[llvm-dev] llvm bug 11944

David Blaikie via llvm-dev
Tue, 27 Apr 2021 00:01:13 -0000

On Mon, Apr 26, 2021 at 4:45 PM pawel k. <pawel.kunio at gmail.com>
wrote:
>
> Hello,
> Yep I meant sal. Id think we coud have printfy sallike annottation added to
llvm libc at least for emulation of planned c++ extension about precond postcond
constraints that need to hold for calls to lib not to give warnings or not to
err. We could try to catch common risks if not errors/holes early via such a
way.

Possibly - as you say, it's something Microsoft's found useful to
have, though hasn't really been picked up in the unix-y world.

> Not sure im 100% right here but if preconds/postconds can be moved fully to
compile time, we could try to get rid of many checks from libc code and for
example move them to asserts in debug only etc.

Maybe - though being able to build existing code that might rely on
these checks is pretty important, so that might not be a feasible
direction.

> You may see by now already im enthusiast of solution of sw devel moving in
bit rusty direction like linux kernel plans to.
>
> Another side question if we got or could use having solution(s) that big
redmond supplier has for catching many
>
> pointer access/mem errors/leaks/usebeforealloc/useafterfree/global and
stack vars usedbefore init
>
> kind of errors with 100% rate assumed all paths are taken. It is runtime
checks only though and might need extending interfaces of malloc free new delete
either via syntax extension or without if assisted by compiler.

You're asking if the LLVM project has tools for finding those kinds of
bugs? The sanitizers catch some of these at runtime (ASan for
accessing memory that you don't have access to (buffer overrun, etc),
MSan for accessing uninitialized memory, LSan for leaks, etc) and some
can be caught at compile time (Clang has a check for variables used
before they are initialized, for instance).

- Dave

>
> Best regards,
> Pawel Kunio
>
> pon., 26.04.2021, 18:30 użytkownik David Blaikie <dblaikie at
gmail.com> napisał:
>>
>> On Fri, Apr 23, 2021 at 9:10 PM pawel k. <pawel.kunio at
gmail.com> wrote:
>> >
>> > Hello,
>> > To clarify on stdlib/libc.
>>
>> For what it's worth, there are several implementations of libc -
GNU
>> libc and LLVM libc (well, the latter is relatively new/incomplete)
>> would be
>>
>> >  I like way ms has annotations in comments of their stdlib/libc.
>>
>> Ah, you're talking about SAL? (
>>
https://docs.microsoft.com/en-us/cpp/c-runtime-library/sal-annotations?view=msvc-160
>> )
>>
>> >  It is used by compiler as config script to something similar to
static analyzer/warningsmodule both release and debug and as base for genning
runtime checks in debug only.
>> >
>> > It helps to catch suspicious or wrong calls to stdlib/libc.
>> >
>> > If You were interested in something similar, id be willing to help
here. You wouldnt have to write a new subcheker/subanalyzer per libc function.
>>
>> Yeah, there's some general attribute annotations (like for
>> printf-style functions, for instance). Not sure if there are
>> attributes for general array+arraybound parameters, for instance.
>>
>> - Dave
>>
>> >
>> > Best regards,
>> > Pawel Kunio
>> >
>> > pt., 23.04.2021, 21:11 użytkownik David Blaikie <dblaikie at
gmail.com> napisał:
>> >>
>> >> On Fri, Apr 23, 2021 at 12:29 AM pawel k. <pawel.kunio at
gmail.com> wrote:
>> >> >
>> >> > Hello,
>> >> > Thank You for kind friendly warning. Just to get it
right. It might be too tough or too pointless for someone to integrate it?
>> >>
>> >> Yeah, my rough guess is that it's a challenging problem -
one that
>> >> many LLVM developers care about (many uses of LLVM as a
library where
>> >> the use of global constructors adds to the application's
startup time)
>> >> and have tried to fix to varying degrees at various times, but
haven't
>> >> managed it.
>> >>
>> >> > As of from ones you proposed i should either stick to
warning point misreporting bugs or something on analyzer or tidy. Id say complex
algos connected with optimizations skipping opportunity etc I could try to catch
them and measure or benchmark them or maybe write tests for but fir hardcore c++
development i could need some time to get back in full shape.
>> >>
>> >> Actually middle end optimizations can sometimes be some of the
simpler
>> >> code to fix/play with - so I wouldn't rule out playing
with/modifying
>> >> them, if it interests you.
>> >>
>> >> > I was thinking of another side project that should be
doable. I could try to get the snapshot of libc api and try to parse it and use
it as config script to what kind of issues look for in analyzer.
>> >>
>> >> Not sure I'm following here - are you referring to GNU
libc, or LLVM's
>> >> recent libc project? & which analyzer?
>> >>
>> >> Clang's Static Analyzer ( https://clang-analyzer.llvm.org/
) - if
>> >> you're talking about that, and thinking of finding bugs
/in/ the
>> >> analyzer (rather than using the analyzer to find bugs in other
code)
>> >> by running it on some existing codebase (like libc, GNU or
LLVM's) and
>> >> reporting what you find - yes, that could be useful. (though
realize
>> >> there's probably lots of already known bugs, so sometimes
bug finding
>> >> isn't the bottleneck/where there's a problem)
>> >>
>> >> > Thus emulating planned c++ feature of postconditions
preconditions checker.
>> >> > If i reget my rolodex, i might be able to get in touch
with few whitehats who could provide data what kind of holes via which api calls
are most common and generally what kind of attacks misusing api are possible if
that would help on this one.
>> >>
>> >> Yeah, if there are new static analysis checks you've got a
use for,
>> >> could try implementing them as either Clang warnings,
clang-tidy
>> >> checks, or Clang Static Analyzer checks.
>> >>
>> >> - Dave
>> >>
>> >> >
>> >> > Best regards,
>> >> > Pawel Kunio
>> >> >
>> >> > pt., 23.04.2021, 04:59 użytkownik David Blaikie
<dblaikie at gmail.com> napisał:
>> >> >>
>> >> >> On Thu, Apr 22, 2021 at 7:47 PM pawel k.
<pawel.kunio at gmail.com> wrote:
>> >> >> >
>> >> >> > Hello,
>> >> >> > Sorry about my bad english and wrecked
communications line. I meant 11944 bug from llvm database. As on our system, we
had similar issue in sense on embedded project around lte and 5g base stations.
Archs finally decided global compound constructors are forbidden due to startup
overhead and call order unpredictability.
>> >> >> >
>> >> >> > Thank You for mentioning warning enabling flag.
That should help me pinpoint all interesting uses of this construct.
>> >> >>
>> >> >> Sure thing - oh, the other thing you might find
interesting, regarding
>> >> >> the issues with global constructor ordering problems
- there's a
>> >> >> dynamic tool to help find those:
>> >> >>
https://github.com/google/sanitizers/wiki/AddressSanitizerInitializationOrderFiasco
>> >> >>
>> >> >> > I will try to lookup how these global
constructors are done in llvmlib. What i saw as general guideline along which
commandline opts component is built i generally like. I dont see yet fully where
is issue in moving it to runtime. Id think we could try to use some pattern like
commandline opts tables from gcc but nit sure yet whether keep much in globals
or store within specific apps/tools classes.
>> >> >> >
>> >> >> > In one of first phases id think of checking
buildflags of clang what targets etc are enabled and register all supported
commands and maybe prescan command line for further flags which enable or
disable other flags.
>> >> >> >
>> >> >> > I will try to build llvm lib with this warning
enabled to see all opts uses and learn the api etc. I need to understand furst
completely based on what logic macros and flags which flags blocks should be
registered and later kept or disabled if we need to scan commandline in multiple
passes etc.
>> >> >>
>> >> >> I will say I wouldn't readily suggest this as the
/best/ place to
>> >> >> start in LLVM - if you're interested in getting
into the project
>> >> >> generally, without any particular preference for
which part - given
>> >> >> this has been thought about a fair bit by folks
throughout the
>> >> >> project's life, the amount of design work/time
involved by core
>> >> >> developers will probably be non-trivial to figure out
the right path
>> >> >> before the mechanical work can begin. Fair warning.
>> >> >>
>> >> >> Usually I suggest starting with clang warnings (or
>> >> >> clang-format/clang-tidy these days) bugs, since they
can be fairly
>> >> >> narrow/sometimes relatively isolated to fix. Source
location bugs or
>> >> >> fixits (warnings that point to the wrong part of the
code, or  errors
>> >> >> that don't offer a fixit hint when they could)
can be interesting
>> >> >> learning experiences about the Clang Abstract Syntax
Tree/source
>> >> >> location information.
>> >> >>
>> >> >> But if you're more interested in LLVM proper -
probably middle-end
>> >> >> misoptimizations (probably easier to fix
bugs/miscompiles than fixing
>> >> >> missed optimizations - the latter are probably more
likely to be
>> >> >> deeply involved work/changes to make an optimization
more powerful for
>> >> >> some reason).
>> >> >>
>> >> >> > Ill try to learn all i can first from code and
gathered warnings and if questions arise ill ring back with them and if not ill
try to propose some solution after fully testing it on current testsuite and all
targets etc.
>> >> >>
>> >> >> Yep, always happy to answer questions/provide
pointers.
>> >> >>
>> >> >> - Dave
>> >> >>
>> >> >> >
>> >> >> > Best regards,
>> >> >> > Pawel Kunio
>> >> >> >
>> >> >> >
>> >> >> > pt., 23.04.2021, 02:30 użytkownik David Blaikie
<dblaikie at gmail.com> napisał:
>> >> >> >>
>> >> >> >> [It might be helpful if you used a few more
words & fully explained
>> >> >> >> what you're referring to - I'm
having a hard time following your
>> >> >> >> emails in this abbreviated writing style]
>> >> >> >>
>> >> >> >> You looked at an existing bug (do you have a
link to it, or bug number
>> >> >> >> on the bugs.llvm.org database) related to
the use of global
>> >> >> >> constructors in the LLVM codebase? Or in
another codebase using LLVM?
>> >> >> >>
>> >> >> >> You had similar issues on another codebase
you worked on where you
>> >> >> >> weren't allowed to use any non-trivial
global constructors?
>> >> >> >>
>> >> >> >> & you're proposing creating a
checker or other tool to help find these
>> >> >> >> cases? Or proposing using an existing tool
for finding such things?
>> >> >> >>
>> >> >> >> Clang does have a warning for this already,
I believe:
>> >> >> >> -Wglobal-constructors. But, yes, the LLVM
codebase isn't remotely
>> >> >> >> ready for that and it's not been a high
enough priority for anyone to
>> >> >> >> really clean it up - mostly because the main
use of global
>> >> >> >> constructors is in the LLVM command line
argument handling code - so
>> >> >> >> it's a non-trivial
design/redesign/refactoring effort to figure out
>> >> >> >> the right new design for that and make all
the changes necessary to
>> >> >> >> migrate to such a design. (after that
there'd probably be a bunch of
>> >> >> >> smaller more incremental changes to cleanup
global constructors and
>> >> >> >> get the codebase to have no
-Wglobal-constructors warnings, then we
>> >> >> >> could turn on the warning to ensure we
didn't regress)
>> >> >> >>
>> >> >> >>  - Dave
>> >> >> >>
>> >> >> >> On Thu, Apr 22, 2021 at 4:50 PM pawel k. via
llvm-dev
>> >> >> >> <llvm-dev at lists.llvm.org> wrote:
>> >> >> >> >
>> >> >> >> > Hello,
>> >> >> >> > I took glimpse of PR llvm and or clang
has ie lib cpp compound constructing global vars which makes objects get
constructed in random order and with nonzero startup cpu/time overhead.
>> >> >> >> >
>> >> >> >> > In one of rather mighty embedded
projects for 4g enodeb and later 5g base stations we had similar issue. It was
forbidden to complex construct global vars.
>> >> >> >> >
>> >> >> >> > We could fancy clang syntax checker
having option to early detect and track those so we could now how many are still
left if any. I would disencourage running it on testsuite though as there were
many false positives reported there.
>> >> >> >> >
>> >> >> >> > Happy if that helps any.
>> >> >> >> >
>> >> >> >> > Best regards,
>> >> >> >> > Pawel Kunio
>> >> >> >> >
_______________________________________________
>> >> >> >> > LLVM Developers mailing list
>> >> >> >> > llvm-dev at lists.llvm.org
>> >> >> >> >