Re: [PATCH v14 8/8] arm64: expose FAR_EL1 tag bits in siginfo

On Mon, Nov 09, 2020 at 07:13:08PM -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, create a new pair of
fields in siginfo._sigfault, and store the tag bits of FAR_EL1 there,
together with a mask specifying which bits are valid.

A flag is added to si_faultflags to allow userspace to determine whether
the values in the fields are valid.

[...]

What prevents adding a sigaction sa_flag SA_EXPOSE_TABITS that when set
causes the high bits to be set, and when clear (the default) will have
the signal delivery code clear those bits.

Thanks for the review so far Eric.

If the SA_EXPOSE_TAGBITS idea works, I'd much rather have that than the
ultra generic xflags approach.

Given that Peter is going to rework this part, could you please pick the
first clean-up patches via your tree? If there are dependencies with the
reworked arm64 patches, I'm also happy for them to go via your tree.

Thanks.

-- 
Catalin