Re: [PATCH v7 7/7] docs: Document Syscall User Dispatch

Florian Weimer
2020-11-18T08:55:57Z

* 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”.