linux-kernel - Re: [PATCH v2 0/4] kasan, arm64, scs, stacktrace: collect stack traces from Shadow Call Stack

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Date:   Mon, 28 Mar 2022 14:36:22 +0200
From:   Marco Elver <elver@...gle.com>
To:     andrey.konovalov@...ux.dev
Cc:     Alexander Potapenko <glider@...gle.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andrey Konovalov <andreyknvl@...il.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Andrey Ryabinin <ryabinin.a.a@...il.com>,
        kasan-dev@...glegroups.com, Mark Rutland <mark.rutland@....com>,
        Vincenzo Frascino <vincenzo.frascino@....com>,
        Sami Tolvanen <samitolvanen@...gle.com>,
        Peter Collingbourne <pcc@...gle.com>,
        Evgenii Stepanov <eugenis@...gle.com>,
        Florian Mayer <fmayer@...gle.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        Andrey Konovalov <andreyknvl@...gle.com>
Subject: Re: [PATCH v2 0/4] kasan, arm64, scs, stacktrace: collect stack
 traces from Shadow Call Stack

On Wed, 23 Mar 2022 at 16:33, <andrey.konovalov@...ux.dev> wrote:
>
> From: Andrey Konovalov <andreyknvl@...gle.com>
>
> kasan, arm64, scs, stacktrace: collect stack traces from Shadow Call Stack
>
> Currently, KASAN always uses the normal stack trace collection routines,
> which rely on the unwinder, when saving alloc and free stack traces.
>
> Instead of invoking the unwinder, collect the stack trace by copying
> frames from the Shadow Call Stack whenever it is enabled. This reduces
> boot time by 30% for all KASAN modes when Shadow Call Stack is enabled.
>
> Stack staces are collected from the Shadow Call Stack via a new
> stack_trace_save_shadow() interface.
>
> Note that the frame of the interrupted function is not included into
> the stack trace, as it is not yet saved on the SCS when an interrupt
> happens.
>
> ---
>
> To deal with this last thing, we could save the interrupted frame address
> in another per-CPU variable. I'll look into implementing this for v3.
>
> I decided to postpone the changes to stack depot that avoid copying
> frames twice until a planned upcoming update for stack depot.

That's fair.

> Changes v1->v2:
> - Provide a kernel-wide stack_trace_save_shadow() interface for collecting
>   stack traces from shadow stack.
> - Use ptrauth_strip_insn_pac() and READ_ONCE_NOCHECK, see the comments.
> - Get SCS pointer from x18, as per-task value is meant to save the SCS
>   value on CPU switches.
> - Collect stack frames from SDEI and IRQ contexts.

Do any of these new changes introduce new (noticeable) overhead (in
particular patch 2)?