[Bug libc/22396] x86-64: siglongjmp does not restore PKRU register

["dave at sr71 dot net" <sourceware-bugzilla at sourceware dot org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=22396

Dave Hansen <dave at sr71 dot net> changed:

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                 CC|                            |dave at sr71 dot net

--- Comment #4 from Dave Hansen <dave at sr71 dot net> ---
It should also be noted that this issue also applies to MPX which has a lot
more state than protection keys.  (BNDSTATUS (8) + BNDCFGU (8) +
BNDREGS0-3(4*16)).  The ABI for BNDREGS probably saves us here at least partly
because it's pretty arguable that the registers are clobber at siglongjmp.  But
BNDCFGU which turns the feature on/off is a problem.

Back to PK... The manpage (http://man7.org/linux/man-pages/man7/pkeys.7.html)
could use some additional clarity here, but it does mention the signal
behavior, and also says:

    The rights of any interrupted context are
    restored when the signal handler returns.

We could clarify that we truly mean sys_sigreturn() and that other mechanisms
don't count as "returning".

The kernel _could_ detect when someone unblocks signals from inside a signal
handler, but without doing a sys_sigreturn().  We could theoretically take some
action there.  Does that do us any good?

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