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

[George Dunlap <george.dunlap@xxxxxxxxxx>]

On 27/03/17 10:56, Andrew Cooper 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.

So memcpy() will actually try to write to dst even if bytes == 0?

That seems a bit strange, but OK:

Acked-by: George Dunlap <george.dunlap@xxxxxxxxxx>

> 
> 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 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/tools/fuzz/x86_instruction_emulator/fuzz-emul.c 
> b/tools/fuzz/x86_instruction_emulator/fuzz-emul.c
> index 890642c..cbdb3dd 100644
> --- 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 )
>      {
>          memcpy(dst,  input.data + data_index, bytes);
>          data_index += bytes;
> 


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