Disable AVX-optimized functions in glibc (LD_HWCAP_MASK, /etc/ld.so.nohwcap) for valgrind & gdb record

Question

Modern x86_64 linux with glibc will detect that CPU has support of AVX extension and will switch many string functions from generic implementation to AVX-optimized version (

Answer 1

I encountered this problem recently as well, and ended up solving it using dynamic CPUID faulting to interrupt execution of the CPUID instruction and override its result, which avoids touching glibc or the dynamic linker. This requires processor support for CPUID faulting (Ivy Bridge+) as well as Linux kernel support (4.12+) for exposing it to userspace through the ARCH_GET_CPUID and ARCH_SET_CPUID subfunctions of arch_prctl(). When this feature is enabled, a SIGSEGV signal will be delivered on each execution of CPUID, allowing a signal handler can emulate execution of the instruction and override the result.

The full solution is a bit involved since I also need to interpose the dynamic linker, because hardware capability detection was moved there starting with glibc 2.26+. I've uploaded the full solution online at https://github.com/ddcc/libcpuidoverride .