[Bug nptl/14744] kill -32 $pid or kill -33 $pid on a process cancels a random thread

["bugdal at aerifal dot cx" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=14744

--- Comment #5 from Rich Felker <bugdal at aerifal dot cx> ---
Allowing cancellation from other applications is certainly not desirable, any
moreso than it would be desirable for one process to call free() on a pointer
malloc'd by another process. And anyway, pthread_t is only meaningful within
the context of the process it belongs to.

Technically you could use the tgkill syscall directly to intentionally send a
cancellation signal to a particular target thread in another process (this
would be less insane than sending it to the process and having the kernel
randomly deliver it to one thread). Perhaps the original intent of the code was
to allow something like this, where users might attempt to cancel a stuck
thread as a way to get a process going again. This seems like a really bad idea
though (most apps don't use or support cancellation, and cancelling a thread
that's not written to be cancelled could cause dangerous misbehavior). If
someone really wants to go to that kind of extent to try to "fix" a hung
application, gdb is the right tool, and in fact you can use gdb to call
pthread_cancel on a particular thread or make the thread itself call
pthread_exit.

So, I really don't think there's any sense in trying to allow cancelling
threads from outside the process.

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