Re: [PATCH v3 23/23] arm64: mte: Add Memory Tagging Extension documentation

Catalin Marinas
2020-05-15T10:38:56Z

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.

--
Catalin