Re: [RFC PATCH v9 05/27] x86/cet/shstk: Add Kconfig option for user-mode Shadow Stack protection

[Yu-cheng Yu]

On Fri, 2020-03-06 at 11:02 -0800, Dave Hansen wrote:
> On 3/6/20 10:37 AM, Yu-cheng Yu wrote:
> > We used to do this for CET instructions, but after adding kernel-mode
> > instructions and inserting ENDBR's, the code becomes cluttered.  I also
> > found an earlier discussion on the ENDBR:
> > 
> > https://lore.kernel.org/lkml/CALCETrVRH8LeYoo7V1VBPqg4WS0Enxtizt=T7dPvgoeWfJrdzA@xxxxxxxxxxxxxx/
> > 
> > It makes sense to let the user know early on that the system cannot support
> > CET and cannot build a CET-enabled kernel.
> > 
> > One thing we can do is to disable CET in Kconfig and not in kernel
> > build, which I will do in the next version.
> 
> I'll go on the record and say I think we should allow building
> CET-enabled kernels on old toolchains.  We need it for build test
> coverage.  We can spit out a warning, but we need to allow building it.

The build test will go through (assembler or .byte), once the opcode patch
is applied [1].  Also, when we enable kernel-mode CET, it is difficult to
build IBT code without the right tool chain.

Yu-cheng

[1] opcode patch: 
https://lore.kernel.org/lkml/20200204171425.28073-1-yu-cheng.yu@xxxxxxxxx/