[libunwind] a9e6f90 - [libunwind][PAuthLR] Remove PC offset when using FEAT_PAuthLR (#164224)

[via cfe-commits]

Author: Jack Styles
Date: 2025-10-29T08:20:11Z
New Revision: a9e6f90867b1c7cc18fe46822148d99f614f9e33

URL: https://github.com/llvm/llvm-project/commit/a9e6f90867b1c7cc18fe46822148d99f614f9e33
DIFF: https://github.com/llvm/llvm-project/commit/a9e6f90867b1c7cc18fe46822148d99f614f9e33.diff

LOG: [libunwind][PAuthLR] Remove PC offset when using FEAT_PAuthLR (#164224)

When originally introduced to libunwind as part of #112171, FEAT_PAuthLR
had its Call Frame Instruction's (CFI's) in a different location to
other Signing Authentication methods. To incorporate this in libunwind,
a 4 byte offset was introduced to work with this. However, this design
was reversed in #121551 so the CFI's are emitted in the same location as
other methods. When making this change, the offset in libunwind was not
removed, so libunwind's PC value would be incorrect.

As the 4 byte offset is no longer needed, that adjustment can be
removed. results->ptrAuthDiversifier will still be set.

Added: 
    

Modified: 
    libunwind/src/DwarfParser.hpp

Removed: 
    


################################################################################
diff  --git a/libunwind/src/DwarfParser.hpp b/libunwind/src/DwarfParser.hpp
index dbd7d65c354aa..2b04ae2831f9a 100644
--- a/libunwind/src/DwarfParser.hpp
+++ b/libunwind/src/DwarfParser.hpp
@@ -842,12 +842,10 @@ bool CFI_Parser<A>::parseFDEInstructions(A &addressSpace,
             results->savedRegisters[UNW_AARCH64_RA_SIGN_STATE].value ^ 0x3;
         results->setRegisterValue(UNW_AARCH64_RA_SIGN_STATE, value,
                                   initialState);
-        // When calculating the value of the PC, it is assumed that the CFI
-        // instruction is placed before the signing instruction, however it is
-        // placed after. Because of this, we need to take into account the CFI
-        // instruction is one instruction call later than expected, and reduce
-        // the PC value by 4 bytes to compensate.
-        results->ptrAuthDiversifier = fdeInfo.pcStart + codeOffset - 0x4;
+        // When using Feat_PAuthLR, the PC value needs to be captured so that
+        // during unwinding, the correct PC value is used for re-authentication.
+        // It is assumed that the CFI is placed before the signing instruction.
+        results->ptrAuthDiversifier = fdeInfo.pcStart + codeOffset;
         _LIBUNWIND_TRACE_DWARF(
             "DW_CFA_AARCH64_negate_ra_state_with_pc(pc=0x%" PRIx64 ")\n",
             static_cast<uint64_t>(results->ptrAuthDiversifier));