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From:   Florian Weimer <fweimer@redhat.com>
To:     Andy Lutomirski <luto@kernel.org>
Cc:     Yu-cheng Yu <yu-cheng.yu@intel.com>,
        linux-arch <linux-arch@vger.kernel.org>, X86 ML <x86@kernel.org>,
        "H. Peter Anvin" <hpa@zytor.com>,
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        Arnd Bergmann <arnd@arndb.de>,
        Balbir Singh <bsingharora@gmail.com>,
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        Dave Hansen <dave.hansen@linux.intel.com>,
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        Jonathan Corbet <corbet@lwn.net>,
        Kees Cook <keescook@chromium.org>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Nadav Amit <nadav.amit@gmail.com>,
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        Peter Zijlstra <peterz@infradead.org>,
        Randy Dunlap <rdunlap@infradead.org>,
        "Ravi V. Shankar" <ravi.v.shankar@intel.com>,
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        Dave Martin <Dave.Martin@arm.com>,
        Weijiang Yang <weijiang.yang@intel.com>,
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        Haitao Huang <haitao.huang@intel.com>
Subject: Re: extending ucontext (Re: [PATCH v26 25/30] x86/cet/shstk: Handle
 signals for shadow stack)
References: <20210427204315.24153-1-yu-cheng.yu@intel.com>
        <20210427204315.24153-26-yu-cheng.yu@intel.com>
        <CALCETrVTeYfzO-XWh+VwTuKCyPyp-oOMGH=QR_msG9tPQ4xPmA@mail.gmail.com>
Date:   Fri, 30 Apr 2021 08:45:40 +0200
In-Reply-To: <CALCETrVTeYfzO-XWh+VwTuKCyPyp-oOMGH=QR_msG9tPQ4xPmA@mail.gmail.com>
        (Andy Lutomirski's message of "Wed, 28 Apr 2021 16:03:55 -0700")
Message-ID: <87a6pgb78r.fsf@oldenburg.str.redhat.com>
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* Andy Lutomirski:

> The kernel has:
>
> struct rt_sigframe {
>     char __user *pretcode;
>     struct ucontext uc;
>     struct siginfo info;
>     /* fp state follows here */
> };
>
> This is roughly the actual signal frame.  But userspace does not have
> this struct declared, and user code does not know the sizes of the
> fields.  So it's accessed in a nonsensical way.  The signal handler
> function is passed a pointer to the whole sigframe implicitly in RSP,
> a pointer to &frame->info in RSI, anda pointer to &frame->uc in RDX.
> User code can *find* the fp state by following a pointer from
> mcontext, which is, in turn, found via uc:
>
> struct ucontext {
>     unsigned long      uc_flags;
>     struct ucontext  *uc_link;
>     stack_t          uc_stack;
>     struct sigcontext uc_mcontext;  <-- fp pointer is in here
>     sigset_t      uc_sigmask;    /* mask last for extensibility */
> };

I must say that I haven't reviewed this in detail, but for historic
reasons, glibc userspace has a differently-sized sigset_t.  So the
kernel ucontext (used in signals) and user ucontext (used for
swapcontext et al.) are already fully decoupled?

Thanks,
Florian