Florian Weimer fw@deneb.enyo.de writes:
- 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”.
It is per-thread, as it doesn't survive across clone/fork syscalls. I will fix the first paragraph of this text.