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SubjectRe: [RFC][PATCH 1/2] x86: Allow breakpoints to emulate call functions
On Tue, 7 May 2019 21:50:52 -0700
Linus Torvalds <torvalds@linux-foundation.org> wrote:

> > It's been a bane of mine for some time.
>
> Guys, I have basically a one-liner patch for your hangups.
>
> It's called "rename 'sp' to 'user_sp' on x86-32".
>
> Then we make the 'sp' field on x86-64 be a union, so that you can call
> it user_sp or sp as you wish.
>
> Yeah, it's really more than one line, because obviously the users will
> need chaning, but honestly, that would be a _real_ cleanup. Make the
> register match what it actually is.

But is it? Sure, it will be a reminder that it's different for x86-32,
but that still doesn't take away the fact that pt_regs on x86_32 is an
anomaly! Where else do we have part of a data structure that can't be
read because it can possibly fault? If regs is a valid pointer, one
would think that simply reading regs->sp (or regs->user_sp) would be no
more dangerous than reading regs->ip.

The difference between entry_32.S from entry_64.S causes it to spill
into C code, making the x86_64 code more difficult to deal with. Sure,
10 to 15 years ago, all your arguments would make sense. But today, who
uses x86_32? Perhaps you may use it in a VM, but I asked a few
developers when was the last time they used one, they told me 5 to 7
years ago. I only boot x86_32 to test to make sure I didn't break it.

Yes, your diffstat is really nice to the changes to entry_32.S, but at
what cost? To make the x86_64 code more complex? That whole returning
the regs in the int3 handler makes no sense on x86_64, but yet we would
need to do it to handle x86_32. Why burden the architecture of today
and tomorrow with the architecture of yesterday? x86_32 is becoming
more obsolete by the day. It baffles me why we wouldn't want to contain
its complexity in a single file then to spread it out like wildfire
across the generic x86 code.

The &regs->sp hack is just one more rung in the complex learning curve
ladder of becoming a Linux kernel developer.

-- Steve

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