The 05/15/2020 11:38, Catalin Marinas wrote: > On Thu, May 14, 2020 at 12:37:22PM +0100, Catalin Marinas wrote: > > On Wed, May 13, 2020 at 04:48:46PM +0100, Dave P Martin wrote: > > > > > > On Wed, Apr 29, 2020 at 05:47:05PM +0100, Dave P Martin wrote: > > > > > > > On Tue, Apr 21, 2020 at 03:26:03PM +0100, Catalin Marinas wrote: > > > > > > > > +excludes all tags other than 0. A user thread can enable specific tags > > > > > > > > +in the randomly generated set using the ``prctl(PR_SET_TAGGED_ADDR_CTRL, > > > > > > > > +flags, 0, 0, 0)`` system call where ``flags`` contains the tags bitmap > > > > > > > > +in the ``PR_MTE_TAG_MASK`` bit-field. > > > > > > > > + > > > > > > > > +**Note**: The hardware uses an exclude mask but the ``prctl()`` > > > > > > > > +interface provides an include mask. An include mask of ``0`` (exclusion > > > > > > > > +mask ``0xffff``) results in the CPU always generating tag ``0``. > > > > > > > > > > > > > > Is there no way to make this default to 1 rather than having a magic > > > > > > > meaning for 0? > > [...] > > > The only configuration that doesn't make sense is "no tags allowed", so > > > I'd argue for explicity blocking that, even if the architeture aliases > > > that encoding to something else. > > > > > > If we prefer 0 as a default value so that init inherits the correct > > > value from the kernel without any special acrobatics, then we make it an > > > exclude mask, with the semantics that the hardware is allowed to > > > generate any of these tags, but does not have to be capable of > > > generating all of them. > > > > That's more of a question to the libc people and their preference. > > We have two options with suboptions: > > > > 1. prctl() gets an exclude mask with 0xffff illegal even though the > > hardware accepts it: > > a) default exclude mask 0, allowing all tags to be generated by IRG > > b) default exclude mask of 0xfffe so that only tag 0 is generated > > > > 2. prctl() gets an include mask with 0 illegal: > > a) default include mask is 0xffff, allowing all tags to be generated > > b) default include mask 0f 0x0001 so that only tag 0 is generated > > > > We currently have (2) with mask 0 but could be changed to (2.b). If we > > are to follow the hardware description (which makes more sense to me but > > I don't write the C library), (1.a) is the most appropriate. > > Thinking some more about this, as we are to expose the GCR_EL1.Excl via > a ptrace interface as a regset, it makes more sense to move back to an > exclude mask here with default 0. That would be option 1.a above. i think the libc has to do a prctl call to set mte up and at that point it will use whatever arguments necessary, so 1.a should work (just like the other options). likely libc will disable 0 for irg and possibly one or two other fixed colors (which will have specific use). the difference i see between 1 vs 2 is forward compatibility if the architecture changes (e.g. adding more tag bits) but then likely new prctl flag will be needed for handling that so it's probably not an issue.
