Re: [PATCH v17 0/3] arm64: expose FAR_EL1 tag bits in siginfo

Peter Collingbourne [off-list ref] writes:

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.

In order to allow userspace to determine whether SA_EXPOSE_TAGBITS
is supported, we first introduce a mechanism for userspace to detect
kernel support for SA_* flags.

These patches need to be applied on top of:
https://git.kernel.org/pub/scm/linux/kernel/git/ebiederm/user-namespace.git signal-for-v5.11

The first two patches look good and I have applied them.

While I was at it I added Link tags to the LKML postings to the entire
series.  I don't think anyone has merged my branch into another so it
should still be safe.

Eric

On Tue, Nov 17, 2020 at 10:17 AM Eric W. Biederman
[off-list ref] 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.

In order to allow userspace to determine whether SA_EXPOSE_TAGBITS
is supported, we first introduce a mechanism for userspace to detect
kernel support for SA_* flags.

These patches need to be applied on top of:
https://git.kernel.org/pub/scm/linux/kernel/git/ebiederm/user-namespace.git signal-for-v5.11

The first two patches look good and I have applied them.

While I was at it I added Link tags to the LKML postings to the entire
series.  I don't think anyone has merged my branch into another so it
should still be safe.

Thanks!

Peter