[PATCH v5 1/1] <sys/tagged-address.h>: An API for tagged address

[H.J. Lu]

On Tue, Aug 17, 2021 at 5:33 AM Szabolcs Nagy via Libc-alpha
<libc-alpha@sourceware.org> wrote:
>
> The 08/12/2021 13:01, Szabolcs Nagy via Libc-alpha wrote:
> > The 08/12/2021 10:36, Florian Weimer wrote:
> > > I still don't see a way how we can split tag address bits used by the
> > > implementation (glibc, sanitizers) and the application.
> ...
> > so one approach is to just disallow user tags, only sanitizer
> > and similar tools can tag (and i think hwasan can coordinate
> > with glibc via less formal api/abi that we can change later)
>
> to expand on this: i think we should just focus on the hwasan
> use-case. there may be other use-cases for tagged addresses,
> but we need more experience before we can design a generic api.
> hwasan can just poke at implementation internals and have
> target specific logic for now.

Agreed.  My motivation is hwasan.

> allowing application code to use the tag bits can break c
> semantics and compiler assumptions too easily.
>
> and we should not require hwasan to use libc apis to work
> with tagged addresses, that would slow it down.

There is no reason why a libc API should be slow.   Currently,
LAM enabled libsanitizer/hwasan has

#include <sys/tagged-address.h>

bool lam_failed = set_tagged_address_mask (TAGGED_ADDRESS_MASK (57))

We only need this function to enable LAM in libsanitizer/hwasan
and inform glibc that LAM is enabled.  The rest of functions in
<sys/tagged-address.h> are used to make memmove and memmove
tests LAM compatible.  We can move them to internal header files.

> so i think we don't need the tagged address representation
> related apis. we may need something __hwasan_init can call
> to set up os support. on aarch64 that's a prctl now, but a
> libc api would allow us to disable hwasan from glibc (e.g.
> if there are elf markings for incompatible dsos), i don't

set_tagged_address_mask can be used to enable and disable
LAM/TBI.  memmove needs to know if LAM/TBI is enabled or not
to work correctly.

> know if that makes sense (does hwasan have a fall back if
> there is no os support?).
>

It is a hard error for hwasan if LAM/TBI isn't available.

Thanks.

-- 
H.J.