Re: Rethinking sigcontext's xfeatures slightly for PKRU's benefit?

[Andy Lutomirski]

On Fri, Dec 18, 2015 at 12:58 PM, H. Peter Anvin <hpa@xxxxxxxxx> wrote:
> On 12/18/2015 12:49 PM, Andy Lutomirski wrote:
>>
>> IOW, I like my idea in which signal delivery always sets PKRU to the
>> application-requested-by-syscall values and sigreturn restores it.
>> Kinda like sigaltstack, but applies to all signals and affects PKRU
>> instead of RSP.
>>
>
> I think this is the only sensible option, with the default being all zero.
>

Or not quite all zero if we do Dave's experimental PROT_EXEC thing.

Actually, I want to introduce a set of per-mm "incompatible" bits.  By
default, they'd be zero.  We can, as needed, define bits that do
something nice but break old code.  I want one of the bits to turn
vsyscalls off entirely.  Another bit could say that the kernel is
allowed to steal a protection key for PROT_EXEC.

These bits would be read and written by prctl, but there could also be
an ELF note mechanism to initialize them on execve without a syscall.

--Andy
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