Re: [PATCH v16 6/6] arm64: expose FAR_EL1 tag bits in siginfo

On Sun, Nov 15, 2020 at 08:08:36AM -0600, Eric W. Biederman wrote:

Peter Collingbourne [off-list ref] writes:

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, introduce a flag in
sigaction.sa_flags, SA_EXPOSE_TAGBITS, and only expose the tag bits
there if the signal handler has this flag set.

For future architectures that implement something similar does it make
sense that to hide tag bits by default?

I think on arm64 this comes from the fact that the tag bits information
is not available in all scenarios.  To keep things clean, the decision
was taken early on to just zero them all the time in si_addr to avoid
software getting confused.  Possibly other arches do something similar,
but that would need digging into.

There seems to be debate on whether these bits are part of the address
or not.  For si_addr I think they probably _should_ be regarded as part
of the address in general, and arches that can always report all these
bits in si_addr should probably do so IMHO.

I am wondering if SA_EXPOSE_TABGITS might make sense as an architecture
specific sa bit.

Perhaps.  Peter, do you see other arches masking out bits in si_addr?

[...]

Cheers
---Dave