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

[Markus Armbruster]

Eric Blake <address@hidden> writes:

> On 08/04/2016 12:46 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
>
> I never understood why glib made that choice on g_malloc(0). I would
> much prefer it to ALWAYS return something, just as glibc malloc(0) does.

I guess it made the choice because unlike malloc(0), it can't cause
confusion.  malloc() returning null can mean either error or nothing.
The caller should pick nothing when it passed zero.  g_malloc()
returning null can only mean nothing.

>> then calling memset(NULL, chr, 0), which is UB.
>
> Indeed, although I really wish POSIX could be loosened to say that the
> source pointer is untouched if the length is 0 (I've debated about
> filing a POSIX bug report to that effect, but have not done so yet), so
> that the UB only happens when passing NULL with a non-zero size.

For me, this is one of those pointless UBs, whose only effect on the
real world is wasting programmer time.  Seriously, what else could a
sane memset(NULL, 0, 0) do?  What could anyone gain from an insane
memset() other than the glee of telling people "your program is wrong,
and you're <insert derogatory word>!".

The mission of this standard is to codify existing practice.  Show me a
real-world implementation of memset() that does something other than
nothing when passed a zero size, and whose maintainers don't consider
that a bug in need of fixing.

>> 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"
>
> My preference - even if we have to special case things to avoid UB at
> the lower level, presenting well-defined behavior at the upper level is
> easier to think about.
>
>> (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)
>
> Doable, but not as fun to audit, and not my preference.

Concur.  We shouldn't slavishly replicate an underlying interface's
complexities when we can just as well provide a more regular interface
instead.