Re: DRAFT: eXIf 2017-03-09

On 12 March 2017 at 16:15, Glenn Randers-Pehrson wrote:
>> eXIf Recommendations for Encoders
>>
>> The EXIF specification allows areas of unspecified content around the
>> TIFF IFD structures. Encoders should not leak uninitialized memory
>> into these areas
>
> Why not?  It's none of our business what goes in those areas.

Oh. I thought you are concerned with encoders (accidentally
serializing memory content out of uninitialized data, not realizing
that they do so, which raises a privacy concern, in a similar manner
to the Heartbleed bug), not with decoders. But if that is not the
case, then never mind.

> All
> I said was that decoders need to be aware that such areas might
> be present.

Actually, the standard-compliant decoders reading valid content are
fine, with or without uninitialized memory content surrounding the
valid memory areas. All that decoders need to do is read the valid
memory content, and not read invalid memory content pointed to by
out-of-bounds offsets.

The decoders need not be concerned with unspecified content around
IFDs, similarly to the C string library functions that operate on
snprintf output, without being concerned with snprintf not filling up
an entire stack-allocated memory buffer.

On the topic of hardening a decoder, the uninitialized memory areas
are not an issue. A malevolent content creator may easily set an
offset well within the bounds of a valid IFD which points to content
that's just as harmful. It's like a pointer to a C structure that
points incorrectly to well-defined, yet harmful content. We cannot
address this in any effective way in our specification, other than by
whacking an always-insufficient bunch of moles here and there. It is
up to the decoders to figure out how to properly sanitize their
inputs.

Sincerely,
Cosmin

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