[cfe-dev] [RFC] automatic variable initialization

Kostya Serebryany via cfe-dev cfe-dev at lists.llvm.org
Tue Dec 18 11:38:55 PST 2018


2019 is going to have one bug class fewer. :)
(unless Jann&Co find new bug classes again)

Looking forward to the followup patches (e.g. padding gaps)
and to doing the measurements.

--kcc

On Tue, Dec 18, 2018 at 10:53 AM JF Bastien <jfbastien at apple.com> wrote:

> Hello fans of the pre-C++11 `auto` keyword!
>
> Thanks to reviews from Peter and Richard, automatic variable
> initialization landed last night:
>
> https://reviews.llvm.org/rL349442
>
>
> Chandler pointed out that I should have circled back to the RFC in case
> any of the finer points needed tuning. Let me know if there’s any follow-up
> you’d like! I’ve made zero-init harder / uglier to use as was requested,
> while allowing folks like Kostya to test it out.
>
> Let me know if you find bugs (in your code, or in the implementation), and
> please file bugs on any missed optimization opportunities. I know of a
> handful of missing optimizations which I intend to tackle in the near
> future. I’d also love to hear what kind of performance / size impact you
> see from various codebases.
>
> As pointed out in the patch there’s a few other hardenings that I’ll want
> to tackle next. I’ll do so in the new year.
>
> Thanks,
>
> JF
>
>
> On Dec 11, 2018, at 3:18 PM, Joe Bialek via cfe-dev <
> cfe-dev at lists.llvm.org> wrote:
>
> We went with zero initialization for production builds in Visual Studio
> because we anticipate zero being the fastest, smallest code gen, and safest
> value for the following cases:
>
>    - Pointer: A non-zero pointer could potentially point at valid code.
>    On Windows x64, the first 64kb of the virtual address space is guaranteed
>    to not be mappable and the last 512GB of the virtual address space (today)
>    has the space guarantee. So null pointer or near-null pointer dereferences
>    are denial of service at worst.
>    - Size: If you forget to set a size as an out parameter we’d rather
>    it’s set to 0 so you don’t index out of bounds.
>    - Index: Same as size.
>
>
> We are using this mitigation to downgrade vulnerabilities from remote code
> execution, elevation of privilege, or information disclosure, down to
> denial of service at worst.
>
> Assuming the denial of service isn’t for a scenario where it matters
> (there are many types of DOS we don’t really worry about for security), we
> will not bother servicing the vulnerabilities down-level and instead only
> fix them for new code.
>
> It is helpful for us to have deterministic behavior to help accomplish
> this goal. If an engineer knows a value is always set to 0, they can
> quickly determine if the bug report is actually exploitable or not. If it
> is initialized to a random or compile time constant value, we’ll need to do
> more work to determine if the bug is exploitable.
>
> We are not zero initializing CHK builds to prevent folks from taking a
> dependency on this feature.
>
>
> Joe
>
> *From:* Kostya Serebryany <kcc at google.com>
> *Sent:* Tuesday, December 11, 2018 3:04 PM
> *To:* Mehdi AMINI <joker.eph at gmail.com>; Jann Horn <jannh at google.com>
> *Cc:* David Blaikie <dblaikie at gmail.com>; Richard Smith <
> richard at metafoo.co.uk>; Clang Dev <cfe-dev at lists.llvm.org>; Joe Bialek <
> jobialek at microsoft.com>
> *Subject:* Re: [cfe-dev] [RFC] automatic variable initialization
>
> One more dimension in this discussion (zero-init vs pattern-init) is what
> will security get from those options.
> My second-hand knowledge here suggests that zero-init may have *on
> average* better security guarantees than non-zero init.
> For example if the uninitialized data is interpreted as a length of
> something, it better be zero than any large number or, worse, a negative
> signed number.
> +Jann Horn <jannh at google.com> and Joe Bialek who have first-hand
> knowledge here.
>
> --kcc
>
>
>
>
> On Thu, Dec 6, 2018 at 3:58 PM Kostya Serebryany <kcc at google.com> wrote:
>
>
> On Thu, Dec 6, 2018 at 3:55 PM Mehdi AMINI <joker.eph at gmail.com> wrote:
>
>
> On Thu, Dec 6, 2018 at 3:43 PM Kostya Serebryany <kcc at google.com> wrote:
>
>
> On Thu, Dec 6, 2018 at 3:03 PM Mehdi AMINI <joker.eph at gmail.com> wrote:
>
>
> On Thu, Dec 6, 2018 at 1:01 PM Kostya Serebryany <kcc at google.com> wrote:
>
>
> On Wed, Dec 5, 2018 at 6:07 PM Mehdi AMINI via cfe-dev <
> cfe-dev at lists.llvm.org> wrote:
>
> I have the same question as David: I don't understand why this isn't just
> an experimental build option disabled by default? (Google, Apple, etc. can
> just enable it in their build script, no need to patch the source).
>
>
> I will need to rebuild half a dozen compiler binaries to do the
> measurements I need.
> This is going to double the cost of the effort for me because it adds too
> many extra moving pieces.
>
>
> Not sure I follow, you have to build the compiler anyway to get it after
> the code is patched?
> The only thing you would have to do is one CL to enable the build flag
> (that expose the command line flag) inside your codebase and then you get
> your toolchain as usual?
>
>
> Once the JF's patch is in, I'll have it in most production compilers I
> care about in 2-6 weeks, w/o any effort on my side.
> None of those builders (easily) support adding custom patches, and even a
> build flag is going to be very non-trivial.
> I can deal with one or two of those builds relatively easily, but not with
> the rest.
>
>
> What builders are you referring to? So far I was assuming we were talking
> about your internal infrastructure that produces your production compilers.
>
>
> I am talking about our internal infra.
> But we have multiple independent builders from multiple independent
> compiler users (e.g. Chrome and Android are using different compiler
> builds).
>
>
>
>
>
>
>
>
>
> --
> Mehdi
>
>
>
>
>
>
>
> The only argument I read in the thread was about allowing people to make
> performance measurement without rebuilding the compiler, but I have a hard
> time reconciliation this with the fact that we're talking about not
> shipping this before performing the actual measurements?
>
> I expect that anyone that cares enough about the perf impact of this to
> influence the development of the feature in clang should already be
> rebuilding their compiler today.
>
> --
> Mehdi
>
>
> On Mon, Dec 3, 2018 at 5:58 PM David Blaikie via cfe-dev <
> cfe-dev at lists.llvm.org> wrote:
>
> Just out of curiosity - what's the major difference between a build-time
> off-by-default feature and a build-time on-by-default-but-off-in-release
> branches feature? If we're only targeting groups that build/release the
> compiler themselves, then they're likely able to opt-in to a build-time
> feature easily enough, I'd think? & then there's no need to make our
> releases different from day-to-day builds?
>
> But sounds like folks are in general agreement of a way forward, so I
> don't want to disrupt/delay that.
> On Wed, Nov 28, 2018 at 11:14 PM Chandler Carruth <chandlerc at gmail.com>
> wrote:
>
> Suggested compromise technique to at least get an initial set of numbers:
>
> 1) Require a special, long, ugly flag name.
> 2) Make it a CC1 flag, requiring -Xclang ... to use.
> 3) Emit a warning by default (that cannot be suppressed with a -Wno-...
> flag) when this flag is enabled.
> 4) Commit to never including this flag in any upstream release. Either we
> remove it before the next release branches or we revert it on the branch.
>
> Most of the folks we're hoping to get performance data with are willing to
> use a not-yet-released build of Clang. They won't have to actually patch it
> in any way. They will have strong reminders to not deploy this in any way
> due to the warning.
>
> Thoughts?
>
> On Wed, Nov 28, 2018 at 4:34 PM Kostya Serebryany via cfe-dev <
> cfe-dev at lists.llvm.org> wrote:
>
> On Wed, Nov 28, 2018 at 3:28 PM David Blaikie <dblaikie at gmail.com> wrote:
>
>
> On Wed, Nov 28, 2018 at 3:17 PM Kostya Serebryany <kcc at google.com> wrote:
>
>
> Seems easier to me to separate the two pieces - move ahead with the
> non-zero options, and separate the discussion on the zero option. You can
> present performance numbers from what you can measure without shipping a
> compiler with the feature - and if those numbers are sufficiently
> compelling compared to the risks of slicing the language, then perhaps we
> go that way.
>
>
> This approach will significantly impair my ability to do the measurements
> I need.
>
>
> I'm aware waht I'm proposing would make it more difficult for some people
> to take measurements - that's a tradeoff to be sure - one where I err in
> this direction.
>
> Specifically for Google though - would it be that difficult for Google to
> opt-in to a certain build configuration of LLVM?
>
>
> Absolutely yes.
> Google is not just a single monolithic code base
> <https://nam06.safelinks.protection.outlook.com/?url=http%3A%2F%2Fdelivery.acm.org%2F10.1145%2F2860000%2F2854146%2Fp78-potvin.pdf%3Fip%3D104.133.8.94%26id%3D2854146%26acc%3DOA%26key%3D4D4702B0C3E38B35%252E4D4702B0C3E38B35%252E4D4702B0C3E38B35%252E5945DC2EABF3343C%26__acm__%3D1543446999_3aadcd36f657e2297430c38bee93f16c&data=02%7C01%7Cjobialek%40microsoft.com%7C533e0fe1444d4b5a3a6b08d65fbcf479%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C636801662510848537&sdata=dd4I1xQ5qfawzLwsnBM3iqsypqZ%2BlFswj5IgKa%2FLHbw%3D&reserved=0>
> .
>
>
> Couldn't access this link ("An error occurred while processing your
> request" - but yeah, I understand there's a bunch of different pieces of
> Google beyond the "stuff that runs in data centers" piece we mostly support.
>
>
> Besides that monolithic thing, we have Android, Chrome, ChromeOS, Fuchsia,
> and a bazillion of smaller efforts that use their own toolchains.
>
>
> Still, most/all of these build their own compilers, I think? But yeah,
> that adds an opt-in overhead to each project, for sure.
>
>
> In some cases the most reliable and complete way of measuring performance
> changes is to submit the changes to revision control,
> and let the performance bots shew it for a couple of days. That's how we
> iterated with the LLVM's CFI in Chrome.
> We will also need to work with the upstream Linux kernel -- it's hard
> enough for them to use clang and a modified clang will cost us much more
> effort.
>
>
> Yeah, I can imagine that one's a bit trickier - how's performance
> evaluation of the kernel done?
>
>
> I don't think anyone knows that. :-|
> And requiring a compiler patch will shift the problem from "hard" to "I'd
> better do something else".
>
>
> (though, again, I imagine a fair amount of progress could be made without
> the zero-init feature - perhaps enough to say "hey, here are all the places
> we have run tests & seen the performance tradeoff is worthwhile for us (&
> possibly that it's close to the zero-init case, but that's sort of
> orthogonal, imho - that it's worthwhile is the main thing) - perhaps other
> folks would be willing to test it (non-zero init) & see if it's worthwhile
> to them - and if it isn't/they're interested in more performance, maybe all
> that evidence we can gain from the places where it's easy for us to rebuild
> compilers, etc, would make it interesting enough to motivate someone to do
> build the kernel with a custom compiler & do some performance measurements,
> etc...
>
> Sorry that was a bit rambly, anyway.
>
> - Dave
>
>
>
>
>
>
> We/Google do build the compiler from scratch, I assume we pick the
> configuration options we build with & some of them probably aren't the
> defaults for a release build of LLVM. So if it was important that Google's
> production compiler had these features enabled (rather than building a test
> compiler for running some experiments), that doesn't seem (at least to me,
> at this moment) especially prohibitive, is it?
>
>
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