On Wed, Aug 21, 2019 at 06:33:53PM +0100, Will Deacon wrote: > 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()``). sigaltstack() is interesting, since we don't support tagged stacks. Do we keep the ss_sp tag in the kernel, but squash it when delivering a signal to the alternate stack? (I can't remember whether this would be compatible with the architectural tag checking semantics...) > 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 :( Time for some creative vagueness? We can write a statement of our overall intent, along with examples of a few cases where the tag should and should not be expected to emerge intact. There is no foolproof rule, unless we can rewrite history... Cheers ---Dave
