On 12/06/2019 15:21, Vincenzo Frascino wrote: > On arm64 the TCR_EL1.TBI0 bit has been always enabled hence > the userspace (EL0) is allowed to set a non-zero value in the > top byte but the resulting pointers are not allowed at the > user-kernel syscall ABI boundary. > > With the relaxed ABI proposed in this set, it is now possible to pass > tagged pointers to the syscalls, when these pointers are in memory > ranges obtained by an anonymous (MAP_ANONYMOUS) mmap(). > > Relax the requirements described in tagged-pointers.txt to be compliant > with the behaviours guaranteed by the ARM64 Tagged Address ABI. > > Cc: Catalin Marinas <catalin.marinas@xxxxxxx> > Cc: Will Deacon <will.deacon@xxxxxxx> > CC: Andrey Konovalov <andreyknvl@xxxxxxxxxx> > Signed-off-by: Vincenzo Frascino <vincenzo.frascino@xxxxxxx> > --- > Documentation/arm64/tagged-pointers.txt | 23 ++++++++++++++++------- > 1 file changed, 16 insertions(+), 7 deletions(-) > > diff --git a/Documentation/arm64/tagged-pointers.txt b/Documentation/arm64/tagged-pointers.txt > index a25a99e82bb1..db58a7e95805 100644 > --- a/Documentation/arm64/tagged-pointers.txt > +++ b/Documentation/arm64/tagged-pointers.txt > @@ -18,7 +18,8 @@ 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 userspace opts-in the ARM64 Tagged > +Address ABI via the PR_SET_TAGGED_ADDR_CTRL prctl(). > > This includes, but is not limited to, addresses found in: > > @@ -31,18 +32,23 @@ 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 opt-in to the ARM64 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 userspace application did not opt-in, 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 > visibility. > > +A definition of the meaning of ARM64 Tagged Address ABI and of the > +guarantees that the ABI provides when the userspace opts-in via prctl() > +can be found in: Documentation/arm64/tagged-address-abi.txt. > + OK. > > Preserving tags > --------------- > @@ -57,6 +63,9 @@ 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 behaviours are preserved even when the the userspace opts-in the ARM64 these behaviours. > +Tagged Address ABI via the PR_SET_TAGGED_ADDR_CTRL prctl(). > + > > Other considerations > -------------------- >
