Or: it cannot be implemented 100% safely and correctly without slowdown.
If you know the use you wish to put this to, and are willing to risk a
permission check somewhere being confused momentarily by a non-atomic
update of a 32-bit number (or the non-atomic update between several
32-bit numbers, which I think is less serious because then you are not
granting more than the union of the two UIDs) go ahead and patch your
kernel.
> So ignore my patch.
For official kernels, I agree. They need to be as safe and
deterministic as possible, especially security-wise, and a semaphore on
every permission check would be ridiculous.
Peter
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