Author: Piotr Kubaj Date: 2026-07-25T12:11:31-07:00 New Revision: 6b0a46958c56d91e3b2fd5ffecfe163dde1fac3a
URL: https://github.com/llvm/llvm-project/commit/6b0a46958c56d91e3b2fd5ffecfe163dde1fac3a DIFF: https://github.com/llvm/llvm-project/commit/6b0a46958c56d91e3b2fd5ffecfe163dde1fac3a.diff LOG: [libunwind][PPC64] Fix unw_getcontext corrupting callee-saved VSX registers on LE (#198371) This is the first of two independent fixes for libunwind on ppc64le (ELFv2 ABI, little-endian), where two separate bugs together cause SIGSEGV during backtracing. This commit addresses the VSX register corruption; the TOC-restore fault is handled in a follow-up. Both were discovered while debugging lang/rust build failures with RUST_BACKTRACE=1 on FreeBSD/powerpc64le (IBM POWER9). On ppc64le, `unw_getcontext` saves each VS register with an in-place `xxswapd n, n` followed by `stxvd2x`. The swap is needed because `stxvd2x` stores doublewords in the wrong order on LE. However, the macro never applies a second `xxswapd` to restore the register after the store, so all 64 VS registers are permanently corrupted on return from `unw_getcontext`. This affects every callee-saved VSX register: f14-f31 (VSR14-VSR31) and VR20-VR31 (VSR52-VSR63). After `_Unwind_Backtrace` returns, any code that uses these registers sees wrong values. In practice this manifests as SIGSEGV inside hashbrown's `reserve_rehash`: VR20-VR31 are corrupted before a SIMD comparison loop runs, producing an out-of-bounds access. Fix: add a second `xxswapd n, n` after the `stxvd2x` store. Since `xxswapd` is its own inverse, the pair is a no-op on the architectural register while still writing the correctly byte-swapped value to memory. Added: Modified: libunwind/src/UnwindRegistersSave.S Removed: ################################################################################ diff --git a/libunwind/src/UnwindRegistersSave.S b/libunwind/src/UnwindRegistersSave.S index 678187fb52f01..37acbf3c69322 100644 --- a/libunwind/src/UnwindRegistersSave.S +++ b/libunwind/src/UnwindRegistersSave.S @@ -424,9 +424,13 @@ LnoR2Fix: // register in the incorrect doubleword order. // FIXME: when supporting targets older than Power9 on LE is no longer required // this can be changed to simply `stxv n, 16 * n(4)`. +// Restore the register after storing: xxswapd is its own inverse, so a second +// xxswapd undoes the in-place corruption of callee-saved VSRs (f14-f31, +// VR20-VR31) that would otherwise persist after unw_getcontext returns. #define PPC64_STVS(n) \ xxswapd n, n ;\ stxvd2x n, 0, 4 ;\ + xxswapd n, n ;\ addi 4, 4, 16 #else #define PPC64_STVS(n) \ _______________________________________________ cfe-commits mailing list [email protected] https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits
