw-point.c b/gdb/nat/aarch64-hw-point.c
index 04674de..08fd230 100644
--- a/gdb/nat/aarch64-hw-point.c
+++ b/gdb/nat/aarch64-hw-point.c
@@ -73,6 +73,31 @@ aarch64_watchpoint_length (unsigned int ctrl)
   return retval;
 }
 
+/* Utility function that returns the type of a watchpoint according to the
+   content of a hardware debug control register CTRL.  */
+
+enum target_hw_bp_type
+aarch64_watchpoint_type (unsigned int ctrl)
+{
+  unsigned int type = DR_CONTROL_TYPE (ctrl);
+
+  switch (type)
+    {
+    case 1:
+      return hw_read;
+    case 2:
+      return hw_write;
+    case 3:
+      return hw_access;
+    case 0:
+      /* Reserved for a watchpoint.  It must behave as if the watchpoint is
+	 disabled.  */
+      return hw_execute;
+    default:
+      gdb_assert_not_reached ("");
+    }
+}
+
 /* Given the hardware breakpoint or watchpoint type TYPE and its
    length LEN, return the expected encoding for a hardware
    breakpoint/watchpoint control register.  */
diff --git a/gdb/nat/aarch64-hw-point.h b/gdb/nat/aarch64-hw-point.h
index 70f71db..bdcca93 100644
--- a/gdb/nat/aarch64-hw-point.h
+++ b/gdb/nat/aarch64-hw-point.h
@@ -73,6 +73,7 @@
 
 #define DR_CONTROL_ENABLED(ctrl)	(((ctrl) & 0x1) == 1)
 #define DR_CONTROL_MASK(ctrl)		(((ctrl) >> 5) & 0xff)
+#define DR_CONTROL_TYPE(ctrl)		(((ctrl) >> 3) & 0x3)
 
 /* Structure for managing the hardware breakpoint/watchpoint resources.
    DR_ADDR_* stores the address, DR_CTRL_* stores the control register
@@ -107,6 +108,7 @@ void aarch64_notify_debug_reg_change (ptid_t ptid, int is_watchpoint,
 
 unsigned int aarch64_watchpoint_offset (unsigned int ctrl);
 unsigned int aarch64_watchpoint_length (unsigned int ctrl);
+enum target_hw_bp_type aarch64_watchpoint_type (unsigned int ctrl);
 
 int aarch64_handle_breakpoint (enum target_hw_bp_type type, CORE_ADDR addr,
 			       int len, int is_insert, ptid_t ptid,