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

Florian Weimer [off-list ref] writes:

* Gabriel Krisman Bertazi:

quoted

+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”.

It is per-thread, as it doesn't survive across clone/fork syscalls.  I
will fix the first paragraph of this text.

-- 
Gabriel Krisman Bertazi