[PATCH] D84502: [AArch64][GlobalISel] Implement __builtin_return_address for PAC-RET

[Ahmed Bougacha via Phabricator via llvm-commits]

ab accepted this revision.
ab added inline comments.
This revision is now accepted and ready to land.


================
Comment at: llvm/lib/Target/AArch64/GISel/AArch64InstructionSelector.cpp:4720
       EntryBuilder.setInstr(*EntryBlock.begin());
-      EntryBuilder.buildCopy({DstReg}, {Register(AArch64::LR)});
+      if (MF.getFunction().hasFnAttribute("sign-return-address")) {
+        if (STI.hasV8_3aOps()) {
----------------
danielkiss wrote:
> chill wrote:
> > ab wrote:
> > > chill wrote:
> > > > ab wrote:
> > > > > I don't know what model you have for the attribute and the command line options, and I guess you're probably aware, but this seems unsafe for depths > 0.  I'm not sure there's a better way to deal with this, short of just forcing the strips in any aarch64 code, which is not an option.  Maybe emitting strips when +pa is available?  I imagine that's not at all reliable either.
> > > > > ... I guess you're probably aware, but this seems unsafe for depths > 0. 
> > > > In what sense is it unsafe? The clear instructions do not require a valid PAC
> > > > to be present, so even if a caller is not using PAC-RET we still get the return
> > > > address.
> > > I'm thinking of the opposite scenario, where a caller in an unrelated library does have a signed return address, but this function wasn't compiled with that support: it wouldn't know to strip.
> > I see. Looks like checking for `+pa`, while not perfect, will work in more cases.
> `XPACLRI` can always be emitted because it is in the NOP space. Besides it is not nice to emit always one extra presumably unnecessary instruction but I don't see why it won't work.
Yeah, it should work to always emit it.  On darwin we know we don't need to, but I imagine it's the exception rather than the rule.  I think we'll address that when merging this into our ptrauth changes: either emit XPACI (as a strip intrinsic) or nothing, based on the target architecture.  Either way this LGTM.


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