Re: CVS commit: src/sys/arch/amd64/amd64

[Maxime Villard <max%m00nbsd.net@localhost>]

Le 22/06/2019 à 22:29, maya%netbsd.org@localhost a écrit :
> On Fri, Dec 01, 2017 at 09:22:45PM +0000, Maxime Villard wrote:
> > Module Name:	src
> > Committed By:	maxv
> > Date:		Fri Dec  1 21:22:45 UTC 2017
> >
> > Modified Files:
> > 	src/sys/arch/amd64/amd64: machdep.c process_machdep.c
> >
> > Log Message:
> > Don't even bother with the trap frame, and force the default values.
>
>
> What's the rationale?
> Not getting the correct values for registers in getmcontext is
> unexpected and weird.

FYI, in 64bit mode, x86 does not enforce segmentation. You can set whatever
base/limit in the segregs, it doesn't get applied.

Moreover, certain instructions like 'syscall' force certain segregs to default
values, regardless of what userland had set.

Also, if you look at the commit before, you can see I had found an info leak
caused by the fact that we were fetching the segregs without masking them
properly.

So, given that (1) these segregs have no functional change, (2) some are forced
to default values anyway, and (3) the way we were fetching them had already
proven to be buggy, I just dropped the thing and set the default values.

This is expected and normal, likely prevents future bugs, and also simplifies
the understanding of USER_LDT.