Hi Peter,

On Wed, Apr 29, 2020 at 02:42:01PM -0700, Peter Collingbourne wrote:

On Wed, Apr 29, 2020 at 2:08 PM Will Deacon [off-list ref] wrote:

quoted

On Fri, Mar 27, 2020 at 12:19:15PM -0700, Peter Collingbourne wrote:

quoted

The kernel currently clears the tag bits (i.e. bits 56-63) in the fault
address exposed via siginfo.si_addr and sigcontext.fault_address. However,
the tag bits may be needed by tools in order to accurately diagnose
memory errors, such as HWASan [1] or future tools based on the Memory
Tagging Extension (MTE).

We should not stop clearing these bits in the existing fault address
fields, because there may be existing userspace applications that are
expecting the tag bits to be cleared. Instead, create a far_context in
sigcontext (similar to the existing esr_context), and store the original
value of FAR_EL1 (including the tag bits) there.

[1] http://clang.llvm.org/docs/HardwareAssistedAddressSanitizerDesign.html

Signed-off-by: Peter Collingbourne <redacted>
---
v3:
- add documentation to tagged-pointers.rst
- update comments in sigcontext.h

Hmm, although the code looks fine, why don't we just expose the tag in the
new field, rather than duplicate the address information? I'm nervous about
exposing privileged registers directly to userspace.

I have no strong opinion on whether this should just contain the tag or not.

A few of us chatted about this today. Please could you spin a v4 where only
the top byte is exposed in the new sigcontext record as a __u8? You'll need
to think of a better name than "FAR"; perhaps something like 'si_addr_top_byte',
'si_addr_63_56' or whatever you fancy. Naming is hard.

For MTE we can add a separate record later on, so as not to overload this
(e.g. si_addr_mte_tag).

Ta,

Will

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