On 07/11/2017 02:51 PM, Ram Pai wrote: > On Wed, Jul 12, 2017 at 07:29:37AM +1000, Benjamin Herrenschmidt wrote: >> On Tue, 2017-07-11 at 11:11 -0700, Dave Hansen wrote: >>> On 07/05/2017 02:21 PM, Ram Pai wrote: >>>> Currently sys_pkey_create() provides the ability to disable read >>>> and write permission on the key, at creation. powerpc has the >>>> hardware support to disable execute on a pkey as well.This patch >>>> enhances the interface to let disable execute at key creation >>>> time. x86 does not allow this. Hence the next patch will add >>>> ability in x86 to return error if PKEY_DISABLE_EXECUTE is >>>> specified. >> >> That leads to the question... How do you tell userspace. >> >> (apologies if I missed that in an existing patch in the series) >> >> How do we inform userspace of the key capabilities ? There are at least >> two things userspace may want to know already: >> >> - What protection bits are supported for a key > > the userspace is the one which allocates the keys and enables/disables the > protection bits on the key. the kernel is just a facilitator. Now if the > use space wants to know the current permissions on a given key, it can > just read the AMR/PKRU register on powerpc/intel respectively. Let's say I want to execute-disable a region. Can I use protection keys? Do I do pkey_mprotect(... PKEY_DISABLE_EXECUTE); and assume that the -EINVAL is because PKEY_DISABLE_EXECUTE is unsupported, or do I do: #ifdef __ppc__ pkey = pkey_aloc(); pkey_mprotect(... PKEY_DISABLE_EXECUTE); #else mprotect(); #endif >> - How many keys exist > > There is no standard way of finding this other than trying to allocate > as many till you fail. A procfs or sysfs file can be added to expose > this information. It's also dynamic. On x86, you lose a key if you've used the execute-only support. We also reserve the right to steal more in the future if we want. >> - Which keys are available for use by userspace. On PowerPC, the >> kernel can reserve some keys for itself, so can the hypervisor. In >> fact, they do. > > this information can be exposed through /proc or /sysfs > > I am sure there will be more demands and requirements as applications > start leveraging these feature. For 5 bits, I think just having someone run pkey_alloc() in a loop is fine. I don't think we really need to enumerate it in some other way. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
