Re: [PATCH v2 6/6] x86: Fix thread_saved_pc()

Josh Poimboeuf
2016-06-20T16:04:16Z

On Sat, Jun 18, 2016 at 04:56:18PM -0400, Brian Gerst wrote:
> thread_saved_pc() was using a completely bogus method to get the return
> address. Since switch_to() was previously inlined, there was no sane way
> to know where on the stack the return address was stored. Now with the
> frame of a sleeping thread well defined, this can be implemented correctly.
>
> Signed-off-by: Brian Gerst <brgerst@gmail.com>
> ---
> arch/x86/include/asm/processor.h | 10 ++--------
> arch/x86/kernel/process.c | 10 ++++++++++
> arch/x86/kernel/process_32.c | 8 --------
> 3 files changed, 12 insertions(+), 16 deletions(-)
>
> diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h
> index 1e7d634..413f4f1 100644
> --- a/arch/x86/include/asm/processor.h
> +++ b/arch/x86/include/asm/processor.h
> @@ -716,8 +716,6 @@ static inline void spin_lock_prefetch(const void *x)
> .io_bitmap_ptr = NULL, \
> }
>
> -extern unsigned long thread_saved_pc(struct task_struct *tsk);
> -
> /*
> * TOP_OF_KERNEL_STACK_PADDING reserves 8 bytes on top of the ring0 stack.
> * This is necessary to guarantee that the entire "struct pt_regs"
> @@ -767,17 +765,13 @@ extern unsigned long thread_saved_pc(struct task_struct *tsk);
> .sp0 = TOP_OF_INIT_STACK \
> }
>
> -/*
> - * Return saved PC of a blocked thread.
> - * What is this good for? it will be always the scheduler or ret_from_fork.
> - */
> -#define thread_saved_pc(t) READ_ONCE_NOCHECK(*(unsigned long *)((t)->thread.sp - 8))
> -
> #define task_pt_regs(tsk) ((struct pt_regs *)(tsk)->thread.sp0 - 1)
> extern unsigned long KSTK_ESP(struct task_struct *task);
>
> #endif /* CONFIG_X86_64 */
>
> +extern unsigned long thread_saved_pc(struct task_struct *tsk);
> +
> extern void start_thread(struct pt_regs *regs, unsigned long new_ip,
> unsigned long new_sp);
>
> diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
> index 00ebab0..db458c4 100644
> --- a/arch/x86/kernel/process.c
> +++ b/arch/x86/kernel/process.c
> @@ -513,6 +513,16 @@ unsigned long arch_randomize_brk(struct mm_struct *mm)
> }
>
> /*
> + * Return saved PC of a blocked thread.
> + */
> +unsigned long thread_saved_pc(struct task_struct *tsk)
> +{
> + struct inactive_task_frame *frame =
> + (struct inactive_task_frame *) READ_ONCE(tsk->thread.sp);
> + return READ_ONCE_NOCHECK(frame->ret_addr);
> +}
> +
> +/*

I would agree with the above (removed) comment:

"What is this good for? it will be always the scheduler or ret_from_fork."

And I'd guess the same is true for all the arches which have to
implement it. Maybe this function (and its single call site in
sched_show_task()) should just be removed altogether?

--
Josh