On Mon, Dec 16, 2019 at 09:30:36AM -0800, Peter Collingbourne wrote: > On Mon, Dec 16, 2019 at 6:20 AM Kevin Brodsky <kevin.brodsky@xxxxxxx> wrote: > > In this patch, the default exclusion mask remains 0 (i.e. all tags can be generated). > > After some more discussions, Branislav and I think that it would be better to start > > with the reverse, i.e. all tags but 0 excluded (mask = 0xfe or 0xff). So with mask 0xff, IRG generates only tag 0? This seems to be the case reading the pseudocode in the ARM ARM. > > This should simplify the MTE setup in the early C runtime quite a bit. Indeed, if all > > tags can be generated, doing any heap or stack tagging before the > > PR_SET_TAGGED_ADDR_CTRL prctl() is issued can cause problems, notably because tagged > > addresses could end up being passed to syscalls. Conversely, if IRG and ADDG never > > set the top byte by default, then tagging operations should be no-ops until the > > prctl() is issued. This would be particularly useful given that it may not be > > straightforward for the C runtime to issue the prctl() before doing anything else. > > > > Additionally, since the default tag checking mode is PR_MTE_TCF_NONE, it would make > > perfect sense not to generate tags by default. > > > > Any thoughts? > > This would indeed allow the early C runtime startup code to pass > tagged addresses to syscalls, but I don't think it would entirely free > the code from the burden of worrying about stack tagging. Either way, > any stack frames that are active at the point when the prctl() is > issued would need to be compiled without stack tagging, because > otherwise those stack frames may use ADDG to rematerialize a stack > object address, which may produce a different address post-prctl. > Setting the exclude mask to 0xffff would at least make it more likely > for this problem to be detected, though. > > If we change the default in this way, maybe it would be worth > considering flipping the meaning of the tag mask and have it be a mask > of tags to allow. That would be consistent with the existing behaviour > where userspace sets bits in tagged_addr_ctrl in order to enable > tagging features. Either option works for me. It's really for the libc people to decide what they need. I think an "include" rather than "exclude" mask makes sense with the default 0 meaning only generate tag 0. Thanks. -- Catalin
