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

Not the best or complete solution, just a smallest bit-editing kludge to allow valgrind and gdb record for the my task.

Lekensteyn asks:

how to mask out AVX/SSE without recompiling glibc

I did full rebuild of unmodified glibc, which is rather easy in debian and ubuntu: just sudo apt-get source glibc, sudo apt-get build-dep glibc and cd glibc-*/; dpkg-buildpackage -us -uc (manual to get the ld.so without stripped debugging information.

Then I did binary (bit) patching of the output ld.so file, in the function used by __get_cpu_features. Target function was compiled from get_common_indeces of source file sysdeps/x86/cpu-features.c under the name of get_common_indeces.constprop.1 (it is just next after the __get_cpu_features in the binary code). It has several cpuids, first one is cpuid eax=1 "Processor Info and Feature Bits"; and later there is check "jle 0x6" and jump down around the code "cpuid eax=7 ecx=0 Extended Features" just to get AVX2 status. There is the code which was compiled into this logic:

get_common_indeces (struct cpu_features *cpu_features,
            unsigned int *family, unsigned int *model,
            unsigned int *extended_model, unsigned int *stepping)
{ ...
  if (cpu_features->max_cpuid >= 7)
    __cpuid_count (7, 0,
           cpu_features->cpuid[COMMON_CPUID_INDEX_7].eax,
           cpu_features->cpuid[COMMON_CPUID_INDEX_7].ebx,
           cpu_features->cpuid[COMMON_CPUID_INDEX_7].ecx,
           cpu_features->cpuid[COMMON_CPUID_INDEX_7].edx);

The cpu_features->max_cpuid was filled in init_cpu_features of the same file in __cpuid (0, cpu_features->max_cpuid, ebx, ecx, edx); line. It was easier to disable the if statement by replacing jle after cmp 0x6 with jg (byte 0x7e to 0x7f). (Actually this binary patch was reapplied manually to the __get_cpu_features function of real system ld-linux.so.2 - first jle before mov 7 eax; xor ecx,ecx; cpuid changed into jg.)

Recompiled package and modified ld.so were not installed into the system; I used commandline syntax of ld.so ./my_program (or mv ld.so /some/short/path.so and patchelf --set-interpreter ./my_program).

Other possible solutions:

• try to use more recent valgrind & gdb record tools
• try to use older glibc
• implement missing instruction emulation in gdb record if it is not done
• do source code patching around if (cpu_features->max_cpuid >= 7) in glibc and recompile
• do source code patching around avx2-enabled string functions in glibc and recompile