[AArch64][PAC] Simplify emission of authenticated pointer check (NFC)
The AArch64AsmPrinter::emitPtrauthCheckAuthenticatedValue method accepts
two arguments, `bool ShouldTrap` and `const MCSymbol *OnFailure`, that
control the behavior of the emitted instruction sequence when the check
fails:
* `ShouldTrap` requests an error to be generated
* `OnFailure` requests branching to the given label after clearing the
PAC field
An assertion in `emitPtrauthCheckAuthenticatedValue` ensures that
`OnFailure` must be null when `ShouldTrap` is set. But the opposite is
also true: when `ShouldTrap` is false, `OnFailure` is always non-null,
as otherwise the entire sequence following `AUT[ID][AB]` instruction
would turn into a very expensive equivalent of XPAC (unless the CPU
implements FEAT_FPAC):
authenticate Xn
inspect PAC field of Xn
if PAC field was not cleared:
clear PAC field
The only caller that makes use of checking-but-not-trapping mode of
emitPtrauthCheckAuthenticatedValue is emitPtrauthAuthResign, and it
passes a non-null pointer as OnFailure when ShouldTrap is false.
This commit makes the invariant explicit by omitting the ShouldTrap
argument and inferring its value from the OnFailure argument instead.
diff --git a/llvm/lib/Target/AArch64/AArch64AsmPrinter.cpp b/llvm/lib/Target/AArch64/AArch64AsmPrinter.cpp
index c31a090..1a2808f 100644
--- a/llvm/lib/Target/AArch64/AArch64AsmPrinter.cpp
+++ b/llvm/lib/Target/AArch64/AArch64AsmPrinter.cpp
@@ -162,8 +162,7 @@
Register ScratchReg,
AArch64PACKey::ID Key,
AArch64PAuth::AuthCheckMethod Method,
- bool ShouldTrap,
- const MCSymbol *OnFailure);
+ const MCSymbol *OnFailure = nullptr);
// Check authenticated LR before tail calling.
void emitPtrauthTailCallHardening(const MachineInstr *TC);
@@ -1939,14 +1938,19 @@
return ScratchReg;
}
-/// Emits a code sequence to check an authenticated pointer value.
+/// Emit a code sequence to check an authenticated pointer value.
///
-/// If OnFailure argument is passed, jump there on check failure instead
-/// of proceeding to the next instruction (only if ShouldTrap is false).
+/// This function emits a sequence of instructions that checks if TestedReg was
+/// authenticated successfully. On success, execution continues at the next
+/// instruction after the sequence.
+///
+/// The action performed on failure depends on the OnFailure argument:
+/// * if OnFailure is not nullptr, control is transferred to that label after
+/// clearing the PAC field
+/// * otherwise, BRK instruction is emitted to generate an error
void AArch64AsmPrinter::emitPtrauthCheckAuthenticatedValue(
Register TestedReg, Register ScratchReg, AArch64PACKey::ID Key,
- AArch64PAuth::AuthCheckMethod Method, bool ShouldTrap,
- const MCSymbol *OnFailure) {
+ AArch64PAuth::AuthCheckMethod Method, const MCSymbol *OnFailure) {
// Insert a sequence to check if authentication of TestedReg succeeded,
// such as:
//
@@ -1983,7 +1987,7 @@
.addReg(getWRegFromXReg(ScratchReg))
.addReg(TestedReg)
.addImm(0));
- assert(ShouldTrap && !OnFailure && "DummyLoad always traps on error");
+ assert(!OnFailure && "DummyLoad always traps on error");
return;
}
@@ -2037,15 +2041,14 @@
llvm_unreachable("Unsupported check method");
}
- if (ShouldTrap) {
- assert(!OnFailure && "Cannot specify OnFailure with ShouldTrap");
+ if (!OnFailure) {
// Trapping sequences do a 'brk'.
// brk #<0xc470 + aut key>
EmitToStreamer(MCInstBuilder(AArch64::BRK).addImm(0xc470 | Key));
} else {
// Non-trapping checked sequences return the stripped result in TestedReg,
- // skipping over success-only code (such as re-signing the pointer) if
- // there is one.
+ // skipping over success-only code (such as re-signing the pointer) by
+ // jumping to OnFailure label.
// Note that this can introduce an authentication oracle (such as based on
// the high bits of the re-signed value).
@@ -2070,12 +2073,9 @@
MCInstBuilder(XPACOpc).addReg(TestedReg).addReg(TestedReg));
}
- if (OnFailure) {
- // b Lend
- EmitToStreamer(
- MCInstBuilder(AArch64::B)
- .addExpr(MCSymbolRefExpr::create(OnFailure, OutContext)));
- }
+ // b Lend
+ const auto *OnFailureExpr = MCSymbolRefExpr::create(OnFailure, OutContext);
+ EmitToStreamer(MCInstBuilder(AArch64::B).addExpr(OnFailureExpr));
}
// If the auth check succeeds, we can continue.
@@ -2102,9 +2102,8 @@
"Neither x16 nor x17 is available as a scratch register");
AArch64PACKey::ID Key =
AArch64FI->shouldSignWithBKey() ? AArch64PACKey::IB : AArch64PACKey::IA;
- emitPtrauthCheckAuthenticatedValue(
- AArch64::LR, ScratchReg, Key, LRCheckMethod,
- /*ShouldTrap=*/true, /*OnFailure=*/nullptr);
+ emitPtrauthCheckAuthenticatedValue(AArch64::LR, ScratchReg, Key,
+ LRCheckMethod);
}
void AArch64AsmPrinter::emitPtrauthAuthResign(
@@ -2178,9 +2177,8 @@
if (IsAUTPAC && !ShouldTrap)
EndSym = createTempSymbol("resign_end_");
- emitPtrauthCheckAuthenticatedValue(AUTVal, Scratch, AUTKey,
- AArch64PAuth::AuthCheckMethod::XPAC,
- ShouldTrap, EndSym);
+ emitPtrauthCheckAuthenticatedValue(
+ AUTVal, Scratch, AUTKey, AArch64PAuth::AuthCheckMethod::XPAC, EndSym);
}
// We already emitted unchecked and checked-but-non-trapping AUTs.
@@ -2519,9 +2517,7 @@
: AArch64PACKey::DA);
emitPtrauthCheckAuthenticatedValue(AArch64::X16, AArch64::X17, AuthKey,
- AArch64PAuth::AuthCheckMethod::XPAC,
- /*ShouldTrap=*/true,
- /*OnFailure=*/nullptr);
+ AArch64PAuth::AuthCheckMethod::XPAC);
}
} else {
EmitToStreamer(MCInstBuilder(AArch64::LDRXui)
@@ -2654,9 +2650,7 @@
(AuthOpcode == AArch64::AUTIA ? AArch64PACKey::IA : AArch64PACKey::DA);
emitPtrauthCheckAuthenticatedValue(AuthResultReg, AArch64::X17, AuthKey,
- AArch64PAuth::AuthCheckMethod::XPAC,
- /*ShouldTrap=*/true,
- /*OnFailure=*/nullptr);
+ AArch64PAuth::AuthCheckMethod::XPAC);
emitMovXReg(DstReg, AuthResultReg);
}
