Inside the noble image the following extension where shown in /proc/cpuinfo:
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The illegal instruction is observed inside glibc's memset implementation at an instruction that uses RISC-V vector extension (vse64.v). The failure is intermittent (~50% reproducer rate). Using a scalar-only memset (libnovecmem.so via LD_PRELOAD) or running under gdb / enabling QEMU_STRACE significantly reduces or eliminates the failure, which strongly suggests a qemu-user/emulation bug (vector handling / code generation / state corruption), not a cmake bug. ## Affects - qemu-user qemu-riscv64-static (as packaged in Ubuntu qemu 10.1.0+ds-5ubuntu3) - Running in Docker container for riscv64 on x86_64 host via binfmt_misc qemu-user static interpreter ## Environment / Context - Host CPU: x86_64 (Docker multiarch running qemu-user for riscv64) - Host OS：multiple Ubuntu releases (22.04, 24.04, 25.10) - Container image: ubuntu:26.04 for riscv64 - qemu package used: - downloaded .deb from Launchpad: qemu-user_10.1.0+ds-5ubuntu3_amd64.deb and on several Debian qemu-user packages (qemu-user_10.2.0~rc1+ds-1, qemu-user_10.0.6+ds-0+deb13u2). - copied qemu-riscv64 binary into /usr/bin/qemu-riscv64-static inside host and registered via /proc/sys/fs/binfmt_misc/register - CMake version used inside container (bootstrap/build may use system-provided cmake binary): cmake 3.x (bootstrapping cmake while building also triggers crash) - Reproduction frequency: intermittent, ~50% (can get large variance: several consecutive successes or failures) - Observed behavior changes when: LD_PRELOAD libnovecmem.so (scalar memset) — almost completely avoids crash; running under gdb or enabling QEMU_STRACE also makes it much harder to reproduce. ## Full reproduction steps 1. On x86_64 host, fetch qemu-user .deb and extract the riscv static binary: wget https://launchpad.net/ubuntu/+source/qemu/1:10.1.0+ds-5ubuntu3/+build/31393935/+files/qemu-user_10.1.0+ds-5ubuntu3_amd64.deb dpkg-deb -x qemu-user_10.1.0+ds-5ubuntu3_amd64.deb qemu-user_10.1.0+ds-5ubuntu3_amd64 sudo cp qemu-user_10.1.0+ds-5ubuntu3_amd64/usr/bin/qemu-riscv64 /usr/bin/qemu-riscv64-static 2. Register qemu-riscv64 with binfmt_misc: echo -1 > /proc/sys/fs/binfmt_misc/qemu-riscv64 echo ':qemu-riscv64:M:0:\x7f\x45\x4c\x46\x02\x01\x01\x00\x00\x00\x00\x00\x00\x00\x00\x00\x02\x00\xf3\x00:\xff\xff\xff\xff\xff\xff\xff\x00\xff\xff\xff\xff\xff\xff\xff\xff\xfe\xff\xff\xff:/usr/bin/qemu-riscv64-static:POCF' >/proc/sys/fs/binfmt_misc/register 3. Start riscv64 ubuntu container: docker run --platform=linux/riscv64 --name ubuntu26 -itd ubuntu:26.04 bash docker exec -it ubuntu26 bash -i 4. Inside container: apt update apt install -y build-essential cmake 5. Reproducer 1: cmake --system-information -> Often fails with: bash: [15: 1 (255)] tcsetattr: Inappropriate ioctl for device Illegal instruction (core dumped) 6. Reproducer 2 (minimal C project): Create test_cmake/CMakeLists.txt: cmake_minimum_required(VERSION 3.10) project(HelloCMake C) add_executable(hello main.c) Create test_cmake/main.c: #include <stdio.h> int main() { printf("Hello, CMake!\n"); return 0; } cd test_cmake cmake . -> Crash with: -- Detecting C compiler ABI info bash: line 1: 8489 Illegal instruction (core dumped) cmake . 7. Reproducer 3 (rebuild cmake from source inside container): apt source cmake cd cmake apt-get build-dep . dpkg-buildpackage -us -uc -b -> Bootstrapping error: Illegal instruction (core dumped) Error when bootstrapping CMake: Problem while running initial CMake 8. Observed crash location (from gdb/QEMU_STRACE when available): - Illegal instruction is in memset@@GLIBC_2.27+0x52 - Faulting instruction: vse64.v v1,(a5) (RISC-V vector store of 64-bit elements) ## Workarounds - LD_PRELOAD a scalar-only memset library (libnovecmem.so) to avoid glibc using vectorized memset. - Run the failing process under gdb (slower) or enable QEMU_STRACE=1 — both make the failure much less likely. Note: The same workload does not reproduce the crash when run under qemu-system (full-system emulation). The issue appears specific to qemu-user To manage notifications about this bug go to: https://bugs.launchpad.net/qemu/+bug/2133188/+subscriptions
