On Wed, Aug 21, 2019 at 05:47:30PM +0100, Catalin Marinas wrote: > From: Vincenzo Frascino <vincenzo.frascino@xxxxxxx> > > On AArch64 the TCR_EL1.TBI0 bit is set by default, allowing userspace > (EL0) to perform memory accesses through 64-bit pointers with a non-zero > top byte. However, such pointers were not allowed at the user-kernel > syscall ABI boundary. > > With the Tagged Address ABI patchset, it is now possible to pass tagged > pointers to the syscalls. Relax the requirements described in > tagged-pointers.rst to be compliant with the behaviours guaranteed by > the AArch64 Tagged Address ABI. > > Cc: Will Deacon <will.deacon@xxxxxxx> > Cc: Szabolcs Nagy <szabolcs.nagy@xxxxxxx> > Cc: Kevin Brodsky <kevin.brodsky@xxxxxxx> > Acked-by: Andrey Konovalov <andreyknvl@xxxxxxxxxx> > Signed-off-by: Vincenzo Frascino <vincenzo.frascino@xxxxxxx> > Co-developed-by: Catalin Marinas <catalin.marinas@xxxxxxx> > Signed-off-by: Catalin Marinas <catalin.marinas@xxxxxxx> > --- > Documentation/arm64/tagged-pointers.rst | 23 ++++++++++++++++------- > 1 file changed, 16 insertions(+), 7 deletions(-) > > diff --git a/Documentation/arm64/tagged-pointers.rst b/Documentation/arm64/tagged-pointers.rst > index 2acdec3ebbeb..04f2ba9b779e 100644 > --- a/Documentation/arm64/tagged-pointers.rst > +++ b/Documentation/arm64/tagged-pointers.rst > @@ -20,7 +20,9 @@ Passing tagged addresses to the kernel > -------------------------------------- > > All interpretation of userspace memory addresses by the kernel assumes > -an address tag of 0x00. > +an address tag of 0x00, unless the application enables the AArch64 > +Tagged Address ABI explicitly > +(Documentation/arm64/tagged-address-abi.rst). > > This includes, but is not limited to, addresses found in: > > @@ -33,13 +35,15 @@ This includes, but is not limited to, addresses found in: > - the frame pointer (x29) and frame records, e.g. when interpreting > them to generate a backtrace or call graph. > > -Using non-zero address tags in any of these locations may result in an > -error code being returned, a (fatal) signal being raised, or other modes > -of failure. > +Using non-zero address tags in any of these locations when the > +userspace application did not enable the AArch64 Tagged Address ABI may > +result in an error code being returned, a (fatal) signal being raised, > +or other modes of failure. > > -For these reasons, passing non-zero address tags to the kernel via > -system calls is forbidden, and using a non-zero address tag for sp is > -strongly discouraged. > +For these reasons, when the AArch64 Tagged Address ABI is disabled, > +passing non-zero address tags to the kernel via system calls is > +forbidden, and using a non-zero address tag for sp is strongly > +discouraged. > > Programs maintaining a frame pointer and frame records that use non-zero > address tags may suffer impaired or inaccurate debug and profiling > @@ -59,6 +63,11 @@ be preserved. > The architecture prevents the use of a tagged PC, so the upper byte will > be set to a sign-extension of bit 55 on exception return. > > +This behaviour is maintained when the AArch64 Tagged Address ABI is > +enabled. In addition, with the exceptions above, the kernel will > +preserve any non-zero tags passed by the user via syscalls and stored in > +kernel data structures (e.g. ``set_robust_list()``, ``sigaltstack()``). Hmm. I can see the need to provide this guarantee for things like set_robust_list(), but the problem is that the statement above is too broad and isn't strictly true: for example, mmap() doesn't propagate the tag of its address parameter into the VMA. So I think we need to nail this down a bit more, but I'm having a really hard time coming up with some wording :( Will
