Re: [PATCH] x86emul: rework wrapping of libc functions in test and fuzzing harnesses

[Jan Beulich <jbeulich@xxxxxxxx>]

On 17.08.2023 14:58, Andrew Cooper wrote:
> On 17/08/2023 12:47 pm, Jan Beulich wrote:
>> Our present approach is working fully behind the compiler's back. This
>> was found to not work with LTO. Employ ld's --wrap= option instead. Note
>> that while this makes the build work at least with new enough gcc (it
>> doesn't with gcc7, for example, due to tool chain side issues afaict),
>> according to my testing things still won't work when building the
>> fuzzing harness with afl-cc: While with the gcc7 tool chain I see afl-as
>> getting invoked, this does not happen with gcc13. Yet without using that
>> assembler wrapper the resulting binary will look uninstrumented to
>> afl-fuzz.
>>
>> While checking the resulting binaries I noticed that we've gained uses
>> of snprintf() and strstr(), which only just so happen to not cause any
>> problems. Add a wrappers for them as well.
>>
>> Since we don't have any actual uses of v{,sn}printf(), no definitions of
>> their wrappers appear (just yet). But I think we want
>> __wrap_{,sn}printf() to properly use __real_v{,sn}printf() right away,
>> which means we need delarations of the latter.
>>
>> Reported-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
>> Suggested-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> 
> This does resolve the build issue.  I do get a binary out of the end, so
> Tested-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>.

Thanks.

>  I presume that you've smoke tested the resulting binary?

The fuzzer one? No. I didn't think it is of any use when not driven by afl.
I did a proper test of the test harness one, albeit not with LTO in use (I
focused on the fuzzer one with the LTO issue).

> However, I do see something else in the logs which is concerning. 
> Likely unrelated.
> 
> make[6]: Entering directory
> '/builddir/build/BUILD/xen-4.18.0/tools/tests/x86_emulator'
> gcc -m32 -march=i686 -DBUILD_ID -fno-strict-aliasing -std=gnu99 -Wall
> -Wstrict-prototypes -Wdeclaration-after-statement
> -Wno-unused-but-set-variable -Wno-unused-local-typedefs -g3 -Werror -Og
> -fno-omit-frame-pointer
> -D__XEN_INTERFACE_VERSION__=__XEN_LATEST_INTERFACE_VERSION__
> -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -mno-tls-direct-seg-refs
> -fno-pie -fno-stack-protector -fno-exceptions
> -fno-asynchronous-unwind-tables -fno-builtin -g0 -D_64f2 -mavx512fp16
> -ffixed-xmm0 -Os -DVEC_SIZE=64 -DFLOAT_SIZE=2 -c avx512fp16.c
> make[6]: Leaving directory
> '/builddir/build/BUILD/xen-4.18.0/tools/tests/x86_emulator'
> /tmp/ccrznrqa.s: Assembler messages:
> /tmp/ccrznrqa.s:98: Error: no such instruction: `vmovw .LC0,%xmm3'
> /tmp/ccrznrqa.s:99: Error: no such instruction: `vmovw %xmm3,58(%esp)'
> /tmp/ccrznrqa.s:105: Error: no such instruction: `vcvtsi2shl
> %eax,%xmm1,%xmm1'
> /tmp/ccrznrqa.s:106: Error: no such instruction: `vmovw
> %xmm3,382(%esp,%eax,2)'
> /tmp/ccrznrqa.s:107: Error: no such instruction: `vmovw
> %xmm1,-2(%edx,%eax,2)'
> /tmp/ccrznrqa.s:108: Error: no such instruction: `vcvtsi2shl
> %ecx,%xmm1,%xmm1'
> /tmp/ccrznrqa.s:109: Error: no such instruction: `vmovw
> %xmm1,318(%esp,%eax,2)'
> /tmp/ccrznrqa.s:113: Error: no such instruction: `vaddph
> 256(%esp),%zmm7,%zmm5'
> <snip many>
> simd-fma.c:208: Error: no such instruction: `vfmaddsub231ph
> 60(%esp){1to32},%zmm6,%zmm5'
> simd-fma.c:209: Error: no such instruction: `vfmaddsub231ph
> 60(%esp){1to32},%zmm6,%zmm1'
> 
> GCC is 12.2.1, binutils is 2.37
> 
> AVX512_FP16 was added in bintuils 2.38 so I understand the simd-fma.c
> complains,

Right. I assume the gcc is not the system one, or else I'd find it
odd to have a compiler backed by a less capable assembler.

> but ccrznrqa.s suggest that there's a bad -m passed.  I
> haven't figured out which source file it logically associated with.

The source file is avx512fp16.c as per the compiler command line,
which in turn is a symlink to simd.c. We don't pass any naming
options from testcase.mk, so the compiler using a made up name is
expected.

Jan