[PATCH 2/5] perf/hw_breakpoint: Modify breakpoint even if the new attr has disabled set

[Jiri Olsa]

We need to change the breakpoint even if the attr with
new fields has disabled set to true.

Current code prevents following user code to change
the breakpoint address:

  ptrace(PTRACE_POKEUSER, child, offsetof(struct user, u_debugreg[0]), addr_1)
  ptrace(PTRACE_POKEUSER, child, offsetof(struct user, u_debugreg[0]), addr_2)
  ptrace(PTRACE_POKEUSER, child, offsetof(struct user, u_debugreg[7]), dr7)

The first PTRACE_POKEUSER creates the breakpoint with
attr.disabled set to true:

  ptrace_set_breakpoint_addr(nr = 0)
    struct perf_event *bp = t->ptrace_bps[nr];

    ptrace_register_breakpoint(..., disabled = true)
      ptrace_fill_bp_fields(..., disabled)
      register_user_hw_breakpoint

So the second PTRACE_POKEUSER will be omitted:

  ptrace_set_breakpoint_addr(nr = 0)
    struct perf_event *bp = t->ptrace_bps[nr];
    struct perf_event_attr attr = bp->attr;

    modify_user_hw_breakpoint(bp, &attr)
      if (!attr->disabled)
        modify_user_hw_breakpoint_check

Acked-by: Oleg Nesterov <o...@redhat.com>
Acked-by: Frederic Weisbecker <frede...@kernel.org>
Reported-by: Milind Chabbi <chabbi.mil...@gmail.com>
Link: http://lkml.kernel.org/n/tip-yjhgplc28gk5gfzt7ceoo...@git.kernel.org
Signed-off-by: Jiri Olsa <jo...@kernel.org>
---
 kernel/events/hw_breakpoint.c | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/kernel/events/hw_breakpoint.c b/kernel/events/hw_breakpoint.c
index b3814fce5ecb..fb229d9c7f3c 100644
--- a/kernel/events/hw_breakpoint.c
+++ b/kernel/events/hw_breakpoint.c
@@ -509,6 +509,8 @@ modify_user_hw_breakpoint_check(struct perf_event *bp, 
struct perf_event_attr *a
  */
 int modify_user_hw_breakpoint(struct perf_event *bp, struct perf_event_attr 
*attr)
 {
+       int err;
+
        /*
         * modify_user_hw_breakpoint can be invoked with IRQs disabled and 
hence it
         * will not be possible to raise IPIs that invoke __perf_event_disable.
@@ -520,11 +522,11 @@ int modify_user_hw_breakpoint(struct perf_event *bp, 
struct perf_event_attr *att
        else
                perf_event_disable(bp);
 
-       if (!attr->disabled) {
-               int err = modify_user_hw_breakpoint_check(bp, attr, false);
+       err = modify_user_hw_breakpoint_check(bp, attr, false);
+       if (err)
+               return err;
 
-               if (err)
-                       return err;
+       if (!attr->disabled) {
                perf_event_enable(bp);
                bp->attr.disabled = 0;
        }
-- 
2.17.1