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

On Sun, 15 Nov 2015, Christos Zoulas wrote:

Sure, that sounds prudent. ...

Also realize that disabling autounload won't prevent manual unloading
of modules via modunload(8).

                     ... 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
- instead of keeping l_sysent, keep l_sysmodflags or something.

It's not only "compat" modules that need to worry about this.  It is
potentially any unloadable module that uses syscall_establish().  (We
could skip allocating/setting the flag for built-in modules, but I
don't think that syscall_establish() is aware of this.)

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