On Mon, Nov 16, 2020 at 03:55:05PM -0600, Eric W. Biederman wrote: > Catalin Marinas <catalin.marinas@xxxxxxx> 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@xxxxxxx> > > > > 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
