Re: [PATCH 3/3] x86/debug: Fix PTRACE_{BLOCK,SINGLE}STEP vs ptrace_get_debugreg(6)

[Kyle Huey]

On Tue, Oct 27, 2020 at 2:44 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>
> Commit d53d9bc0cf78 ("x86/debug: Change thread.debugreg6 to
> thread.virtual_dr6") changed the semantics of the variable from random
> collection of bits, to exactly only those bits that ptrace() needs.
>
> Unfortunately we lost DR_STEP for PTRACE_{BLOCK,SINGLE}STEP.
>
> Fixes: d53d9bc0cf78 ("x86/debug: Change thread.debugreg6 to thread.virtual_dr6")
> Reported-by: Kyle Huey <me@xxxxxxxxxxxx>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
> ---
>  arch/x86/include/asm/ptrace.h |    2 ++
>  arch/x86/kernel/step.c        |    9 +++++++++
>  arch/x86/kernel/traps.c       |    2 +-
>  3 files changed, 12 insertions(+), 1 deletion(-)
>
> --- a/arch/x86/include/asm/ptrace.h
> +++ b/arch/x86/include/asm/ptrace.h
> @@ -355,6 +355,8 @@ static inline unsigned long regs_get_ker
>  #define arch_has_block_step()  (boot_cpu_data.x86 >= 6)
>  #endif
>
> +extern unsigned long user_dr_step(unsigned long dr6);
> +
>  #define ARCH_HAS_USER_SINGLE_STEP_REPORT
>
>  struct user_desc;
> --- a/arch/x86/kernel/step.c
> +++ b/arch/x86/kernel/step.c
> @@ -235,3 +235,12 @@ void user_disable_single_step(struct tas
>         if (test_and_clear_tsk_thread_flag(child, TIF_FORCED_TF))
>                 task_pt_regs(child)->flags &= ~X86_EFLAGS_TF;
>  }
> +
> +unsigned long user_dr_step(unsigned long dr6)
> +{
> +       if (test_thread_flag(TIF_BLOCKSTEP) ||
> +           test_thread_flag(TIF_SINGLESTEP))
> +               return dr6 & DR_STEP;
> +
> +       return 0;
> +}
> --- a/arch/x86/kernel/traps.c
> +++ b/arch/x86/kernel/traps.c
> @@ -940,7 +940,7 @@ static __always_inline void exc_debug_us
>          * Clear the virtual DR6 value, ptrace() routines will set bits here
>          * for things it wants signals for.
>          */
> -       current->thread.virtual_dr6 = 0;
> +       current->thread.virtual_dr6 = user_dr_step(dr6);
>
>         /*
>          * The SDM says "The processor clears the BTF flag when it
>
>

Tested-by: Kyle Huey <me@xxxxxxxxxxxx>

Confirmed that this patch series fixes rr when applied on top of the
v5.10-rc1 tag.

- Kyle