arm64: ptrace: avoid using HW_BREAKPOINT_EMPTY for disabled events

Commit 8f34a1da35ae ("arm64: ptrace: use HW_BREAKPOINT_EMPTY type for
disabled breakpoints") fixed an issue with GDB trying to zero breakpoint
control registers. The problem there is that the arch hw_breakpoint code
will attempt to create a (disabled), execute breakpoint of length 0.

This will fail validation and report unexpected failure to GDB. To avoid
this, we treated disabled breakpoints as HW_BREAKPOINT_EMPTY, but that
seems to have broken with recent kernels, causing watchpoints to be
treated as TYPE_INST in the core code and returning ENOSPC for any
further breakpoints.

This patch fixes the problem by prioritising the `enable' field of the
breakpoint: if it is cleared, we simply update the perf_event_attr to
indicate that the thing is disabled and don't bother changing either the
type or the length. This reinforces the behaviour that the breakpoint
control register is essentially read-only apart from the enable bit
when disabling a breakpoint.

Cc: <stable@vger.kernel.org>
Reported-by: Aaron Liu <liucy214@gmail.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

1 files changed, 18 insertions, 20 deletions

diff --git a/arch/arm64/kernel/ptrace.c b/arch/arm64/kernel/ptrace.c
index 6777a2192b83..6a8928bba03c 100644
--- a/arch/arm64/kernel/ptrace.c
+++ b/arch/arm64/kernel/ptrace.c
@@ -214,31 +214,29 @@ static int ptrace_hbp_fill_attr_ctrl(unsigned int note_type,
 {
         int err, len, type, disabled = !ctrl.enabled;
 
-        if (disabled) {
-                len = 0;
-                type = HW_BREAKPOINT_EMPTY;
-        } else {
-                err = arch_bp_generic_fields(ctrl, &len, &type);
-                if (err)
-                        return err;
-
-                switch (note_type) {
-                case NT_ARM_HW_BREAK:
-                        if ((type & HW_BREAKPOINT_X) != type)
-                                return -EINVAL;
-                        break;
-                case NT_ARM_HW_WATCH:
-                        if ((type & HW_BREAKPOINT_RW) != type)
-                                return -EINVAL;
-                        break;
-                default:
+        attr->disabled = disabled;
+        if (disabled)
+                return 0;
+
+        err = arch_bp_generic_fields(ctrl, &len, &type);
+        if (err)
+                return err;
+
+        switch (note_type) {
+        case NT_ARM_HW_BREAK:
+                if ((type & HW_BREAKPOINT_X) != type)
                         return -EINVAL;
-                }
+                break;
+        case NT_ARM_HW_WATCH:
+                if ((type & HW_BREAKPOINT_RW) != type)
+                        return -EINVAL;
+                break;
+        default:
+                return -EINVAL;
         }
 
         attr->bp_len    = len;
         attr->bp_type   = type;
-        attr->disabled  = disabled;
 
         return 0;
 }