[PATCH 3.8 097/166] arm64: ptrace: avoid using HW_BREAKPOINT_EMPTY for disabled events

Kamal Mostafa
2014-01-15T21:51:51Z

From: Will Deacon <will.deacon@arm.com>

commit cdc27c27843248ae7eb0df5fc261dd004eaa5670 upstream.

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.

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>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
---
arch/arm64/kernel/ptrace.c | 38 ++++++++++++++++++--------------------
1 file changed, 18 insertions(+), 20 deletions(-)

diff --git a/arch/arm64/kernel/ptrace.c b/arch/arm64/kernel/ptrace.c
index 6e1e77f..5341534 100644
--- a/arch/arm64/kernel/ptrace.c
+++ b/arch/arm64/kernel/ptrace.c
@@ -236,31 +236,29 @@ 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;
}
--
1.8.3.2