linux-kernel - Re: [PATCH v2] riscv: VMAP_STACK overflow detection thread-safe

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Message-ID: <CABCJKueNhc8qCbZbHJqdCB+PHHy0u5ETP4uWfpWBRaOMX6U6hA@mail.gmail.com>
Date:   Mon, 24 Jul 2023 09:34:04 -0700
From:   Sami Tolvanen <samitolvanen@...gle.com>
To:     Guo Ren <guoren@...nel.org>
Cc:     Deepak Gupta <debug@...osinc.com>, palmer@...belt.com,
        linux-kernel@...r.kernel.org, linux-riscv@...ts.infradead.org,
        Jisheng Zhang <jszhang@...nel.org>
Subject: Re: [PATCH v2] riscv: VMAP_STACK overflow detection thread-safe

On Thu, Jul 20, 2023 at 8:06 AM Guo Ren <guoren@...nel.org> wrote:
>
> On Thu, Jul 20, 2023 at 8:19 AM Sami Tolvanen <samitolvanen@...gle.com> wrote:
> >
> > Are you planning on resending this patch? I see it didn't gain much
> > traction last time, but this looks like a much cleaner solution for
> > selecting the overflow stack than having a `shadow_stack` and calling
> > to C to compute the per-CPU offset. The asm_per_cpu macro also would
> > come in handy when implementing CONFIG_SHADOW_CALL_STACK, which we'd
> > like to have on RISC-V too.
> I remember we ended up with an atomic lock mechanism instead of percpu
> offset, so what's the benefit of percpu style in overflow_stack path?

The benefit is not needing a separate temporary stack and locks just
to compute the per-CPU offset. With CONFIG_SHADOW_CALL_STACK, we would
also need a "shadow" shadow call stack in this case before calling to
C code, at which point computing the offsets directly in assembly is
just significantly cleaner and without concurrency issues.

Sami