Re: MPK: removing a pkey

[Vlastimil Babka <vbabka@suse.cz>]

On 11/22/2017 01:15 PM, Florian Weimer wrote:
> On 11/22/2017 09:18 AM, Vlastimil Babka wrote:
>> And, was the pkey == -1 internal wiring supposed to be exposed to the
>> pkey_mprotect() signal, or should there have been a pre-check returning
>> EINVAL in SYSCALL_DEFINE4(pkey_mprotect), before calling
>> do_mprotect_pkey())? I assume it's too late to change it now anyway (or
>> not?), so should we also document it?
> 
> I think the -1 case to the set the default key is useful because it 
> allows you to use a key value of -1 to mean a??MPK is not supporteda??, and 
> still call pkey_mprotect.

Hmm the current manpage says then when MPK is not supported, pkey has to
be specified 0. Which is a value that doesn't work when MPK *is*
supported. So -1 is more universal indeed.

> I plan to document this behavior on the glibc side, and glibc will call 
> mprotect (not pkey_mprotect) for key -1, so that you won't get ENOSYS 
> with kernels which do not support pkey_mprotect.

Fair enough. What will you do about pkey_alloc() in that case, emulate
ENOSPC? Oh, the manpage already suggests so. And the return value in
that case is... -1. Makes sense :)

> Thanks,
> Florian
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