[cfe-dev] [RFC] automatic variable initialization

[Kostya Serebryany via cfe-dev]

On Tue, Nov 27, 2018 at 6:12 PM Richard Smith <richard at metafoo.co.uk> wrote:

> On Tue, 27 Nov 2018 at 11:52, Kostya Serebryany via cfe-dev <
> cfe-dev at lists.llvm.org> wrote:
>
>> On Tue, Nov 27, 2018 at 10:43 AM Sean McBride <sean at rogue-research.com>
>> wrote:
>>
>>> On Tue, 27 Nov 2018 10:19:03 -0800, Kostya Serebryany via cfe-dev said:
>>>
>>> >One more data point: among the bugs found by MSAN in Chrome over the
>>> past
>>> >few years 449 were uninitialized heap and 295 were uninitialized stack.
>>> >So, the proposed functionality would prevent ~40% (i.e. quite a bit!) of
>>> >all UUMs in software like Chrome.
>>>
>>> I just lurk here, but I think the proposed functionality would be
>>> greatly appreciated by C/C++/Obj-C developers on macOS, where
>>> MemorySanitizer is not supported and valgrind can't even launch TextEdit.
>>> If I'm not mistaken, it would be the *only* tool on macOS to catch UUMs.
>>>
>>
>> It won't catch anything -- but it will prevent the stack UUMs from
>> hurting you in production.
>>
>
> Well, it will prevent them from resulting in unbounded UB, yes, but that's
> not the only thing that hurts you.
>

My statement above is applicable to both zero-initialize and
pattern-/random-intialize.


>
> A few years back I improved clang's -Wuninitialized and it found a few
> hundred bugs in one codebase. Of those, in only about half of the cases was
> the correct fix to zero-initialize; the uninitialized read was very often
> symptomatic of a logic bug in the function. Now suppose a compiler adds a
> flag to automatically zero-initialize. This will likely catch on, just like
> -fno-strict-aliasing did, because it makes it easier to write wrong code
> that appears to work, and there's a tendency to value code appearing to
> work more than you value it actually working. And before you know it, ~all
> large projects need to be built with that flag enabled all the time,
> because they depend on some code that expects uninitialized variables to be
> zero-initialized (say, in inline functions or templates). Now we lose an
> opportunity to catch lots of bugs (either at compile time or runtime), and
> the benefit is that we define away a similar number of bugs. I don't think
> it's clear that that's a good tradeoff.
>

I have absolutely no disagreement with what you say here.
I'd love to pass the bikeshedding phase and get the performance numbers,
then come back to the discussion if the numbers show that zero-init is much
faster.


>
> Also, as others have noted, adding an "initialize to zero" flag will
> create an incompatible language dialect, just like -fno-strict-aliasing and
> -fno-exceptions and -fwrapv (etc) did. We have a general policy that we
> don't want to do that. Initializing to a pseudo-random or
> intentionally-chosen-to-often-trap bit-pattern seems fine to me, though,
> and an entirely reasonable security measure.
>
> Half-baked idea: what if we made it possible to enable a "zero-initialize
> all uninitialized variables" mode internally within the compiler but didn't
> expose it at all? (That way, you could turn this feature on with a compiler
> plugin, but a stock clang binary can't do it no matter what you write on
> the command line, unless you have such a plugin, which we don't ship with
> clang.)
>

Interesting, but I don't know how easy it is to use a plugin in all
environments where we need to measure perf.
IMHO, a scary-named flag that we periodically change (more frequently than
every release) should work well enough.
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