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@xxxxxxx> With Eric's ack, I'm happy to take the series through the arm64 tree, otherwise Eric's tree is fine as well. Thanks. -- Catalin
