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

[Catalin Marinas <catalin.marinas@arm.com>]

On Mon, Nov 16, 2020 at 03:55:05PM -0600, Eric W. Biederman wrote:
> Catalin Marinas <catalin.marinas@arm.com> writes:
> > On Thu, Nov 12, 2020 at 06:53:36PM -0800, Peter Collingbourne wrote:
> >> diff --git a/Documentation/arm64/tagged-pointers.rst b/Documentation/arm64/tagged-pointers.rst
> >> index eab4323609b9..19d284b70384 100644
> >> --- a/Documentation/arm64/tagged-pointers.rst
> >> +++ b/Documentation/arm64/tagged-pointers.rst
> >> @@ -53,12 +53,25 @@ visibility.
> >>  Preserving tags
> >>  ---------------
> >>  
> >> -Non-zero tags are not preserved when delivering signals. This means that
> >> -signal handlers in applications making use of tags cannot rely on the
> >> -tag information for user virtual addresses being maintained for fields
> >> -inside siginfo_t. One exception to this rule is for signals raised in
> >> -response to watchpoint debug exceptions, where the tag information will
> >> -be preserved.
> >> +When delivering signals, non-zero tags are not preserved in
> >> +siginfo.si_addr unless the flag SA_EXPOSE_TAGBITS was set in
> >> +sigaction.sa_flags when the signal handler was installed. This means
> >> +that signal handlers in applications making use of tags cannot rely
> >> +on the tag information for user virtual addresses being maintained
> >> +in these fields unless the flag was set.
> >> +
> >> +Due to architecture limitations, bits 63:60 of the fault address
> >> +are not preserved in response to synchronous tag check faults
> >> +(SEGV_MTESERR) even if SA_EXPOSE_TAGBITS was set. Applications should
> >> +treat the values of these bits as undefined in order to accommodate
> >> +future architecture revisions which may preserve the bits.
> >
> > If future architecture versions will preserve these bits, most likely
> > we'll add a new HWCAP bit so that the user knows what's going on. But
> > the user shouldn't rely on them being 0, just in case.
> >
> >> +For signals raised in response to watchpoint debug exceptions, the
> >> +tag information will be preserved regardless of the SA_EXPOSE_TAGBITS
> >> +flag setting.
> >> +
> >> +Non-zero tags are never preserved in sigcontext.fault_address
> >> +regardless of the SA_EXPOSE_TAGBITS flag setting.
> >
> > We could've done it the other way around (fault_address tagged, si_addr
> > untagged) but that would be specific to arm64, so I think we should
> > solve it for other architectures that implement (or plan to) tagging.
> > The fault_address in the arm64 sigcontext was an oversight, we should
> > have removed it but when we realised it was already ABI.
> >
> > Anyway, I'm fine with the arm64 changes here:
> >
> > Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
> >
> > With Eric's ack, I'm happy to take the series through the arm64 tree,
> > otherwise Eric's tree is fine as well.
> 
> In general I am fine with the last two patches.
> 
> I want to understand where the value for SA_UNSUPPORTED comes from, and
> while I have good answers I am still digesting the question of if
> SA_EXPOSE_TAGBITS should be implemented in the arch specific header or
> in a generic header.  I quite agree it should have a generic
> definition/implementation.  I just don't know if it makes sense to make
> the value available to userspace if the architecture does not have
> tagbits.  Mostly my concern is about bit consumption as we only have
> 30ish sigaction bits.

An alternative would be to make this opt-in per process (or thread)
based on a prctl() call. We already have one for PR_TAGGED_ADDR_ENABLE
to allow tagged addresses from user at the syscall ABI level. Another
bit in there would allow si_addr to be tagged. The disadvantage is that
this is quite coarse control affecting other signal handlers.

> I will follow with my acks when I have resolved those issues.

Thanks.

-- 
Catalin