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?
Repository:
rG LLVM Github Monorepo
CHANGES SINCE LAST ACTION
https://reviews.llvm.org/D112941/new/
https://reviews.llvm.org/D112941
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