Re: [Xen-devel] [PATCH 03/10] tools/insn-fuzz: Don't use memcpy() for zero-length reads

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 27.03.17 at 11:56, <andrew.cooper3@xxxxxxxxxx> wrote:
> For control-flow changes, the emulator needs to perform a zero-length
> instruction fetch at the target offset.  It also passes NULL for the
> destination buffer, as there is no instruction stream to collect.
> 
> This trips up UBSAN, even with a size of 0.  Exclude zero-length reads from
> using memcpy(), rather than excluding NULL destination pointers, to still
> catch unintentional uses of NULL.
> 
> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> ---
> CC: Jan Beulich <JBeulich@xxxxxxxx>
> CC: George Dunlap <george.dunlap@xxxxxxxxxxxxx>
> CC: Ian Jackson <Ian.Jackson@xxxxxxxxxxxxx>
> CC: Wei Liu <wei.liu2@xxxxxxxxxx>
> ---
>  tools/fuzz/x86_instruction_emulator/fuzz-emul.c | 2 +-

Btw., shouldn't this entire directory, just like tools/tests/x86_emulator/,
fall under X86 in ./MAINTAINERS?

> --- a/tools/fuzz/x86_instruction_emulator/fuzz-emul.c
> +++ b/tools/fuzz/x86_instruction_emulator/fuzz-emul.c
> @@ -77,7 +77,7 @@ static int data_read(const char *why, void *dst, unsigned 
> int bytes)
>      else
>          rc = maybe_fail(why, true);
>  
> -    if ( rc == X86EMUL_OKAY )
> +    if ( rc == X86EMUL_OKAY && bytes )

So if we really want to work around this oddity, then I think we want
- a comment here
- the check moved into fuzz_insn_fetch()

Jan


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