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From:   Florian Weimer <fw@deneb.enyo.de>
To:     Gabriel Krisman Bertazi <krisman@collabora.com>
Cc:     luto@kernel.org, tglx@linutronix.de, keescook@chromium.org,
        christian.brauner@ubuntu.com, peterz@infradead.org,
        willy@infradead.org, shuah@kernel.org,
        linux-kernel@vger.kernel.org, linux-api@vger.kernel.org,
        linux-kselftest@vger.kernel.org, x86@kernel.org, gofmanp@gmail.com,
        kernel@collabora.com
Subject: Re: [PATCH v7 7/7] docs: Document Syscall User Dispatch
References: <20201118032840.3429268-1-krisman@collabora.com>
        <20201118032840.3429268-8-krisman@collabora.com>
Date:   Wed, 18 Nov 2020 09:48:41 +0100
In-Reply-To: <20201118032840.3429268-8-krisman@collabora.com> (Gabriel Krisman
        Bertazi's message of "Tue, 17 Nov 2020 22:28:40 -0500")
Message-ID: <875z63yrrq.fsf@mid.deneb.enyo.de>
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* Gabriel Krisman Bertazi:

> +Interface
> +---------
> +
> +A process can setup this mechanism on supported kernels
> +CONFIG_SYSCALL_USER_DISPATCH) by executing the following prctl:
> +
> +  prctl(PR_SET_SYSCALL_USER_DISPATCH, <op>, <offset>, <length>, [selector])
> +
> +<op> is either PR_SYS_DISPATCH_ON or PR_SYS_DISPATCH_OFF, to enable and
> +disable the mechanism globally for that thread.  When
> +PR_SYS_DISPATCH_OFF is used, the other fields must be zero.
> +
> +<offset> and <offset+length> delimit a closed memory region interval
> +from which syscalls are always executed directly, regardless of the
> +userspace selector.  This provides a fast path for the C library, which
> +includes the most common syscall dispatchers in the native code
> +applications, and also provides a way for the signal handler to return
> +without triggering a nested SIGSYS on (rt_)sigreturn.  Users of this
> +interface should make sure that at least the signal trampoline code is
> +included in this region. In addition, for syscalls that implement the
> +trampoline code on the vDSO, that trampoline is never intercepted.
> +
> +[selector] is a pointer to a char-sized region in the process memory
> +region, that provides a quick way to enable disable syscall redirection
> +thread-wide, without the need to invoke the kernel directly.  selector
> +can be set to PR_SYS_DISPATCH_ON or PR_SYS_DISPATCH_OFF.  Any other
> +value should terminate the program with a SIGSYS.

Is this a process property or a task/thread property?  The last
paragraph says “thread-wide”, but the first paragraph says “process”.