On Thu, Nov 19, 2020 at 2:40 AM Szabolcs Nagy <szabolcs.nagy@xxxxxxx> wrote: > > The 11/18/2020 21:20, Peter Collingbourne via Libc-alpha 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 > > i would mention in the commit that the overhead of the kernel entry > code change was measured to be small. Done. > > --- > > v3: > > - fix some style nits > > - move kernel entry ISB to after setting EnIA > > - rename sctlr -> sctlr_user > > - remove init_sctlr > > > > v2: > > - added prctl(PR_PAC_GET_ENABLED_KEYS) > > - added ptrace APIs for getting and setting the set of enabled > > keys > > - optimized the instruction sequence for kernel entry/exit > > - rebased on top of MTE series > ... > > +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. > > please document how the setting is inherited across clone, fork, exec > and the setting at process startup (since it wont be inherited across > exec and not every reader knows what setting is required for bw compat). Done. (Sorry, I said that I would do that then forgot to actually do it.) While I was here I also documented the perf difference between IA and the other keys. Peter
