On Tue, Nov 17, 2020 at 10:14 AM Szabolcs Nagy <szabolcs.nagy@xxxxxxx> wrote: > > The 11/17/2020 17:29, Catalin Marinas wrote: > > Adding Szabolcs and libc-alpha. The original patch below and also here: > > > > https://lore.kernel.org/r/20201014055106.25164-1-pcc@xxxxxxxxxx > > > > The patch looks fine to me but I'd like the glibc community to confirm > > that they are happy with this ABI addition. I'd also like Dave Martin to > > ack the patch since he was involved in the discussion for v1. > > > > Thanks. > > > > Catalin > > > > On Tue, Oct 13, 2020 at 10:51:06PM -0700, Peter Collingbourne wrote: > > > This prctl allows the user program to control which PAC keys are enabled > > > in a particular task. The main reason why this is useful is to enable a > > > userspace ABI that uses PAC to sign and authenticate function pointers > > > and other pointers exposed outside of the function, while still allowing > > > binaries conforming to the ABI to interoperate with legacy binaries that > > > do not sign or authenticate pointers. > > > > > > The idea is that a dynamic loader or early startup code would issue > > > this prctl very early after establishing that a process may load legacy > > > binaries, but before executing any PAC instructions. > > > > > > Signed-off-by: Peter Collingbourne <pcc@xxxxxxxxxx> > > > Link: https://linux-review.googlesource.com/id/Ibc41a5e6a76b275efbaa126b31119dc197b927a5 > > > --- > > > This patch must be applied on top of Catalin's MTE series: > > > > > > git://git.kernel.org/pub/scm/linux/kernel/git/arm64/linux for-next/mte > > > > > > Alternatively the series with this patch on top may be > > > downloaded from Gerrit by following the link above. > ... > > > +Enabling and disabling keys > > > +--------------------------- > > > + > > > +The prctl PR_PAC_SET_ENABLED_KEYS allows the user program to control which > > > +PAC keys are enabled in a particular task. It takes two arguments, the > > > +first being a bitmask of PR_PAC_APIAKEY, PR_PAC_APIBKEY, PR_PAC_APDAKEY > > > +and PR_PAC_APDBKEY specifying which keys shall be affected by this prctl, > > > +and the second being a bitmask of the same bits specifying whether the key > > > +should be enabled or disabled. For example:: > > > + > > > + prctl(PR_PAC_SET_ENABLED_KEYS, > > > + PR_PAC_APIAKEY | PR_PAC_APIBKEY | PR_PAC_APDAKEY | PR_PAC_APDBKEY, > > > + PR_PAC_APIBKEY, 0, 0); > > > + > > > +disables all keys except the IB key. > > > + > > > +The main reason why this is useful is to enable a userspace ABI that uses PAC > > > +instructions to sign and authenticate function pointers and other pointers > > > +exposed outside of the function, while still allowing binaries conforming to > > > +the ABI to interoperate with legacy binaries that do not sign or authenticate > > > +pointers. > > > + > > > +The idea is that a dynamic loader or early startup code would issue this > > > +prctl very early after establishing that a process may load legacy binaries, > > > +but before executing any PAC instructions. > > which keys are enabled by default when HWCAP_PACA is > set in AT_HWCAPS ? i think that would be useful to > point out here. For compatibility with previous kernel versions all keys are enabled by default at process startup. But I agree it would be useful to point it out here. > per process enable/disable was deemed costly to do when > pac support was added, did this change? (i'm happy if it > did, because even without a new PAC specific pointer ABI, > plain PAC-RET can cause problems if a process tries to do > its own DWARF unwinding but does not understand the new > opcode that toggles PAC status of the return address, > possibly in a third party library, so a way to opt-out of > PAC is useful. currently it's a kernel config option and > system wide on or off.) My understanding is that it was considered too expensive to do without a use case, but we did come up with one. In general we only need to update the system register on process switch as well as on entry/exit if we need to disable IA which is also needed by in-kernel PAC. I did benchmark a variant of the code sequences on existing non-PAC HW [1] which showed that the entry/exit overhead in the common case where IA is left enabled is expected to be low. But with the new Apple chips I think it should be possible to benchmark the real entry/exit code sequences on supporting HW in the enabled and disabled cases. Peter [1] https://lore.kernel.org/linux-arm-kernel/CAMn1gO7HCJiXEzDvBb-=T7znATqyaxE_t-zezqKL0dyXRCG-nQ@xxxxxxxxxxxxxx/
