Re: [Xen-devel] [PATCH v3 21/28] tools/libxl: Infrastructure for writing a v2 stream

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 13/07/15 16:21, Ian Jackson wrote:
> Andrew Cooper writes ("[PATCH v3 21/28] tools/libxl: Infrastructure for 
> writing a v2 stream"):
> ...
>> +void libxl__stream_write_start(libxl__egc *egc,
>> +                               libxl__stream_write_state *stream)
>> +{
>> +    libxl__datacopier_state *dc = &stream->dc;
>> +    STATE_AO_GC(stream->ao);
>> +    struct libxl__sr_hdr hdr = { 0 };
> ...
>> +    libxl__datacopier_prefixdata(egc, dc, &hdr, sizeof(hdr));
> Sadly this pattern is not correct.  I don't think this initialisation
> ensures that the memory in hdr is all-bits-0.

It guarantees that all object have their default values, which is 0 for
PoD integers.  (On a POSIX system, it is only floats/doubles/_Complex
which have default representations with not all bits 0)

From the standard,

[6.7.8.21] If there are fewer initializers in a brace-enclosed list than
there are elements or members of an aggregate, or fewer characters in a
string literal used to initialize an array of known size than there are
elements in the array, the remainder of the aggregate shall be
initialized implicitly the same as objects that have static storage
duration.

i.e. everything will get the same value it would have done had it been
declared static.

>   So you may leak
> arbitrary data from the toolstack process stack into the migration or
> save stream.

I am fairly certain that it cannot leak process stack.

>
> I think you need to use FILLZERO.
>
>> +    struct libxl__sr_rec_hdr rec = { REC_TYPE_LIBXC_CONTEXT };
> ...
>> +    setup_write(egc, stream, "libxc header",
>> +                &rec, NULL, libxc_header_done);
> Same again.
>
> (I haven't searched thoroughly for these.)
>
>
>> +/*----- Success/error/cleanup handling. -----*/
> As with read, I would prefer these to be unified into one
> stream_complete function.

They can't, because of differing function signatures from callbacks.

~Andrew

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