linux-kernel - Re: [PATCH v4 07/17] scs: add support for stack usage debugging

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Message-ID: <CABCJKueoJs7hS7VrVoz6CY_oAjTGcV-W61v9GAdwg+zk0W5ErA@mail.gmail.com>
Date:   Mon, 4 Nov 2019 13:35:28 -0800
From:   Sami Tolvanen <samitolvanen@...gle.com>
To:     Mark Rutland <mark.rutland@....com>
Cc:     Will Deacon <will@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Dave Martin <Dave.Martin@....com>,
        Kees Cook <keescook@...omium.org>,
        Laura Abbott <labbott@...hat.com>,
        Marc Zyngier <maz@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Jann Horn <jannh@...gle.com>,
        Miguel Ojeda <miguel.ojeda.sandonis@...il.com>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v4 07/17] scs: add support for stack usage debugging

On Mon, Nov 4, 2019 at 4:40 AM Mark Rutland <mark.rutland@....com> wrote:
> > +#ifdef CONFIG_DEBUG_STACK_USAGE
> > +static inline unsigned long scs_used(struct task_struct *tsk)
> > +{
> > +     unsigned long *p = __scs_base(tsk);
> > +     unsigned long *end = scs_magic(tsk);
> > +     uintptr_t s = (uintptr_t)p;
>
> As previously, please use unsigned long for consistency.

Ack.

> > +     while (p < end && *p)
> > +             p++;
>
> I think this is the only place where we legtimately access the shadow
> call stack directly.

There's also scs_corrupted, which checks that the end magic is intact.

> When using SCS and KASAN, are the
> compiler-generated accesses to the SCS instrumented?
>
> If not, it might make sense to make this:
>
>         while (p < end && READ_ONCE_NOCKECK(*p))
>
> ... and poison the allocation from KASAN's PoV, so that we can find
> unintentional accesses more easily.

Sure, that makes sense. I can poison the allocation for the
non-vmalloc case, I'll just need to refactor scs_set_magic to happen
before the poisoning.

Sami