Re: RFC: userspace exception fixups

[Linus Torvalds]

On Thu, Nov 1, 2018 at 11:52 AM Rich Felker <dalias@xxxxxxxx> wrote:
>
> There's no need to chain if the handler is specific to the context
> where the fault happens. You just replace the handler with the one
> relevant to the code you're about to run before you run it.

That's much too expensive to do as a system call.

Maybe an rseq-like "register an area where exception information will
be found" and then you can just swap in a pointer there (and nest with
previous pointers).

But even that doesn't work. Maybe some library wants to capture page
faults because they write-protected some area and want to log writes
and then emulate them (or just enable them after logging - statistical
logging is a thing).

And then another library (or just nested code) wants to handle the
eenter fault, so it overwrites the page handler fault. What do you do
if you now get a page fault before you even do the eenter?

The whole "one global error handler" model is broken. It's broken even
if the "global" one is just per-thread. Don't do it.

Even signals didn't make *that* bad a mistake, and signals are horrible.

                        Linus