On Wed, Nov 02, 2016 at 12:15:50PM -0700, Dave Hansen wrote:
On 10/31/2016 05:08 PM, Mark Rutland wrote:
quoted
When an architecture does not select CONFIG_ARCH_HAS_PKEYS, the pkey_alloc
syscall will return -ENOSPC for all (otherwise well-formed) requests, as the
generic implementation of mm_pkey_alloc() returns -1. The other pkey syscalls
perform some work before always failing, in a similar fashion.
This implies the absence of keys, but otherwise functional pkey support. This
is odd, since the architecture provides no such support. Instead, it would be
preferable to indicate that the syscall is not implemented, since this is
effectively the case.
This makes the behavior of an x86 cpu without pkeys and an arm cpu
without pkeys differ. Is that what we want?
My rationale was that we have no idea whether architectures will have pkey
support in future, and if/when they do, we may have to apply additional checks
anyhow. i.e. in cases we'd return -ENOSPC today, we might want to return
another error code.
Returning -ENOSYS retains the current behaviour, and allows us to handle that
ABI issue when we know what architecture support looks like.
Other architectures not using the generic syscalls seem to handle this with
-ENOSYS, e.g. parisc with commit 18088db042dd9ae2, so there's differing
behaviour regardless of arm specifically.
An application that _wants_ to use protection keys but can't needs to handle
-ENOSPC anyway.
Sure, and that application *also* has to handle -ENOSYS, given current kernels.
On an architecture that will never support pkeys, it makes sense to do
-ENOSYS, but that's not the case for arm, right?
I don't know whether arm or other architectures will have (user-accessible)
pkey-like suport.
Thanks,
Mark.
--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org. For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>