Re: [Qemu-devel] qtest protocol: should memset/read/write etc of a size of 0 bytes be permitted?

[John Snow]

On 08/04/2016 02:46 PM, Peter Maydell wrote:
> I've upgraded to a more recent version of clang, which now produces
> undefined-behaviour warnings for passing NULL pointers to some library
> functions. One of the things it has shown up is that some of the
> qtest tests ask for "memset" with size zero. In our current implementation
> this results in qtest.c calling g_malloc(0), which returns NULL, and
> then calling memset(NULL, chr, 0), which is UB.
>
> So should we:
> (1) declare the qtest protocol commands 'memset', 'read', 'write'
> etc which operate on a lump of guest memory of specified size to
> support size == 0 as meaning "do nothing"

This would be easy to do.

> (2) declare that size == 0 is not valid and make it return a failure
> code back down the qtest pipe (and fix the offending tests)

This is probably the nicer thing to do -- if memset of length 0 is 
undefined, probably qmemset and friends should also be undefined by 
extension.

I reserve the right to change my mind depending on how gnarly it is to 
untangle.

I assume you're hoping for 2.7.

> ?
>
> The offending tests are i386/ahci/flush/simple and i386/ahci/max
> (because ahci_io() calls qmemset() with a zero size.)
>
> thanks
> -- PMM

--js