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Sender: Ingo Molnar <mingo.kernel.org@gmail.com>
Date:   Wed, 17 Mar 2021 11:06:40 +0100
From:   Ingo Molnar <mingo@kernel.org>
To:     "Chang S. Bae" <chang.seok.bae@intel.com>
Cc:     bp@suse.de, tglx@linutronix.de, luto@kernel.org, x86@kernel.org,
        len.brown@intel.com, dave.hansen@intel.com, hjl.tools@gmail.com,
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        carlos@redhat.com, tony.luck@intel.com, ravi.v.shankar@intel.com,
        libc-alpha@sourceware.org, linux-arch@vger.kernel.org,
        linux-api@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v7 0/6] x86: Improve Minimum Alternate Stack Size
Message-ID: <20210317100640.GC1724119@gmail.com>
References: <20210316065215.23768-1-chang.seok.bae@intel.com>
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* Chang S. Bae <chang.seok.bae@intel.com> wrote:

> During signal entry, the kernel pushes data onto the normal userspace
> stack. On x86, the data pushed onto the user stack includes XSAVE state,
> which has grown over time as new features and larger registers have been
> added to the architecture.
> 
> MINSIGSTKSZ is a constant provided in the kernel signal.h headers and
> typically distributed in lib-dev(el) packages, e.g. [1]. Its value is
> compiled into programs and is part of the user/kernel ABI. The MINSIGSTKSZ
> constant indicates to userspace how much data the kernel expects to push on
> the user stack, [2][3].
> 
> However, this constant is much too small and does not reflect recent
> additions to the architecture. For instance, when AVX-512 states are in
> use, the signal frame size can be 3.5KB while MINSIGSTKSZ remains 2KB.
> 
> The bug report [4] explains this as an ABI issue. The small MINSIGSTKSZ can
> cause user stack overflow when delivering a signal.

>   uapi: Define the aux vector AT_MINSIGSTKSZ
>   x86/signal: Introduce helpers to get the maximum signal frame size
>   x86/elf: Support a new ELF aux vector AT_MINSIGSTKSZ
>   selftest/sigaltstack: Use the AT_MINSIGSTKSZ aux vector if available
>   x86/signal: Detect and prevent an alternate signal stack overflow
>   selftest/x86/signal: Include test cases for validating sigaltstack

So this looks really complicated, is this justified?

Why not just internally round up sigaltstack size if it's too small? 
This would be more robust, as it would fix applications that use 
MINSIGSTKSZ but don't use the new AT_MINSIGSTKSZ facility.

I.e. does AT_MINSIGSTKSZ have any other uses than avoiding the 
segfault if MINSIGSTKSZ is used to create a small signal stack?

Thanks,

	Ingo