Re: valgrind support for glibc master

[Ramana Radhakrishnan <ramana dot gcc at googlemail dot com>]

On Thu, Jun 22, 2017 at 12:14 PM, Florian Weimer <fweimer@redhat.com> wrote:
> glibc performs a quick test run using valgrind as part of the build process.
>
> Lately, this started crashing:
>
> + elf/ld.so --library-path .:elf:nptl:dlfcn /usr/bin/valgrind elf/ld.so
> --library-path .:elf:nptl:dlfcn /usr/bin/true
> ==924== Memcheck, a memory error detector
> ==924== Copyright (C) 2002-2017, and GNU GPL'd, by Julian Seward et al.
> ==924== Using Valgrind-3.13.0 and LibVEX; rerun with -h for copyright info
> ==924== Command: elf/ld.so --library-path .:elf:nptl:dlfcn /usr/bin/true
> ==924==
> ARM64 front end: branch_etc
> disInstr(arm64): unhandled instruction 0xD5380000
> disInstr(arm64): 1101'0101 0011'1000 0000'0000 0000'0000
> ==924== valgrind: Unrecognised instruction at address 0x11f548.
> ==924==    at 0x11F548: init_cpu_features (cpu-features.c:32)
> ==924==    by 0x11F548: dl_platform_init (dl-machine.h:241)
> ==924==    by 0x11F548: _dl_sysdep_start (dl-sysdep.c:231)
> ==924==    by 0x10981B: _dl_start_final (rtld.c:412)
> ==924==    by 0x109AAB: _dl_start (rtld.c:520)
> ==924==    by 0x108F47: ??? (in
> /builddir/build/BUILD/glibc-2.25-545-g9649350/build-aarch64-redhat-linux/elf/ld.so)
>
> The line in question is:
>
>       asm volatile ("mrs %0, midr_el1" : "=r"(id));

This instruction actually traps to the kernel and comes back with the
right value for MIDR with some emulation in the kernel. I suspect you
are looking at a valgrind issue here.

regards
Ramana

>
> That seems to match the instruction bit pattern, too.  There is a check
> around it:
>
>   if (hwcap & HWCAP_CPUID)
>     {
>       register uint64_t id = 0;
>       asm volatile ("mrs %0, midr_el1" : "=r"(id));
>       cpu_features->midr_el1 = id;
>     }
>   else
>     cpu_features->midr_el1 = 0;
>
> I think this code is fine.  Unfortunately, I don't know if I'll be able
> to get a disassembly or debug this any further.  There are a couple of
> potential causes (GLRO (dl_hwcap) is not initialized correctly in glibc,
> HWCAP_CPUID is not masked by the kernel or valgrind despite the lack of
> support, GCC schedule the volatile asm statement before the condition).
>
> Is anyone else seeing this?
>
> I will disable the valgrind sanity test during the Fedora build for the
> time being.
>
> Thanks,
> Florian