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SubjectRe: [PATCH 2/2] ppc64le save_stack_trace_tsk_reliable (Was: HAVE_RELIABLE_STACKTRACE)
On Tue, 27 Feb 2018 17:09:24 +0100
Torsten Duwe <duwe@lst.de> wrote:

> On Tue, Dec 12, 2017 at 01:12:37PM +0100, Miroslav Benes wrote:
> >
> > I think that this is not enough. You need to also implement
> > save_stack_trace_tsk_reliable() for powerpc defined as __weak in
> > kernel/stacktrace.c.
>
> So here is my initial proposal. I'd really like to get the successful
> exit stricter, i.e. hit the initial stack value exactly instead of just
> a window. Also, the check for kernel code looks clumsy IMHO. IOW:
> Comments more than welcome!
>
> Most of it is Copy&Waste, nonetheless:

:)

>
> Signed-off-by: Torsten Duwe <duwe@suse.de>
>
>
> diff --git a/arch/powerpc/kernel/stacktrace.c b/arch/powerpc/kernel/stacktrace.c
> index d534ed901538..e08af49e71d0 100644
> --- a/arch/powerpc/kernel/stacktrace.c
> +++ b/arch/powerpc/kernel/stacktrace.c
> @@ -13,6 +13,7 @@
> #include <linux/export.h>
> #include <linux/sched.h>
> #include <linux/sched/debug.h>
> +#include <linux/sched/task_stack.h>
> #include <linux/stacktrace.h>
> #include <asm/ptrace.h>
> #include <asm/processor.h>
> @@ -76,3 +77,58 @@ save_stack_trace_regs(struct pt_regs *regs, struct stack_trace *trace)
> save_context_stack(trace, regs->gpr[1], current, 0);
> }
> EXPORT_SYMBOL_GPL(save_stack_trace_regs);
> +
> +#ifdef CONFIG_HAVE_RELIABLE_STACKTRACE
> +int
> +save_stack_trace_tsk_reliable(struct task_struct *tsk,
> + struct stack_trace *trace)

Just double checking this is called under the task_rq_lock, so its safe
to call task_stack_page() as opposed to try_get_task_stack()

> +{
> + unsigned long sp;
> + unsigned long stack_page = (unsigned long)task_stack_page(tsk);
> + /* the last frame (unwinding first) may not yet have saved its LR onto the stack. */
> + int firstframe = 1;
> +
> + if (tsk == current)
> + sp = current_stack_pointer();
> + else
> + sp = tsk->thread.ksp;
> +
> + if (sp < stack_page + sizeof(struct thread_struct)
> + || sp > stack_page + THREAD_SIZE - STACK_FRAME_OVERHEAD)
> + return 1;

Some of this is already present in validate_sp(), it also validates
irq stacks, should we just reuse that?

> +
> + for (;;) {
> + unsigned long *stack = (unsigned long *) sp;
> + unsigned long newsp, ip;
> +
> + newsp = stack[0];
> + /* Stack grows downwards; unwinder may only go up */
> + if (newsp <= sp)
> + return 1;
> +
> + if (newsp >= stack_page + THREAD_SIZE)
> + return 1; /* invalid backlink, too far up! */
> +
> + /* Examine the saved LR: it must point into kernel code. */
> + ip = stack[STACK_FRAME_LR_SAVE];
> + if ( (ip & 0xEFFF000000000000) != CONFIG_KERNEL_START
> + && !firstframe)
> + return 1;
> + firstframe = 0;
> +
> + if (!trace->skip)
> + trace->entries[trace->nr_entries++] = ip;
> + else
> + trace->skip--;
> +
> + if (newsp > stack_page + THREAD_SIZE - STACK_FRAME_OVERHEAD)
> + break; /* hit the window for last frame */
> +
> + if (trace->nr_entries >= trace->max_entries)
> + return -E2BIG;
> +
> + sp = newsp;
> + }
> + return 0;
> +}
> +#endif /* CONFIG_HAVE_RELIABLE_STACKTRACE */
>

Looks good to me otherwise.

Acked-by: Balbir Singh <bsingharora@gmail.com>

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