Re: [patch 08/10] x86/entry/32: Remove the 0/-1 distinction from exception entries

[Andy Lutomirski]

On 2/25/20 1:36 PM, Thomas Gleixner wrote:
> Nothing cares about the -1 "mark as interrupt" in the errorcode anymore. Just
> use 0 for all excpetions which do not have an errorcode consistently.
> 

I sincerely wish this were the case.  But look at collect_syscall() in
lib/syscall.c.

It would be really quite nice to address this for real in some
low-overhead way.  My suggestion would be to borrow a trick from 32-bit:
split regs->cs into ->cs and ->__csh, and stick CS_FROM_SYSCALL into
__csh for syscalls.  This will only add any overhead at all to the int80
case.  Then we could adjust syscall_get_nr() to look for CS_FROM_SYSCALL.

What do you think?  An alternative would be to use the stack walking
machinery in collect_syscall(), since the mere existence of that
function is abomination and we may not care about performance.

--Andy