Re: [PATCH 2/5] KVM: selftests: Compare insn opcodes directly in fix_hypercall_test

[Oliver Upton <oliver.upton@linux.dev>]

On Thu, Sep 08, 2022 at 11:31:31PM +0000, Sean Christopherson wrote:
> Directly compare the expected versus observed hypercall instructions when
> verifying that KVM patched in the native hypercall (FIX_HYPERCALL_INSN
> quirk enabled).  gcc rightly complains that doing a 4-byte memcpy() with
> an "unsigned char" as the source generates an out-of-bounds accesses.
> 
> Alternatively, "exp" and "obs" could be declared as 3-byte arrays, but
> there's no known reason to copy locally instead of comparing directly.

I was trying to print just the instruction bytes if such a comparison
failed, but that's already a bust given that it was a 4-byte copy.

Having said that, the assertion should be clear enough.

> In function ‘assert_hypercall_insn’,
>     inlined from ‘guest_main’ at x86_64/fix_hypercall_test.c:91:2:
> x86_64/fix_hypercall_test.c:63:9: error: array subscript ‘unsigned int[0]’
>  is partly outside array bounds of ‘unsigned char[1]’ [-Werror=array-bounds]
>    63 |         memcpy(&exp, exp_insn, sizeof(exp));
>       |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> x86_64/fix_hypercall_test.c: In function ‘guest_main’:
> x86_64/fix_hypercall_test.c:42:22: note: object ‘vmx_hypercall_insn’ of size 1
>    42 | extern unsigned char vmx_hypercall_insn;
>       |                      ^~~~~~~~~~~~~~~~~~
> x86_64/fix_hypercall_test.c:25:22: note: object ‘svm_hypercall_insn’ of size 1
>    25 | extern unsigned char svm_hypercall_insn;
>       |                      ^~~~~~~~~~~~~~~~~~
> In function ‘assert_hypercall_insn’,
>     inlined from ‘guest_main’ at x86_64/fix_hypercall_test.c:91:2:
> x86_64/fix_hypercall_test.c:64:9: error: array subscript ‘unsigned int[0]’
>  is partly outside array bounds of ‘unsigned char[1]’ [-Werror=array-bounds]
>    64 |         memcpy(&obs, obs_insn, sizeof(obs));
>       |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> x86_64/fix_hypercall_test.c: In function ‘guest_main’:
> x86_64/fix_hypercall_test.c:25:22: note: object ‘svm_hypercall_insn’ of size 1
>    25 | extern unsigned char svm_hypercall_insn;
>       |                      ^~~~~~~~~~~~~~~~~~
> x86_64/fix_hypercall_test.c:42:22: note: object ‘vmx_hypercall_insn’ of size 1
>    42 | extern unsigned char vmx_hypercall_insn;
>       |                      ^~~~~~~~~~~~~~~~~~
> cc1: all warnings being treated as errors
> make: *** [../lib.mk:135: tools/testing/selftests/kvm/x86_64/fix_hypercall_test] Error 1
> 
> Fixes: 6c2fa8b20d0c ("selftests: KVM: Test KVM_X86_QUIRK_FIX_HYPERCALL_INSN")
> Cc: Oliver Upton <oliver.upton@linux.dev>
> Signed-off-by: Sean Christopherson <seanjc@google.com>

Reviewed-by: Oliver Upton <oliver.upton@linux.dev>

--
Thanks,
Oliver