[PATCH] D112941: [clang] Add support for the new pointer authentication builtins.

Kristof Beyls via Phabricator via cfe-commits cfe-commits at lists.llvm.org
Wed Nov 10 00:28:49 PST 2021

kristof.beyls added inline comments.

Comment at: clang/include/clang/Driver/Options.td:2865-2872
+let Group = f_Group in {
+  let Flags = [CC1Option] in {
+    def fptrauth_intrinsics : Flag<["-"], "fptrauth-intrinsics">,
+      HelpText<"Enable pointer-authentication intrinsics">;
+  }
+  def fno_ptrauth_intrinsics : Flag<["-"], "fno-ptrauth-intrinsics">;
bruno wrote:
> ab wrote:
> > rjmccall wrote:
> > > kristof.beyls wrote:
> > > > My impression is that generally for `__builtin_XXX` intrinsics, there are no compiler flags to make them available or remove their availability.
> > > > Is there a good reason why a command line option is needed for the `__builtin_ptrauth` intrinsics, but not (IIUC) for most or any other existing `__builtin_XXX` intrinsic?
> > > > If there is no good reason, it seems better to me to not have a command line option so there is better consistency across all `__builtin_XXX` intrinsics?
> > > > 
> > > > (after having read more of the patch): my impression has changed now that the f(no-)ptrauth-intrinsics flag rather selects whether the ptrauth intrinsics get lowered to PAuth hardware instructions, or to "regular" instructions emulating the behavior of authenticated pointers. If that is correct (and assuming it's a useful option to have), I would guess a different name for the command line option could be less misleading. As is, it suggests this selects whether ptrauth_ intrinsics are available or not. If instead, as I'm guessing above, this selects whether ptrauth_ intrinsics get lowered to PAuth instructions or not, maybe something like '-femulate-ptrauth' would describe the effect of the command line switch a bit better?
> > > The ptrauth features were implemented gradually, beginning with the intrinsics.  Originally we needed a way to enable the intrinsics feature without relying on target information.  We do still need a way to enable them without necessarily enabling automatic type/qualifier-based pointer authentication.  I don't know if we need to be able to *disable* them when the target supports them; I agree that that would be a little strange.
> > > 
> > > If not, we could just enable the intrinsics whenever either the target says they're okay or software emulation (a separate, experimental feature) is enabled.  The AArch64 target has a `+pauth` target feature.  However, I don't know if `-arch arm64e` actually adds that feature on Apple targets.  Also, the `HasPAuth` field in the clang `TargetInfo` does not appear to be properly initialized to `false` when `+pauth` *isn't* present; fortunately, that field never used.
> > > 
> > > I'm not sure if it would actually be okay to remove the `-fptrauth-intrinsics` driver option if we just enabled the intrinsics based on the target feature.  That does feel cleaner, but unfortunately, we at Apple probably have explicit uses of the option that we'd have to clean up before we could remove the option.  We could treat that as an Apple problem and keep it out of the open source tree, though, and maybe remove the option altogether someday.
> > > 
> > > Ahmed, thoughts?
> > Hmm, I agree it would be strange to need to disable the intrinsics, but we do also gate the various higher-level qualifiers (and intrinsics) on `ptrauth_intrinsics`.  So, in `ptrauth.h` (and in various users) the feature now really means "we're in a 'ptrauth-aware' environment".  And it does make more sense to keep that separate from "we're running on a CPU that theoretically could support ptrauth".  It comes down to what "ptrauth-aware" really means, and that's probably also an Apple problem, and all current users of `ptrauth_intrinsics` should use something like `__arm64e__` instead.
> > 
> > That still means there's no equivalent for other targets and/or software emulation, but that seems okay: `ptrauth.h` already needs changes to be usable from anywhere other than arm64e (cf. the discussion about keys), and we can cross that bridge when we get there.
> > 
> > (One could argue that all the language-feature-specific qualifiers and intrinsics should be gated on the appropriate ptrauth_whatever feature, but the qualifiers are often used in precisely the glue/runtime code that doesn't build in the appropriate mode, so doesn't have the feature enabled.)
> > 
> > 
> > So, concretely, we could:
> > - continue gating these plain intrinsics on `ptrauth_intrinsics` in ptrauth.h (IIRC there's an ACLE feature macro but it's specific to return address signing and BTI defaults; I'll check)
> > - enable the feature when `+pauth`
> > - replace all other uses of `ptrauth_intrinsics` with `__arm64e__`, in both ptrauth.h (gating the ABI qualifiers) and gradually in our internal codebases
> > 
> > and keep `-fptrauth-intrinsics` downstream for the transition (or, depending on how much the flag is really used, just keep the old behavior of enabling the feature for arm64e only;  but yeah, that's my downstream problem)
> I'd vote for keeping as is: `-fptrauth-intrinsics` allows a nice limited use of the pauth feature pack. Has actually been useful for us (non-Apple targets) in code that needs signing capabilities but not want itself to be codegen'd using pauth - e.g. a dynamic linker for a system that is migrating codebase to pauth in small steps.
> `ptrauth_intrinsics` gates have been equally useful. By doing this downstream we would need to duplicate this logic as well, so I don't really it benefiting the community as much.
> Enabling the feature when `+pauth` sounds like overall goodness regardless imo.
Thanks for sharing your experience @bruno .
I have to confess I  do not fully understand "code that needs signing capabilities but not want itself to be codegen'd using pauth." I'm assuming this means "the ptrauth intrinsics must be available, but the compiler must not automatically insert pointer signing/authenticating instructions"?

If I understand that correctly, I'm still wondering if it's useful to have a command line switch that removes ptrauth intrinsics, rather than relying on ptrauth.h having the appropriate ifdefs to remove intrinsics when targeting something where they cannot work?

With the above, I guess "-f(no)ptrauth-intrinsics" then actually means "let the compiler automatically insert signing/authenticating instructions as defined by the default signing scheme for the target triple you're targeting"?

Maybe my confusion would be less if this patch also adds documentation for the command line switch.
I'm not sure where that documentation would best live. Maybe at https://clang.llvm.org/docs/UsersManual.html#command-line-options?

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