System call number masking

[Ben Hutchings <ben@decadent.org.uk>]

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I'm updating my x32-as-boot-time-option patch for 4.6, and I noticed a
subtle change in system call number masking on x86_64 as a result of
moving the slow path into C.

Previously we would mask out the upper 32 bits before doing anything
with the system call number, both on the slow and fast paths, if and
only if x32 was enabled.

Now we always mask out the upper 32 bits on the slow path, so it's not
quite consistent with the fast path if x32 is disabled.  A system call
that would be rejected by the fast path can succeed on the slow path.
I don't know whether this causes any problems, but it seems
undesirable.

But it's also undesirable that the behaviour of system call numbers not
assigned to x32 also varies depending on whether x32 is enabled.
Should we always mask out the upper 32 bits on the fast path?

Ben.

-- 
Ben Hutchings
In a hierarchy, every employee tends to rise to his level of incompetence.

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