https://github.com/pkubaj updated https://github.com/llvm/llvm-project/pull/198371
>From c20f287696d793c08097001ad4be945ce679991f Mon Sep 17 00:00:00 2001 From: Piotr Kubaj <[email protected]> Date: Mon, 18 May 2026 00:00:00 +0000 Subject: [PATCH] Fix unw_getcontext corrupting callee-saved VSX registers on LE On ppc64le, the PPC64_STVS macro saves each VS register by doing an in-place `xxswapd n, n` to swap its two 64-bit elements into the correct memory layout before calling stxvd2x. It never applies a second xxswapd to restore the register before returning. Since xxswapd is its own inverse, this permanently corrupts all 64 VS registers saved by unw_getcontext -- including the callee-saved f14-f31 (VSR14-VSR31) and VR20-VR31 (VSR52-VSR63). The corruption persists after unw_getcontext returns into the caller, so any code that uses these vector registers after an unwind backtrace sees wrong values. In practice this causes SIGSEGV inside hashbrown's reserve_rehash when RUST_BACKTRACE=1 is set, because every panic calls _Unwind_Backtrace -> unw_getcontext, corrupting VR20-VR31 before hashbrown's SIMD comparison loop runs and producing an out-of-bounds access. Fix by adding a second `xxswapd n, n` immediately after the stxvd2x store. The pair is a no-op on the architectural register while still writing the correctly-ordered value to memory. The existing FIXME noting that this whole dance goes away once pre-P9 LE support is dropped is retained and remains accurate. --- libunwind/src/UnwindRegistersSave.S | 4 ++++ 1 file changed, 4 insertions(+) diff --git a/libunwind/src/UnwindRegistersSave.S b/libunwind/src/UnwindRegistersSave.S index ca9a97b18e764..ffec44452ff60 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
