On Mon, Dec 18, 2017 at 10:54:26AM -0800, Dave Hansen wrote:
On 11/06/2017 12:57 AM, Ram Pai wrote:
Expose useful information for programs using memory protection keys. Provide implementation for powerpc and x86.
On a powerpc system with pkeys support, here is what is shown:
$ head /sys/kernel/mm/protection_keys/*
==> /sys/kernel/mm/protection_keys/disable_access_supported <== true
This is cute, but I don't think it should be part of the ABI. Put it in
thanks :)
debugfs if you want it for cute tests. The stuff that this tells you can and should come from pkey_alloc() for the ABI.
Applications can make system calls with different parameters and on failure determine indirectly that such a feature may not be available in the kernel/hardware. But from an application point of view, I think, it is a very clumsy/difficult way to determine that.
For example, an application can keep making pkey_alloc() calls and count till the call fails, to determine the number of keys supported by the system. And then the application has to release those keys too. Too much side-effect just to determine a simple thing. Do we want the application to endure this pain?
I think we should aim to provide sufficient API/ABI for the application to consume the feature efficiently, and not any more.
I do not claim that the ABI exposed by this patch is sufficiently optimal. But I do believe it is tending towards it.
currently the following ABI is exposed.
a) total number of keys available in the system. This information may not be useful and can possibly be dropped.
b) minimum number of keys available to the application. if libraries consumes a few, they could provide a library interface to the application informing the number available to the application. The library interface can leverage (b) to provide the information.
c) types of disable-rights supported by keys. Helps the application to determine the types of disable-features available. This is helpful, otherwise the app has to make pkey_alloc() call with the corresponding parameter set and see if it suceeds or fails. Painful from an application point of view, in my opinion.
http://man7.org/linux/man-pages/man7/pkeys.7.html
Any application wanting to use protection keys needs to be able to function without them. They might be unavailable because the hardware that the application runs on does not support them, the kernel code does not contain support, the kernel support has been disabled, or because the keys have all been allocated, perhaps by a library the application is using. It is recommended that applications wanting to use protection keys should simply call pkey_alloc(2) and test whether the call succeeds, instead of attempting to detect support for the feature in any other way.Do you really not have standard way on ppc to say whether hardware features are supported by the kernel? For instance, how do you know if a given set of registers are known to and are being context-switched by the kernel?
I think on x86 you look for some hardware registers to determine which hardware features are enabled by the kernel.
We do not have generic support for something like that on ppc. The kernel looks at the device tree to determine what hardware features are available. But does not have mechanism to tell the hardware to track which of its features are currently enabled/used by the kernel; atleast not for the memory-key feature.
RP
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