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Date:   Fri, 13 Nov 2020 17:47:01 -0600
From:   Segher Boessenkool <segher@kernel.crashing.org>
To:     Nick Desaulniers <ndesaulniers@google.com>
Cc:     Alexandre Chartre <alexandre.chartre@oracle.com>,
        kbuild-all@lists.01.org,
        clang-built-linux <clang-built-linux@googlegroups.com>,
        linux-toolchains@vger.kernel.org,
        kernel test robot <lkp@intel.com>,
        Arvind Sankar <nivedita@alum.mit.edu>,
        Ard Biesheuvel <ardb@kernel.org>,
        Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>
Subject: Re: [RFC][PATCH 21/24] x86/entry: Disable stack-protector for IST entry C handlers
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Hi!

On Fri, Nov 13, 2020 at 10:59:26AM -0800, Nick Desaulniers wrote:
> The `optimize` attribute is both non-portable across toolchains (hence
> this warning)

Like *all* GCC extensions.

> and a little quirky in GCC.

How so?  Don't spread FUD please, say what *is* wrong, then people can
decide for themselves whether they want it or not.

We (GCC) do document it as:

     Not every optimization option that starts with the -f prefix
     specified by the attribute necessarily has an effect on the
     function.  The 'optimize' attribute should be used for debugging
     purposes only.  It is not suitable in production code.

The optimize attribute is for setting/resetting flags on a function
granularity.  Not all flags can be flipped per function.  The SSP flags
work fine though, AFAIK.  But don't use it for production, there are no
guarantees.

Cheers,


Segher