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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Wed, 16 Jun 2021 13:00:52 -0700
Message-ID: <CAHk-=wiTEZN_3ipf51sh-csdusW4uGzAXq9m1JcMHu_c8OJ+pQ@mail.gmail.com>
Subject: Re: [PATCH 1/2] alpha/ptrace: Record and handle the absence of switch_stack
To:     "Eric W. Biederman" <ebiederm@xmission.com>
Cc:     Michael Schmitz <schmitzmic@gmail.com>,
        linux-arch <linux-arch@vger.kernel.org>,
        Jens Axboe <axboe@kernel.dk>, Oleg Nesterov <oleg@redhat.com>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Richard Henderson <rth@twiddle.net>,
        Ivan Kokshaysky <ink@jurassic.park.msu.ru>,
        Matt Turner <mattst88@gmail.com>,
        alpha <linux-alpha@vger.kernel.org>,
        Geert Uytterhoeven <geert@linux-m68k.org>,
        linux-m68k <linux-m68k@lists.linux-m68k.org>,
        Arnd Bergmann <arnd@kernel.org>,
        Ley Foon Tan <ley.foon.tan@intel.com>,
        Tejun Heo <tj@kernel.org>, Kees Cook <keescook@chromium.org>
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On Wed, Jun 16, 2021 at 11:32 AM Eric W. Biederman
<ebiederm@xmission.com> wrote:
>
> Prevent security holes by recording when all of the registers are
> available so generic code changes do not result in security holes
> on alpha.

Please no, not this way. ldl/stc is extremely expensive on some alpha cpus.

I really think thatTIF_ALLREGS_SAVED bit isn't worth it, except
perhaps for debugging.

And even for debugging, I think it would be both easier and cheaper to
just add a magic word to the entry stack instead.

              Linus