[cfe-dev] [EXTERNAL] Re: making -ftrivial-auto-var-init=zero a first-class option

Richard Smith via cfe-dev cfe-dev at lists.llvm.org
Wed Apr 22 13:08:03 PDT 2020

On Wed, 22 Apr 2020 at 10:49, Joe Bialek <jobialek at microsoft.com> wrote:

> How are you going to efficiently check that something wasn't initialized
> at runtime? In a way that results in better codegen than just doing pattern
> initialization? I'm happy to see a solution but I don't see how this can be
> done in a way that doesn't involve metadata and checks. If you could do
> this at compile-time, you'd just issue a warning rather than let the issue
> hang around for someone to discover at runtime.

Consider a case such as:

int f(int &r) { return r; }
int g() {
  int n;
  return f(n);

We certainly won't warn on this during compilation, but it's easy for us to
turn this into a trap after inlining.

> Also not clear to me what the OS is expected to do with this trap. We have
> a number of information leak vulnerabilities where force initialization
> kills the bug silently.

Do you really mean "kills the bug"? I would certainly believe you have a
number of information leak vulnerabilities where zero-init fixes the
*vulnerability* (and we should definitely provide tools to harden programs
against such vulnerabilities), but the program is still using an
uninitialized value and still has a bug. The idea that this compiler change
fixes or removes the bug is precisely the language dialect problem that I'm
concerned about. Developers must still think that reading an uninitialized
value is a bug (even if it's not a vulnerability any more) or they're
writing a program in a language dialect where doing that is not a bug.

If you have a non-recoverable trap you are now turning these bugs in to
> kernel crashes which is sort of a crappy user experience compared to just
> silently fixing the bug and allowing the OS to work as normal. As it is
> right now, we can just ignore the issues because they have no security or
> reliability impact which is great because it saves us time and money not
> having to service things, and customers don't have to install a code update
> either.
> Joe
> ------------------------------
> *From:* Kees Cook <keescook at chromium.org>
> *Sent:* Wednesday, April 22, 2020 10:40 AM
> *To:* Richard Smith <richard at metafoo.co.uk>
> *Cc:* Arthur O'Dwyer <arthur.j.odwyer at gmail.com>; Joe Bialek <
> jobialek at microsoft.com>; Clang Dev <cfe-dev at lists.llvm.org>
> *Subject:* [EXTERNAL] Re: [cfe-dev] making -ftrivial-auto-var-init=zero a
> first-class option
> On Tue, Apr 21, 2020 at 04:54:25PM -0700, Richard Smith wrote:
> > The existence of the
> > --long-ugly-flag-name-that-says-we'll-remove-the-feature is the way we
> > currently try to avoid introducing a language dialect. If we remove that
> > flag as is proposed, then we are effectively relitigating the question of
> > whether to have the feature at all.
> What about renaming the enable flag so it doesn't imply that zero-init
> is going to be removed?
> > And indeed it might even be OK if the initial behavior is that we
> *always*
> > zero-initialize (as Philip asked), so long as our documentation clearly
> > says that we do not guarantee that the value will be zero (only that we
> > guarantee that *if the program continues*, the value will be zero), and
> our
> > intent is that we may still produce traps or otherwise abort the
> > computation.
> Right -- I would see adding a trap path as a nice improvement. I still
> think it'll be be too much overhead, though, given needing to check all
> corners of a struct: accessing any padding bytes would need to trap,
> etc.
> --
> Kees Cook
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