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Re: kern/50430: syscall_disestablish() can remove active syscalls

On Mon, 16 Nov 2015, Masao Uebayashi wrote:

Sure, that sounds prudent. It is difficult to fix properly. One way to do
this would be to mark all lwps that have used compat syscalls with a bit
depending on the module they have used, and refuse to unload the module
until the lwp is gone.

- when load a module that has compat syscalls, assign to it a bit.
- mark a flags field of all syscalls that were loaded with that module
  with that bit.
- or the lwp flags with the syscall flags on each syscall.
- when it is time to unload check that no lwp has that bit in the flags set.
- instead of keeping l_sysent, keep l_sysmodflags or something.

What happens if signal handler does longjmp(3) and interrupted syscall
never returns?

Whether or not the interrupted syscall returns, as long as the lwp is
still alive it will prevent the syscall from being disestablished.

In effect, it is a "false positive" but it allows us to err on the side
of caution.  I'd rather have the module remain loaded even if nothing is
currently referencing its resources, rather than have it get unloaded
and then the kernel crashes.

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