Re: [PATCH 000/141] Fix fall-through warnings for Clang

Re: [PATCH 000/141] Fix fall-through warnings for Clang

[Joe Perches]

On Fri, 2020-11-20 at 12:21 -0600, Gustavo A. R. Silva wrote:
> Hi all,
> 
> This series aims to fix almost all remaining fall-through warnings in
> order to enable -Wimplicit-fallthrough for Clang.
> 
> In preparation to enable -Wimplicit-fallthrough for Clang, explicitly
> add multiple break/goto/return/fallthrough statements instead of just
> letting the code fall through to the next case.
> 
> Notice that in order to enable -Wimplicit-fallthrough for Clang, this
> change[1] is meant to be reverted at some point. So, this patch helps
> to move in that direction.

This was a bit hard to parse for a second or three.

Thanks Gustavo.

How was this change done?


_______________________________________________
Virtualization mailing list
Virtualization@lists.linux-foundation.org
https://lists.linuxfoundation.org/mailman/listinfo/virtualization

Re: [PATCH 000/141] Fix fall-through warnings for Clang

[Gustavo A. R. Silva]

On 11/20/20 12:28, Joe Perches wrote:
> On Fri, 2020-11-20 at 12:21 -0600, Gustavo A. R. Silva wrote:
>> Hi all,
>>
>> This series aims to fix almost all remaining fall-through warnings in
>> order to enable -Wimplicit-fallthrough for Clang.
>>
>> In preparation to enable -Wimplicit-fallthrough for Clang, explicitly
>> add multiple break/goto/return/fallthrough statements instead of just
>> letting the code fall through to the next case.
>>
>> Notice that in order to enable -Wimplicit-fallthrough for Clang, this
>> change[1] is meant to be reverted at some point. So, this patch helps
>> to move in that direction.
> 
> This was a bit hard to parse for a second or three.
> 
> Thanks Gustavo.
> 
> How was this change done?

I audited case by case in order to determine the best fit for each
situation. Depending on the surrounding logic, sometimes it makes
more sense a goto or a fallthrough rather than merely a break.

Thanks
--
Gustavo
_______________________________________________
Virtualization mailing list
Virtualization@lists.linux-foundation.org
https://lists.linuxfoundation.org/mailman/listinfo/virtualization

Re: [PATCH 000/141] Fix fall-through warnings for Clang

[Gustavo A. R. Silva]

Hi,

On 11/20/20 12:53, Jakub Kicinski wrote:
> On Fri, 20 Nov 2020 12:21:39 -0600 Gustavo A. R. Silva wrote:
>> This series aims to fix almost all remaining fall-through warnings in
>> order to enable -Wimplicit-fallthrough for Clang.
>>
>> In preparation to enable -Wimplicit-fallthrough for Clang, explicitly
>> add multiple break/goto/return/fallthrough statements instead of just
>> letting the code fall through to the next case.
>>
>> Notice that in order to enable -Wimplicit-fallthrough for Clang, this
>> change[1] is meant to be reverted at some point. So, this patch helps
>> to move in that direction.
>>
>> Something important to mention is that there is currently a discrepancy
>> between GCC and Clang when dealing with switch fall-through to empty case
>> statements or to cases that only contain a break/continue/return
>> statement[2][3][4].
> 
> Are we sure we want to make this change? Was it discussed before?
> 
> Are there any bugs Clangs puritanical definition of fallthrough helped
> find?
> 
> IMVHO compiler warnings are supposed to warn about issues that could
> be bugs. Falling through to default: break; can hardly be a bug?!

The justification for this is explained in this same changelog text:

Now that the -Wimplicit-fallthrough option has been globally enabled[5],
any compiler should really warn on missing either a fallthrough annotation
or any of the other case-terminating statements (break/continue/return/
goto) when falling through to the next case statement. Making exceptions
to this introduces variation in case handling which may continue to lead
to bugs, misunderstandings, and a general lack of robustness. The point
of enabling options like -Wimplicit-fallthrough is to prevent human error
and aid developers in spotting bugs before their code is even built/
submitted/committed, therefore eliminating classes of bugs. So, in order
to really accomplish this, we should, and can, move in the direction of
addressing any error-prone scenarios and get rid of the unintentional
fallthrough bug-class in the kernel, entirely, even if there is some minor
redundancy. Better to have explicit case-ending statements than continue to
have exceptions where one must guess as to the right result. The compiler
will eliminate any actual redundancy.

Note that there is already a patch in mainline that addresses almost
40,000 of these issues[6].

[1] commit e2079e93f562c ("kbuild: Do not enable -Wimplicit-fallthrough for clang for now")
[2] ClangBuiltLinux#636
[3] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91432
[4] https://godbolt.org/z/xgkvIh
[5] commit a035d552a93b ("Makefile: Globally enable fall-through warning")
[6] commit 4169e889e588 ("include: jhash/signal: Fix fall-through warnings for Clang")

Thanks
--
Gustavo
_______________________________________________
Virtualization mailing list
Virtualization@lists.linux-foundation.org
https://lists.linuxfoundation.org/mailman/listinfo/virtualization

Re: [PATCH 000/141] Fix fall-through warnings for Clang

[Kees Cook]

On Fri, Nov 20, 2020 at 10:53:44AM -0800, Jakub Kicinski wrote:
> On Fri, 20 Nov 2020 12:21:39 -0600 Gustavo A. R. Silva wrote:
> > This series aims to fix almost all remaining fall-through warnings in
> > order to enable -Wimplicit-fallthrough for Clang.
> > 
> > In preparation to enable -Wimplicit-fallthrough for Clang, explicitly
> > add multiple break/goto/return/fallthrough statements instead of just
> > letting the code fall through to the next case.
> > 
> > Notice that in order to enable -Wimplicit-fallthrough for Clang, this
> > change[1] is meant to be reverted at some point. So, this patch helps
> > to move in that direction.
> > 
> > Something important to mention is that there is currently a discrepancy
> > between GCC and Clang when dealing with switch fall-through to empty case
> > statements or to cases that only contain a break/continue/return
> > statement[2][3][4].
> 
> Are we sure we want to make this change? Was it discussed before?
> 
> Are there any bugs Clangs puritanical definition of fallthrough helped
> find?
> 
> IMVHO compiler warnings are supposed to warn about issues that could
> be bugs. Falling through to default: break; can hardly be a bug?!

It's certainly a place where the intent is not always clear. I think
this makes all the cases unambiguous, and doesn't impact the machine
code, since the compiler will happily optimize away any behavioral
redundancy.


-- 
Kees Cook
_______________________________________________
Virtualization mailing list
Virtualization@lists.linux-foundation.org
https://lists.linuxfoundation.org/mailman/listinfo/virtualization

Re: [PATCH 000/141] Fix fall-through warnings for Clang

[Kees Cook]

On Fri, Nov 20, 2020 at 11:51:42AM -0800, Jakub Kicinski wrote:
> On Fri, 20 Nov 2020 11:30:40 -0800 Kees Cook wrote:
> > On Fri, Nov 20, 2020 at 10:53:44AM -0800, Jakub Kicinski wrote:
> > > On Fri, 20 Nov 2020 12:21:39 -0600 Gustavo A. R. Silva wrote:  
> > > > This series aims to fix almost all remaining fall-through warnings in
> > > > order to enable -Wimplicit-fallthrough for Clang.
> > > > 
> > > > In preparation to enable -Wimplicit-fallthrough for Clang, explicitly
> > > > add multiple break/goto/return/fallthrough statements instead of just
> > > > letting the code fall through to the next case.
> > > > 
> > > > Notice that in order to enable -Wimplicit-fallthrough for Clang, this
> > > > change[1] is meant to be reverted at some point. So, this patch helps
> > > > to move in that direction.
> > > > 
> > > > Something important to mention is that there is currently a discrepancy
> > > > between GCC and Clang when dealing with switch fall-through to empty case
> > > > statements or to cases that only contain a break/continue/return
> > > > statement[2][3][4].  
> > > 
> > > Are we sure we want to make this change? Was it discussed before?
> > > 
> > > Are there any bugs Clangs puritanical definition of fallthrough helped
> > > find?
> > > 
> > > IMVHO compiler warnings are supposed to warn about issues that could
> > > be bugs. Falling through to default: break; can hardly be a bug?!  
> > 
> > It's certainly a place where the intent is not always clear. I think
> > this makes all the cases unambiguous, and doesn't impact the machine
> > code, since the compiler will happily optimize away any behavioral
> > redundancy.
> 
> If none of the 140 patches here fix a real bug, and there is no change
> to machine code then it sounds to me like a W=2 kind of a warning.

I'd like to avoid splitting common -W options between default and W=2
just based on the compiler. Getting -Wimplicit-fallthrough enabled found
plenty of bugs, so making sure it works correctly for both compilers
feels justified to me. (This is just a subset of the same C language
short-coming.)

> I think clang is just being annoying here, but if I'm the only one who
> feels this way chances are I'm wrong :)

It's being pretty pedantic, but I don't think it's unreasonable to
explicitly state how every case ends. GCC's silence for the case of
"fall through to a break" doesn't really seem justified.

-- 
Kees Cook
_______________________________________________
Virtualization mailing list
Virtualization@lists.linux-foundation.org
https://lists.linuxfoundation.org/mailman/listinfo/virtualization

Re: [PATCH 000/141] Fix fall-through warnings for Clang

[Kees Cook]

On Fri, Nov 20, 2020 at 11:51:42AM -0800, Jakub Kicinski wrote:
> On Fri, 20 Nov 2020 11:30:40 -0800 Kees Cook wrote:
> > On Fri, Nov 20, 2020 at 10:53:44AM -0800, Jakub Kicinski wrote:
> > > On Fri, 20 Nov 2020 12:21:39 -0600 Gustavo A. R. Silva wrote:  
> > > > This series aims to fix almost all remaining fall-through warnings in
> > > > order to enable -Wimplicit-fallthrough for Clang.
> > > > 
> > > > In preparation to enable -Wimplicit-fallthrough for Clang, explicitly
> > > > add multiple break/goto/return/fallthrough statements instead of just
> > > > letting the code fall through to the next case.
> > > > 
> > > > Notice that in order to enable -Wimplicit-fallthrough for Clang, this
> > > > change[1] is meant to be reverted at some point. So, this patch helps
> > > > to move in that direction.
> > > > 
> > > > Something important to mention is that there is currently a discrepancy
> > > > between GCC and Clang when dealing with switch fall-through to empty case
> > > > statements or to cases that only contain a break/continue/return
> > > > statement[2][3][4].  
> > > 
> > > Are we sure we want to make this change? Was it discussed before?
> > > 
> > > Are there any bugs Clangs puritanical definition of fallthrough helped
> > > find?
> > > 
> > > IMVHO compiler warnings are supposed to warn about issues that could
> > > be bugs. Falling through to default: break; can hardly be a bug?!  
> > 
> > It's certainly a place where the intent is not always clear. I think
> > this makes all the cases unambiguous, and doesn't impact the machine
> > code, since the compiler will happily optimize away any behavioral
> > redundancy.
> 
> If none of the 140 patches here fix a real bug, and there is no change
> to machine code then it sounds to me like a W=2 kind of a warning.

FWIW, this series has found at least one bug so far:
https://lore.kernel.org/lkml/CAFCwf11izHF=g1mGry1fE5kvFFFrxzhPSM6qKAO8gxSp=Kr_CQ@mail.gmail.com/

-- 
Kees Cook
_______________________________________________
Virtualization mailing list
Virtualization@lists.linux-foundation.org
https://lists.linuxfoundation.org/mailman/listinfo/virtualization

Re: [PATCH 000/141] Fix fall-through warnings for Clang

[Joe Perches]

On Sun, 2020-11-22 at 10:21 -0800, James Bottomley wrote:
> Please tell me
> our reward for all this effort isn't a single missing error print.

There were quite literally dozens of logical defects found
by the fallthrough additions.  Very few were logging only.



_______________________________________________
Virtualization mailing list
Virtualization@lists.linux-foundation.org
https://lists.linuxfoundation.org/mailman/listinfo/virtualization

Re: [PATCH 000/141] Fix fall-through warnings for Clang

[Joe Perches]

On Sun, 2020-11-22 at 11:12 -0800, James Bottomley wrote:
> On Sun, 2020-11-22 at 10:25 -0800, Joe Perches wrote:
> > On Sun, 2020-11-22 at 10:21 -0800, James Bottomley wrote:
> > > Please tell me our reward for all this effort isn't a single
> > > missing error print.
> > 
> > There were quite literally dozens of logical defects found
> > by the fallthrough additions.  Very few were logging only.
> 
> So can you give us the best examples (or indeed all of them if someone
> is keeping score)?  hopefully this isn't a US election situation ...

Gustavo?  Are you running for congress now?

https://lwn.net/Articles/794944/


_______________________________________________
Virtualization mailing list
Virtualization@lists.linux-foundation.org
https://lists.linuxfoundation.org/mailman/listinfo/virtualization

Re: [Intel-wired-lan] [PATCH 000/141] Fix fall-through warnings for Clang

[Kees Cook]

On Mon, Nov 23, 2020 at 08:31:30AM -0800, James Bottomley wrote:
> Really, no ... something which produces no improvement has no value at
> all ... we really shouldn't be wasting maintainer time with it because
> it has a cost to merge.  I'm not sure we understand where the balance
> lies in value vs cost to merge but I am confident in the zero value
> case.

What? We can't measure how many future bugs aren't introduced because the
kernel requires explicit case flow-control statements for all new code.

We already enable -Wimplicit-fallthrough globally, so that's not the
discussion. The issue is that Clang is (correctly) even more strict
than GCC for this, so these are the remaining ones to fix for full Clang
coverage too.

People have spent more time debating this already than it would have
taken to apply the patches. :)

This is about robustness and language wrangling. It's a big code-base,
and this is the price of our managing technical debt for permanent
robustness improvements. (The numbers I ran from Gustavo's earlier
patches were that about 10% of the places adjusted were identified as
legitimate bugs being fixed. This final series may be lower, but there
are still bugs being found from it -- we need to finish this and shut
the door on it for good.)

-- 
Kees Cook
_______________________________________________
Virtualization mailing list
Virtualization@lists.linux-foundation.org
https://lists.linuxfoundation.org/mailman/listinfo/virtualization

Re: [Intel-wired-lan] [PATCH 000/141] Fix fall-through warnings for Clang

[Miguel Ojeda]

On Tue, Nov 24, 2020 at 11:24 PM Finn Thain <fthain@telegraphics.com.au> wrote:
>
> These statements are not "missing" unless you presume that code written
> before the latest de facto language spec was written should somehow be
> held to that spec.

There is no "language spec" the kernel adheres to. Even if it did,
kernel code is not frozen. If an improvement is found, it should be
applied.

> If the 'fallthrough' statement is not part of the latest draft spec then
> we should ask why not before we embrace it. Being that the kernel still
> prefers -std=gnu89 you might want to consider what has prevented
> -std=gnu99 or -std=gnu2x etc.

The C standard has nothing to do with this. We use compiler extensions
of several kinds, for many years. Even discounting those extensions,
the kernel is not even conforming to C due to e.g. strict aliasing. I
am not sure what you are trying to argue here.

But, since you insist: yes, the `fallthrough` attribute is in the
current C2x draft.

Cheers,
Miguel
_______________________________________________
Virtualization mailing list
Virtualization@lists.linux-foundation.org
https://lists.linuxfoundation.org/mailman/listinfo/virtualization

Re: [Intel-wired-lan] [PATCH 000/141] Fix fall-through warnings for Clang

[Miguel Ojeda]

On Wed, Nov 25, 2020 at 12:53 AM Finn Thain <fthain@telegraphics.com.au> wrote:
>
> I'm saying that supporting the official language spec makes more sense
> than attempting to support a multitude of divergent interpretations of the
> spec (i.e. gcc, clang, coverity etc.)

Making the kernel strictly conforming is a ship that sailed long ago,
for several reasons. Anyway, supporting several compilers and other
tools, regardless of extensions, is valuable.

> I'm also saying that the reason why we use -std=gnu89 is that existing
> code was written in that language, not in ad hoc languages comprised of
> collections of extensions that change with every release.

No, we aren't particularly tied to `gnu89` or anything like that. We
could actually go for `gnu11` already, since the minimum GCC and Clang
support it. Even if a bit of code needs fixing, that shouldn't be a
problem if someone puts the work.

In other words, the kernel code is not frozen, nor are the features it
uses from compilers. They do, in fact, change from time to time.

> Thank you for checking. I found a free version that's only 6 weeks old:

You're welcome! There are quite a few new attributes coming, mostly
following C++ ones.

> It will be interesting to see whether 6.7.11.5 changes once the various
> implementations reach agreement.

Not sure what you mean. The standard does not evolve through
implementations' agreement (although standardizing existing practice
is one of the best arguments to back a change).

Cheers,
Miguel
_______________________________________________
Virtualization mailing list
Virtualization@lists.linux-foundation.org
https://lists.linuxfoundation.org/mailman/listinfo/virtualization

Re: [Intel-wired-lan] [PATCH 000/141] Fix fall-through warnings for Clang

[Nick Desaulniers via Virtualization]

On Tue, Nov 24, 2020 at 11:05 PM James Bottomley
<James.Bottomley@hansenpartnership.com> wrote:
>
> On Tue, 2020-11-24 at 13:32 -0800, Kees Cook wrote:
> > We already enable -Wimplicit-fallthrough globally, so that's not the
> > discussion. The issue is that Clang is (correctly) even more strict
> > than GCC for this, so these are the remaining ones to fix for full
> > Clang coverage too.
> >
> > People have spent more time debating this already than it would have
> > taken to apply the patches. :)
>
> You mean we've already spent 90% of the effort to come this far so we
> might as well go the remaining 10% because then at least we get some
> return? It's certainly a clinching argument in defence procurement ...

So developers and distributions using Clang can't have
-Wimplicit-fallthrough enabled because GCC is less strict (which has
been shown in this thread to lead to bugs)?  We'd like to have nice
things too, you know.

I even agree that most of the churn comes from

case 0:
  ++x;
default:
  break;

which I have a patch for: https://reviews.llvm.org/D91895.  I agree
that can never lead to bugs.  But that's not the sole case of this
series, just most of them.

Though, note how the reviewer (C++ spec editor and clang front end
owner) in https://reviews.llvm.org/D91895 even asks in that review how
maybe a new flag would be more appropriate for a watered
down/stylistic variant of the existing behavior.  And if the current
wording of Documentation/process/deprecated.rst around "fallthrough"
is a straightforward rule of thumb, I kind of agree with him.

>
> > This is about robustness and language wrangling. It's a big code-
> > base, and this is the price of our managing technical debt for
> > permanent robustness improvements. (The numbers I ran from Gustavo's
> > earlier patches were that about 10% of the places adjusted were
> > identified as legitimate bugs being fixed. This final series may be
> > lower, but there are still bugs being found from it -- we need to
> > finish this and shut the door on it for good.)
>
> I got my six patches by analyzing the lwn.net report of the fixes that
> was cited which had 21 of which 50% didn't actually change the emitted
> code, and 25% didn't have a user visible effect.
>
> But the broader point I'm making is just because the compiler people
> come up with a shiny new warning doesn't necessarily mean the problem

That's not what this is though; you're attacking a strawman.  I'd
encourage you to bring that up when that actually occurs, unlike this
case since it's actively hindering getting -Wimplicit-fallthrough
enabled for Clang.  This is not a shiny new warning; it's already on
for GCC and has existed in both compilers for multiple releases.

And I'll also note that warnings are warnings and not errors because
they cannot be proven to be bugs in 100% of cases, but they have led
to bugs in the past.  They require a human to review their intent and
remove ambiguities.  If 97% of cases would end in a break ("Expert C
Programming: Deep C Secrets" - Peter van der Linden), then it starts
to look to me like a language defect; certainly an incorrectly chosen
default.  But the compiler can't know those 3% were intentional,
unless you're explicit for those exceptional cases.

> it's detecting is one that causes us actual problems in the code base.
> I'd really be happier if we had a theory about what classes of CVE or
> bug we could eliminate before we embrace the next new warning.

We don't generally file CVEs and waiting for them to occur might be
too reactive, but I agree that pointing to some additional
documentation in commit messages about how a warning could lead to a
bug would make it clearer to reviewers why being able to enable it
treewide, even if there's no bug in their particular subsystem, is in
the general interest of the commons.

On Mon, Nov 23, 2020 at 7:58 AM James Bottomley
<James.Bottomley@hansenpartnership.com> wrote:
>
> We're also complaining about the inability to recruit maintainers:
>
> https://www.theregister.com/2020/06/30/hard_to_find_linux_maintainers_says_torvalds/
>
> And burn out:
>
> http://antirez.com/news/129
>
> The whole crux of your argument seems to be maintainers' time isn't
> important so we should accept all trivial patches ... I'm pushing back
> on that assumption in two places, firstly the valulessness of the time
> and secondly that all trivial patches are valuable.

It's critical to the longevity of any open source project that there
are not single points of failure.  If someone is not expendable or
replaceable (or claims to be) then that's a risk to the project and a
bottleneck.  Not having a replacement in training or some form of
redundancy is short sighted.

If trivial patches are adding too much to your workload, consider
training a co-maintainer or asking for help from one of your reviewers
whom you trust.  I don't doubt it's hard to find maintainers, but
existing maintainers should go out of their way to entrust
co-maintainers especially when they find their workload becomes too
high.  And reviewing/picking up trivial patches is probably a great
way to get started.  If we allow too much knowledge of any one
subsystem to collect with one maintainer, what happens when that
maintainer leaves the community (which, given a finite lifespan, is an
inevitability)?
-- 
Thanks,
~Nick Desaulniers
_______________________________________________
Virtualization mailing list
Virtualization@lists.linux-foundation.org
https://lists.linuxfoundation.org/mailman/listinfo/virtualization

Re: [Intel-wired-lan] [PATCH 000/141] Fix fall-through warnings for Clang

[Miguel Ojeda]

On Wed, Nov 25, 2020 at 5:24 PM Jakub Kicinski <kuba@kernel.org> wrote:
>
> And just to spell it out,
>
> case ENUM_VALUE1:
>         bla();
>         break;
> case ENUM_VALUE2:
>         bla();
> default:
>         break;
>
> is a fairly idiomatic way of indicating that not all values of the enum
> are expected to be handled by the switch statement.

It looks like a benign typo to me -- `ENUM_VALUE2` does not follow the
same pattern like `ENUM_VALUE1`. To me, the presence of the `default`
is what indicates (explicitly) that not everything is handled.

> Applying a real patch set and then getting a few follow ups the next day
> for trivial coding things like fallthrough missing or static missing,
> just because I didn't have the full range of compilers to check with
> before applying makes me feel pretty shitty, like I'm not doing a good
> job. YMMV.

The number of compilers, checkers, static analyzers, tests, etc. we
use keeps going up. That, indeed, means maintainers will miss more
things (unless maintainers do more work than before). But catching
bugs before they happen is *not* a bad thing.

Perhaps we could encourage more rebasing in -next (while still giving
credit to bots and testers) to avoid having many fixing commits
afterwards, but that is orthogonal.

I really don't think we should encourage the feeling that a maintainer
is doing a bad job if they don't catch everything on their reviews.
Any review is worth it. Maintainers, in the end, are just the
"guaranteed" reviewers that decide when the code looks reasonable
enough. They should definitely not feel pressured to be perfect.

Cheers,
Miguel
_______________________________________________
Virtualization mailing list
Virtualization@lists.linux-foundation.org
https://lists.linuxfoundation.org/mailman/listinfo/virtualization

Re: [Intel-wired-lan] [PATCH 000/141] Fix fall-through warnings for Clang

[Nick Desaulniers via Virtualization]

On Wed, Nov 25, 2020 at 8:24 AM Jakub Kicinski <kuba@kernel.org> wrote:
>
> Applying a real patch set and then getting a few follow ups the next day
> for trivial coding things like fallthrough missing or static missing,
> just because I didn't have the full range of compilers to check with
> before applying makes me feel pretty shitty, like I'm not doing a good
> job. YMMV.

I understand. Everyone feels that way, except maybe Bond villains and
robots.  My advice in that case is don't take it personally.  We're
working with a language that's more error prone relative to others.
While one would like to believe they are flawless, over time they
can't beat the aggregate statistics.  A balance between Imposter
Syndrome and Dunning Kruger is walked by all software developers, and
the fear of making mistakes in public is one of the number one reasons
folks don't take the plunge contributing to open source software or
even the kernel.  My advice to them is "don't sweat the small stuff."
-- 
Thanks,
~Nick Desaulniers
_______________________________________________
Virtualization mailing list
Virtualization@lists.linux-foundation.org
https://lists.linuxfoundation.org/mailman/listinfo/virtualization

Re: [Intel-wired-lan] [PATCH 000/141] Fix fall-through warnings for Clang

[Nick Desaulniers via Virtualization]

On Wed, Nov 25, 2020 at 1:33 PM Finn Thain <fthain@telegraphics.com.au> wrote:
>
> Or do you think that a codebase can somehow satisfy multiple checkers and
> their divergent interpretations of the language spec?

Have we found any cases yet that are divergent? I don't think so.  It
sounds to me like GCC's cases it warns for is a subset of Clang's.
Having additional coverage with Clang then should ensure coverage for
both.

> > This is not a shiny new warning; it's already on for GCC and has existed
> > in both compilers for multiple releases.
> >
>
> Perhaps you're referring to the compiler feature that lead to the
> ill-fated, tree-wide /* fallthrough */ patch series.
>
> When the ink dries on the C23 language spec and the implementations figure
> out how to interpret it then sure, enforce the warning for new code -- the
> cost/benefit analysis is straight forward. However, the case for patching
> existing mature code is another story.

I don't think we need to wait for the ink to dry on the C23 language
spec to understand that implicit fallthrough is an obvious defect of
the C language.  While the kernel is a mature codebase, it's not
immune to bugs.  And its maturity has yet to slow its rapid pace of
development.
-- 
Thanks,
~Nick Desaulniers
_______________________________________________
Virtualization mailing list
Virtualization@lists.linux-foundation.org
https://lists.linuxfoundation.org/mailman/listinfo/virtualization

Re: [Intel-wired-lan] [PATCH 000/141] Fix fall-through warnings for Clang

[Kees Cook]

On Tue, Nov 24, 2020 at 11:05:35PM -0800, James Bottomley wrote:
> Now, what we have seems to be about 6 cases (at least what's been shown
> in this thread) where a missing break would cause potentially user
> visible issues.  That means the value of this isn't zero, but it's not
> a no-brainer massive win either.  That's why I think asking what we've
> invested vs the return isn't a useless exercise.

The number is much higher[1]. If it were 6 in the entire history of the
kernel, I would agree with you. :) Some were fixed _before_ Gustavo's
effort too, which I also count towards the idea of "this is a dangerous
weakness in C, and now we have stopped it forever."

> But the broader point I'm making is just because the compiler people
> come up with a shiny new warning doesn't necessarily mean the problem
> it's detecting is one that causes us actual problems in the code base. 
> I'd really be happier if we had a theory about what classes of CVE or
> bug we could eliminate before we embrace the next new warning.

But we did! It was long ago justified and documented[2], and even links to
the CWE[3] for it. This wasn't random joy over discovering a new warning
we could turn on, this was turning on a warning that the compiler folks
finally gave us to handle an entire class of flaws. If we need to update
the code-base to address it not a useful debate -- that was settled
already, even if you're only discovering it now. :P. This last patch
set is about finishing that work for Clang, which is correctly even
more strict than GCC.

-Kees

[1] https://outflux.net/slides/2019/lss/kspp.pdf calls out specific
    numbers (about 6.5% of the patches fixed missing breaks):
	v4.19:  3 of 129
	v4.20:  2 of  59
	v5.0:   3 of  56
	v5.1:  10 of 100
	v5.2:   6 of  71
	v5.3:   7 of  69

    And in the history of the kernel, it's been an ongoing source of
    flaws:

    $ l --no-merges | grep -i 'missing break' | wc -l
    185

    The frequency of such errors being "naturally" found was pretty
    steady until the static checkers started warning, and then it was
    on the rise, but the full effort flushed the rest out, and now it's
    dropped to almost zero:

      1 v2.6.12
      3 v2.6.16.28
      1 v2.6.17
      1 v2.6.19
      2 v2.6.21
      1 v2.6.22
      3 v2.6.24
      3 v2.6.29
      1 v2.6.32
      1 v2.6.33
      1 v2.6.35
      4 v2.6.36
      3 v2.6.38
      2 v2.6.39
      7 v3.0
      2 v3.1
      2 v3.2
      2 v3.3
      3 v3.4
      1 v3.5
      8 v3.6
      7 v3.7
      3 v3.8
      6 v3.9
      3 v3.10
      2 v3.11
      5 v3.12
      5 v3.13
      2 v3.14
      4 v3.15
      2 v3.16
      3 v3.17
      2 v3.18
      2 v3.19
      1 v4.0
      2 v4.1
      5 v4.2
      4 v4.5
      5 v4.7
      6 v4.8
      1 v4.9
      3 v4.10
      2 v4.11
      6 v4.12
      3 v4.13
      2 v4.14
      5 v4.15
      2 v4.16
      7 v4.18
      2 v4.19
      6 v4.20
      3 v5.0
     12 v5.1
      3 v5.2
      4 v5.3
      2 v5.4
      1 v5.8


    And the reason it's fully zero, is because we still have the cases we're
    cleaning up right now. Even this last one from v5.8 is specifically of
    the same type this series addresses:

        case 4:
                color_index = TrueCModeIndex;
+               break;
        default:
                return;
        }


[2] https://www.kernel.org/doc/html/latest/process/deprecated.html#implicit-switch-case-fall-through

	All switch/case blocks must end in one of:

	break;
	fallthrough;
	continue;
	goto <label>;
	return [expression];

[3] https://cwe.mitre.org/data/definitions/484.html

-- 
Kees Cook
_______________________________________________
Virtualization mailing list
Virtualization@lists.linux-foundation.org
https://lists.linuxfoundation.org/mailman/listinfo/virtualization

Re: [PATCH 000/141] Fix fall-through warnings for Clang

[Miguel Ojeda]

On Sun, Nov 22, 2020 at 7:22 PM James Bottomley
<James.Bottomley@hansenpartnership.com> wrote:
>
> Well, it's a problem in an error leg, sure, but it's not a really
> compelling reason for a 141 patch series, is it?  All that fixing this
> error will do is get the driver to print "oh dear there's a problem"
> under four more conditions than it previously did.
>
> We've been at this for three years now with nearly a thousand patches,
> firstly marking all the fall throughs with /* fall through */ and later
> changing it to fallthrough.  At some point we do have to ask if the
> effort is commensurate with the protection afforded.  Please tell me
> our reward for all this effort isn't a single missing error print.

It isn't that much effort, isn't it? Plus we need to take into account
the future mistakes that it might prevent, too. So even if there were
zero problems found so far, it is still a positive change.

I would agree if these changes were high risk, though; but they are
almost trivial.

Cheers,
Miguel
_______________________________________________
Virtualization mailing list
Virtualization@lists.linux-foundation.org
https://lists.linuxfoundation.org/mailman/listinfo/virtualization

Re: [PATCH 000/141] Fix fall-through warnings for Clang

[Miguel Ojeda]

On Sun, Nov 22, 2020 at 11:54 PM Finn Thain <fthain@telegraphics.com.au> wrote:
>
> We should also take into account optimisim about future improvements in
> tooling.

Not sure what you mean here. There is no reliable way to guess what
the intention was with a missing fallthrough, even if you parsed
whitespace and indentation.

> It is if you want to spin it that way.

How is that a "spin"? It is a fact that we won't get *implicit*
fallthrough mistakes anymore (in particular if we make it a hard
error).

> But what we inevitably get is changes like this:
>
>  case 3:
>         this();
> +       break;
>  case 4:
>         hmmm();
>
> Why? Mainly to silence the compiler. Also because the patch author argued
> successfully that they had found a theoretical bug, often in mature code.

If someone changes control flow, that is on them. Every kernel
developer knows what `break` does.

> But is anyone keeping score of the regressions? If unreported bugs count,
> what about unreported regressions?

Introducing `fallthrough` does not change semantics. If you are really
keen, you can always compare the objects because the generated code
shouldn't change.

Cheers,
Miguel
_______________________________________________
Virtualization mailing list
Virtualization@lists.linux-foundation.org
https://lists.linuxfoundation.org/mailman/listinfo/virtualization

Re: [PATCH 000/141] Fix fall-through warnings for Clang

[Joe Perches]

On Tue, 2020-11-24 at 11:58 +1100, Finn Thain wrote:
> it's not for me to prove that such patches don't affect code 
> generation. That's for the patch author and (unfortunately) for reviewers.

Ideally, that proof would be provided by the compilation system itself
and not patch authors nor reviewers nor maintainers.

Unfortunately gcc does not guarantee repeatability or deterministic output.
To my knowledge, neither does clang.


_______________________________________________
Virtualization mailing list
Virtualization@lists.linux-foundation.org
https://lists.linuxfoundation.org/mailman/listinfo/virtualization

Re: [PATCH 000/141] Fix fall-through warnings for Clang

[Miguel Ojeda]

On Tue, Nov 24, 2020 at 1:58 AM Finn Thain <fthain@telegraphics.com.au> wrote:
>
> What I meant was that you've used pessimism as if it was fact.

"future mistakes that it might prevent" is neither pessimism nor states a fact.

> For example, "There is no way to guess what the effect would be if the
> compiler trained programmers to add a knee-jerk 'break' statement to avoid
> a warning".

It is only knee-jerk if you think you are infallible.

> Moreover, what I meant was that preventing programmer mistakes is a
> problem to be solved by development tools

This warning comes from a development tool -- the compiler.

> The idea that retro-fitting new
> language constructs onto mature code is somehow necessary to "prevent
> future mistakes" is entirely questionable.

The kernel is not a frozen codebase.

Further, "mature code vs. risk of change" arguments don't apply here
because the semantics of the program and binary output isn't changing.

> Sure. And if you put -Wimplicit-fallthrough into the Makefile and if that
> leads to well-intentioned patches that cause regressions, it is partly on
> you.

Again: adding a `fallthrough` does not change the program semantics.
If you are a maintainer and want to cross-check, compare the codegen.

> Have you ever considered the overall cost of the countless
> -Wpresume-incompetence flags?

Yeah: negative. On the other hand, the overall cost of the countless
-fI-am-infallible flags is very noticeable.

> Perhaps you pay the power bill for a build farm that produces logs that
> no-one reads? Perhaps you've run git bisect, knowing that the compiler
> messages are not interesting? Or compiled software in using a language
> that generates impenetrable messages? If so, here's a tip:
>
> # grep CFLAGS /etc/portage/make.conf
> CFLAGS="... -Wno-all -Wno-extra ..."
> CXXFLAGS="${CFLAGS}"
>
> Now allow me some pessimism: the hardware upgrades, gigawatt hours and
> wait time attributable to obligatory static analyses are a net loss.

If you really believe compiler warnings and static analysis are
useless and costly, I think there is not much point in continuing the
discussion.

> No, it's not for me to prove that such patches don't affect code
> generation. That's for the patch author and (unfortunately) for reviewers.

I was not asking you to prove it. I am stating that proving it is very easy.

Cheers,
Miguel
_______________________________________________
Virtualization mailing list
Virtualization@lists.linux-foundation.org
https://lists.linuxfoundation.org/mailman/listinfo/virtualization

Re: [PATCH 000/141] Fix fall-through warnings for Clang

[Miguel Ojeda]

On Sun, Nov 22, 2020 at 11:36 PM James Bottomley
<James.Bottomley@hansenpartnership.com> wrote:
>
> Well, it seems to be three years of someone's time plus the maintainer
> review time and series disruption of nearly a thousand patches.  Let's
> be conservative and assume the producer worked about 30% on the series
> and it takes about 5-10 minutes per patch to review, merge and for
> others to rework existing series.  So let's say it's cost a person year
> of a relatively junior engineer producing the patches and say 100h of
> review and application time.  The latter is likely the big ticket item
> because it's what we have in least supply in the kernel (even though
> it's 20x vs the producer time).

How are you arriving at such numbers? It is a total of ~200 trivial lines.

> It's not about the risk of the changes it's about the cost of
> implementing them.  Even if you discount the producer time (which
> someone gets to pay for, and if I were the engineering manager, I'd be
> unhappy about), the review/merge/rework time is pretty significant in
> exchange for six minor bug fixes.  Fine, when a new compiler warning
> comes along it's certainly reasonable to see if we can benefit from it
> and the fact that the compiler people think it's worthwhile is enough
> evidence to assume this initially.  But at some point you have to ask
> whether that assumption is supported by the evidence we've accumulated
> over the time we've been using it.  And if the evidence doesn't support
> it perhaps it is time to stop the experiment.

Maintainers routinely review 1-line trivial patches, not to mention
internal API changes, etc.

If some company does not want to pay for that, that's fine, but they
don't get to be maintainers and claim `Supported`.

Cheers,
Miguel
_______________________________________________
Virtualization mailing list
Virtualization@lists.linux-foundation.org
https://lists.linuxfoundation.org/mailman/listinfo/virtualization

Re: [PATCH 000/141] Fix fall-through warnings for Clang

[Rafael J. Wysocki]

On Mon, Nov 23, 2020 at 4:58 PM James Bottomley
<James.Bottomley@hansenpartnership.com> wrote:
>
> On Mon, 2020-11-23 at 15:19 +0100, Miguel Ojeda wrote:
> > On Sun, Nov 22, 2020 at 11:36 PM James Bottomley
> > <James.Bottomley@hansenpartnership.com> wrote:

[cut]

> >
> > Maintainers routinely review 1-line trivial patches, not to mention
> > internal API changes, etc.
>
> We're also complaining about the inability to recruit maintainers:
>
> https://www.theregister.com/2020/06/30/hard_to_find_linux_maintainers_says_torvalds/
>
> And burn out:
>
> http://antirez.com/news/129

Right.

> The whole crux of your argument seems to be maintainers' time isn't
> important so we should accept all trivial patches ... I'm pushing back
> on that assumption in two places, firstly the valulessness of the time
> and secondly that all trivial patches are valuable.
>
> > If some company does not want to pay for that, that's fine, but they
> > don't get to be maintainers and claim `Supported`.
>
> What I'm actually trying to articulate is a way of measuring value of
> the patch vs cost ... it has nothing really to do with who foots the
> actual bill.
>
> One thesis I'm actually starting to formulate is that this continual
> devaluing of maintainers is why we have so much difficulty keeping and
> recruiting them.

Absolutely.

This is just one of the factors involved, but a significant one IMV.
_______________________________________________
Virtualization mailing list
Virtualization@lists.linux-foundation.org
https://lists.linuxfoundation.org/mailman/listinfo/virtualization

Re: [PATCH 000/141] Fix fall-through warnings for Clang

[Joe Perches]

On Mon, 2020-11-23 at 07:58 -0800, James Bottomley wrote:
> We're also complaining about the inability to recruit maintainers:
> 
> https://www.theregister.com/2020/06/30/hard_to_find_linux_maintainers_says_torvalds/
> 
> And burn out:
> 
> http://antirez.com/news/129

https://www.wired.com/story/open-source-coders-few-tired/

> What I'm actually trying to articulate is a way of measuring value of
> the patch vs cost ... it has nothing really to do with who foots the
> actual bill.

It's unclear how to measure value in consistency.

But one way that costs can be reduced is by automation and _not_
involving maintainers when the patch itself is provably correct.

> One thesis I'm actually starting to formulate is that this continual
> devaluing of maintainers is why we have so much difficulty keeping and
> recruiting them.

The linux kernel has something like 1500 different maintainers listed
in the MAINTAINERS file.  That's not a trivial number.

$ git grep '^M:' MAINTAINERS | sort | uniq -c | wc -l
1543
$ git grep '^M:' MAINTAINERS| cut -f1 -d'<' | sort | uniq -c | wc -l
1446

I think the question you are asking is about trust and how it
effects development.

And back to that wired story, the actual number of what you might
be considering to be maintainers is likely less than 10% of the
listed numbers above.


_______________________________________________
Virtualization mailing list
Virtualization@lists.linux-foundation.org
https://lists.linuxfoundation.org/mailman/listinfo/virtualization

Re: [PATCH 000/141] Fix fall-through warnings for Clang

[Miguel Ojeda]

On Mon, Nov 23, 2020 at 4:58 PM James Bottomley
<James.Bottomley@hansenpartnership.com> wrote:
>
> Well, I used git.  It says that as of today in Linus' tree we have 889
> patches related to fall throughs and the first series went in in
> october 2017 ... ignoring a couple of outliers back to February.

I can see ~10k insertions over ~1k commits and 15 years that mention a
fallthrough in the entire repo. That is including some commits (like
the biggest one, 960 insertions) that have nothing to do with C
fallthrough. A single kernel release has an order of magnitude more
changes than this...

But if we do the math, for an author, at even 1 minute per line change
and assuming nothing can be automated at all, it would take 1 month of
work. For maintainers, a couple of trivial lines is noise compared to
many other patches.

In fact, this discussion probably took more time than the time it
would take to review the 200 lines. :-)

> We're also complaining about the inability to recruit maintainers:
>
> https://www.theregister.com/2020/06/30/hard_to_find_linux_maintainers_says_torvalds/
>
> And burn out:
>
> http://antirez.com/news/129

Accepting trivial and useful 1-line patches is not what makes a
voluntary maintainer quit... Thankless work with demanding deadlines is.

> The whole crux of your argument seems to be maintainers' time isn't
> important so we should accept all trivial patches

I have not said that, at all. In fact, I am a voluntary one and I
welcome patches like this. It takes very little effort on my side to
review and it helps the kernel overall. Paid maintainers are the ones
that can take care of big features/reviews.

> What I'm actually trying to articulate is a way of measuring value of
> the patch vs cost ... it has nothing really to do with who foots the
> actual bill.

I understand your point, but you were the one putting it in terms of a
junior FTE. In my view, 1 month-work (worst case) is very much worth
removing a class of errors from a critical codebase.

> One thesis I'm actually starting to formulate is that this continual
> devaluing of maintainers is why we have so much difficulty keeping and
> recruiting them.

That may very well be true, but I don't feel anybody has devalued
maintainers in this discussion.

Cheers,
Miguel
_______________________________________________
Virtualization mailing list
Virtualization@lists.linux-foundation.org
https://lists.linuxfoundation.org/mailman/listinfo/virtualization

Re: [PATCH 000/141] Fix fall-through warnings for Clang

[Miguel Ojeda]

On Mon, Nov 23, 2020 at 9:38 PM James Bottomley
<James.Bottomley@hansenpartnership.com> wrote:
>
> So you think a one line patch should take one minute to produce ... I
> really don't think that's grounded in reality.

No, I have not said that. Please don't put words in my mouth (again).

I have said *authoring* lines of *this* kind takes a minute per line.
Specifically: lines fixing the fallthrough warning mechanically and
repeatedly where the compiler tells you to, and doing so full-time for
a month.

For instance, take the following one from Gustavo. Are you really
saying it takes 12 minutes (your number) to write that `break;`?

diff --git a/drivers/gpu/drm/via/via_irq.c b/drivers/gpu/drm/via/via_irq.c
index 24cc445169e2..a3e0fb5b8671 100644
--- a/drivers/gpu/drm/via/via_irq.c
+++ b/drivers/gpu/drm/via/via_irq.c
@@ -364,6 +364,7 @@ int via_wait_irq(struct drm_device *dev, void
*data, struct drm_file *file_priv)
                irqwait->request.sequence +=
                        atomic_read(&cur_irq->irq_received);
                irqwait->request.type &= ~_DRM_VBLANK_RELATIVE;
+               break;
        case VIA_IRQ_ABSOLUTE:
                break;
        default:

>  I suppose a one line
> patch only takes a minute to merge with b4 if no-one reviews or tests
> it, but that's not really desirable.

I have not said that either. I said reviewing and merging those are
noise compared to any complex patch. Testing should be done by the
author comparing codegen.

> Part of what I'm trying to measure is the "and useful" bit because
> that's not a given.

It is useful since it makes intent clear. It also catches actual bugs,
which is even more valuable.

> Well, you know, subsystems are very different in terms of the amount of
> patches a maintainer has to process per release cycle of the kernel.
> If a maintainer is close to capacity, additional patches, however
> trivial, become a problem.  If a maintainer has spare cycles, trivial
> patches may look easy.

First of all, voluntary maintainers choose their own workload.
Furthermore, we already measure capacity in the `MAINTAINERS` file:
maintainers can state they can only handle a few patches. Finally, if
someone does not have time for a trivial patch, they are very unlikely
to have any time to review big ones.

> You seem to be saying that because you find it easy to merge trivial
> patches, everyone should.

Again, I have not said anything of the sort.

Cheers,
Miguel
_______________________________________________
Virtualization mailing list
Virtualization@lists.linux-foundation.org
https://lists.linuxfoundation.org/mailman/listinfo/virtualization

Re: [PATCH 000/141] Fix fall-through warnings for Clang

[Miguel Ojeda]

On Wed, Nov 25, 2020 at 11:44 PM Edward Cree <ecree.xilinx@gmail.com> wrote:
>
> To make the intent clear, you have to first be certain that you
>  understand the intent; otherwise by adding either a break or a
>  fallthrough to suppress the warning you are just destroying the
>  information that "the intent of this code is unknown".

If you don't know what the intent of your own code is, then you
*already* have a problem in your hands.

> Figuring out the intent of a piece of unfamiliar code takes more
>  than 1 minute; just because
>     case foo:
>         thing;
>     case bar:
>         break;
>  produces identical code to
>     case foo:
>         thing;
>         break;
>     case bar:
>         break;
>  doesn't mean that *either* is correct — maybe the author meant

What takes 1 minute is adding it *mechanically* by the author, i.e. so
that you later compare whether codegen is the same.

>  to write
>     case foo:
>         return thing;
>     case bar:
>         break;
>  and by inserting that break you've destroyed the marker that
>  would direct someone who knew what the code was about to look
>  at that point in the code and spot the problem.

Then it means you already have a bug. This patchset gives the
maintainer a chance to notice it, which is a good thing. The "you've
destroyed the market" claim is bogus, because:
  1. you were not looking into it
  2. you didn't notice the bug so far
  3. is implicit -- harder to spot
  4. is only useful if you explicitly take a look at this kind of bug.
So why don't you do it now?

> Thus, you *always* have to look at more than just the immediate
>  mechanical context of the code, to make a proper judgement that
>  yes, this was the intent.

I find that is the responsibility of the maintainers and reviewers for
tree-wide patches like this, assuming they want. They can also keep
the behavior (and the bugs) without spending time. Their choice.

> If you think that that sort of thing
>  can be done in an *average* time of one minute, then I hope you
>  stay away from code I'm responsible for!

Please don't accuse others of recklessness or incompetence, especially
if you didn't understand what they said.

> A warning is only useful because it makes you *think* about the
>  code.  If you suppress the warning without doing that thinking,
>  then you made the warning useless; and if the warning made you
>  think about code that didn't *need* it, then the warning was
>  useless from the start.

We are not suppressing the warning. Quite the opposite, in fact.

> So make your mind up: does Clang's stricter -Wimplicit-fallthrough
>  flag up code that needs thought (in which case the fixes take
>  effort both to author and to review)

As I said several times already, it does take time to review if the
maintainer wants to take the chance to see if they had a bug to begin
with, but it does not require thought for the author if they just go
for equivalent codegen.

> or does it flag up code
>  that can be mindlessly "fixed" (in which case the warning is
>  worthless)?  Proponents in this thread seem to be trying to
>  have it both ways.

A warning is not worthless just because you can mindlessly fix it.
There are many counterexamples, e.g. many
checkpatch/lint/lang-format/indentation warnings, functional ones like
the `if (a = b)` warning...

Cheers,
Miguel
_______________________________________________
Virtualization mailing list
Virtualization@lists.linux-foundation.org
https://lists.linuxfoundation.org/mailman/listinfo/virtualization

Re: [PATCH 000/141] Fix fall-through warnings for Clang

[Geert Uytterhoeven]

Hi Miguel,

On Thu, Nov 26, 2020 at 3:54 PM Miguel Ojeda
<miguel.ojeda.sandonis@gmail.com> wrote:
> On Wed, Nov 25, 2020 at 11:44 PM Edward Cree <ecree.xilinx@gmail.com> wrote:
> > To make the intent clear, you have to first be certain that you
> >  understand the intent; otherwise by adding either a break or a
> >  fallthrough to suppress the warning you are just destroying the
> >  information that "the intent of this code is unknown".
>
> If you don't know what the intent of your own code is, then you
> *already* have a problem in your hands.

The maintainer is not necessarily the owner/author of the code, and
thus may not know the intent of the code.

> > or does it flag up code
> >  that can be mindlessly "fixed" (in which case the warning is
> >  worthless)?  Proponents in this thread seem to be trying to
> >  have it both ways.
>
> A warning is not worthless just because you can mindlessly fix it.
> There are many counterexamples, e.g. many
> checkpatch/lint/lang-format/indentation warnings, functional ones like
> the `if (a = b)` warning...

BTW, you cannot mindlessly fix the latter, as you cannot know if
"(a == b)" or "((a = b))" was intended, without understanding the code
(and the (possibly unavailable) data sheet, and the hardware, ...).

P.S. So far I've stayed out of this thread, as I like it if the compiler
     flags possible mistakes.  After all I was the one fixing new
     "may be used uninitialized" warnings thrown up by gcc-4.1, until
     (a bit later than) support for that compiler was removed...

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds
_______________________________________________
Virtualization mailing list
Virtualization@lists.linux-foundation.org
https://lists.linuxfoundation.org/mailman/listinfo/virtualization

Re: [PATCH 000/141] Fix fall-through warnings for Clang

[Miguel Ojeda]

On Thu, Nov 26, 2020 at 4:28 PM Geert Uytterhoeven <geert@linux-m68k.org> wrote:
>
> The maintainer is not necessarily the owner/author of the code, and
> thus may not know the intent of the code.

Agreed, I was not blaming maintainers -- just trying to point out that
the problem is there :-)

In those cases, it is still very useful: we add the `fallthrough` and
a comment saying `FIXME: fallthrough intended? Figure this out...`.
Thus a previous unknown unknown is now a known unknown. And no new
unknown unknowns will be introduced since we enabled the warning
globally.

> BTW, you cannot mindlessly fix the latter, as you cannot know if
> "(a == b)" or "((a = b))" was intended, without understanding the code
> (and the (possibly unavailable) data sheet, and the hardware, ...).

That's right, I was referring to the cases where the compiler saves
someone time from a typo they just made.

Cheers,
Miguel
_______________________________________________
Virtualization mailing list
Virtualization@lists.linux-foundation.org
https://lists.linuxfoundation.org/mailman/listinfo/virtualization

Re: [PATCH 000/141] Fix fall-through warnings for Clang

[Andy Shevchenko]

On Mon, Nov 23, 2020 at 10:39 PM James Bottomley
<James.Bottomley@hansenpartnership.com> wrote:
> On Mon, 2020-11-23 at 19:56 +0100, Miguel Ojeda wrote:
> > On Mon, Nov 23, 2020 at 4:58 PM James Bottomley
> > <James.Bottomley@hansenpartnership.com> wrote:

...

> > But if we do the math, for an author, at even 1 minute per line
> > change and assuming nothing can be automated at all, it would take 1
> > month of work. For maintainers, a couple of trivial lines is noise
> > compared to many other patches.
>
> So you think a one line patch should take one minute to produce ... I
> really don't think that's grounded in reality.  I suppose a one line
> patch only takes a minute to merge with b4 if no-one reviews or tests
> it, but that's not really desirable.

In my practice most of the one line patches were either to fix or to
introduce quite interesting issues.
1 minute is 2-3 orders less than usually needed for such patches.
That's why I don't like churn produced by people who often even didn't
compile their useful contributions.

-- 
With Best Regards,
Andy Shevchenko
_______________________________________________
Virtualization mailing list
Virtualization@lists.linux-foundation.org
https://lists.linuxfoundation.org/mailman/listinfo/virtualization

Re: [PATCH 000/141] Fix fall-through warnings for Clang

[Sam Ravnborg]

Hi James.

> > > If none of the 140 patches here fix a real bug, and there is no
> > > change to machine code then it sounds to me like a W=2 kind of a
> > > warning.
> > 
> > FWIW, this series has found at least one bug so far:
> > https://lore.kernel.org/lkml/CAFCwf11izHF=g1mGry1fE5kvFFFrxzhPSM6qKAO8gxSp=Kr_CQ@mail.gmail.com/
> 
> 
> Well, it's a problem in an error leg, sure, but it's not a really
> compelling reason for a 141 patch series, is it?  All that fixing this
> error will do is get the driver to print "oh dear there's a problem"
> under four more conditions than it previously did.

You are asking the wrong question here.

Yuo should ask  how many hours could have been saved by all the bugs
people have been fighting with and then fixed *before* the code
hit the kernel at all.

My personal experience is that I, more than once, have had errors
related to a missing break in my code. So this warnings is IMO a win.

And if we are only ~100 patches to have it globally enabled then it is a
no-brainer in my book.

	Sam
_______________________________________________
Virtualization mailing list
Virtualization@lists.linux-foundation.org
https://lists.linuxfoundation.org/mailman/listinfo/virtualization

Re: [PATCH 000/141] Fix fall-through warnings for Clang

[Nick Desaulniers via Virtualization]

On Sun, Nov 22, 2020 at 8:17 AM Kees Cook <keescook@chromium.org> wrote:
>
> On Fri, Nov 20, 2020 at 11:51:42AM -0800, Jakub Kicinski wrote:
> > If none of the 140 patches here fix a real bug, and there is no change
> > to machine code then it sounds to me like a W=2 kind of a warning.
>
> FWIW, this series has found at least one bug so far:
> https://lore.kernel.org/lkml/CAFCwf11izHF=g1mGry1fE5kvFFFrxzhPSM6qKAO8gxSp=Kr_CQ@mail.gmail.com/

So looks like the bulk of these are:
switch (x) {
  case 0:
    ++x;
  default:
    break;
}

I have a patch that fixes those up for clang:
https://reviews.llvm.org/D91895

There's 3 other cases that don't quite match between GCC and Clang I
observe in the kernel:
switch (x) {
  case 0:
    ++x;
  default:
    goto y;
}
y:;

switch (x) {
  case 0:
    ++x;
  default:
    return;
}

switch (x) {
  case 0:
    ++x;
  default:
    ;
}

Based on your link, and Nathan's comment on my patch, maybe Clang
should continue to warn for the above (at least the `default: return;`
case) and GCC should change?  While the last case looks harmless,
there were only 1 or 2 across the tree in my limited configuration
testing; I really think we should just add `break`s for those.
-- 
Thanks,
~Nick Desaulniers
_______________________________________________
Virtualization mailing list
Virtualization@lists.linux-foundation.org
https://lists.linuxfoundation.org/mailman/listinfo/virtualization

Re: [PATCH 000/141] Fix fall-through warnings for Clang

[Kees Cook]

On Mon, Nov 23, 2020 at 05:32:51PM -0800, Nick Desaulniers wrote:
> On Sun, Nov 22, 2020 at 8:17 AM Kees Cook <keescook@chromium.org> wrote:
> >
> > On Fri, Nov 20, 2020 at 11:51:42AM -0800, Jakub Kicinski wrote:
> > > If none of the 140 patches here fix a real bug, and there is no change
> > > to machine code then it sounds to me like a W=2 kind of a warning.
> >
> > FWIW, this series has found at least one bug so far:
> > https://lore.kernel.org/lkml/CAFCwf11izHF=g1mGry1fE5kvFFFrxzhPSM6qKAO8gxSp=Kr_CQ@mail.gmail.com/
> 
> So looks like the bulk of these are:
> switch (x) {
>   case 0:
>     ++x;
>   default:
>     break;
> }
> 
> I have a patch that fixes those up for clang:
> https://reviews.llvm.org/D91895

I still think this isn't right -- it's a case statement that runs off
the end without an explicit flow control determination. I think Clang is
right to warn for these, and GCC should also warn.

-- 
Kees Cook
_______________________________________________
Virtualization mailing list
Virtualization@lists.linux-foundation.org
https://lists.linuxfoundation.org/mailman/listinfo/virtualization

Re: [PATCH 000/141] Fix fall-through warnings for Clang

[Dan Carpenter]

On Mon, Nov 23, 2020 at 05:32:51PM -0800, Nick Desaulniers wrote:
> On Sun, Nov 22, 2020 at 8:17 AM Kees Cook <keescook@chromium.org> wrote:
> >
> > On Fri, Nov 20, 2020 at 11:51:42AM -0800, Jakub Kicinski wrote:
> > > If none of the 140 patches here fix a real bug, and there is no change
> > > to machine code then it sounds to me like a W=2 kind of a warning.
> >
> > FWIW, this series has found at least one bug so far:
> > https://lore.kernel.org/lkml/CAFCwf11izHF=g1mGry1fE5kvFFFrxzhPSM6qKAO8gxSp=Kr_CQ@mail.gmail.com/
> 
> So looks like the bulk of these are:
> switch (x) {
>   case 0:
>     ++x;
>   default:
>     break;
> }

This should not generate a warning.

> 
> I have a patch that fixes those up for clang:
> https://reviews.llvm.org/D91895
> 
> There's 3 other cases that don't quite match between GCC and Clang I
> observe in the kernel:
> switch (x) {
>   case 0:
>     ++x;
>   default:
>     goto y;
> }
> y:;

This should generate a warning.

> 
> switch (x) {
>   case 0:
>     ++x;
>   default:
>     return;
> }

Warn for this.


> 
> switch (x) {
>   case 0:
>     ++x;
>   default:
>     ;
> }

Don't warn for this.

If adding a break statement changes the flow of the code then warn about
potentially missing break statements, but if it doesn't change anything
then don't warn about it.

regards,
dan carpenter
_______________________________________________
Virtualization mailing list
Virtualization@lists.linux-foundation.org
https://lists.linuxfoundation.org/mailman/listinfo/virtualization

Re: [PATCH 000/141] Fix fall-through warnings for Clang

[Dan Carpenter]

On Sun, Nov 22, 2020 at 08:17:03AM -0800, Kees Cook wrote:
> On Fri, Nov 20, 2020 at 11:51:42AM -0800, Jakub Kicinski wrote:
> > On Fri, 20 Nov 2020 11:30:40 -0800 Kees Cook wrote:
> > > On Fri, Nov 20, 2020 at 10:53:44AM -0800, Jakub Kicinski wrote:
> > > > On Fri, 20 Nov 2020 12:21:39 -0600 Gustavo A. R. Silva wrote:  
> > > > > This series aims to fix almost all remaining fall-through warnings in
> > > > > order to enable -Wimplicit-fallthrough for Clang.
> > > > > 
> > > > > In preparation to enable -Wimplicit-fallthrough for Clang, explicitly
> > > > > add multiple break/goto/return/fallthrough statements instead of just
> > > > > letting the code fall through to the next case.
> > > > > 
> > > > > Notice that in order to enable -Wimplicit-fallthrough for Clang, this
> > > > > change[1] is meant to be reverted at some point. So, this patch helps
> > > > > to move in that direction.
> > > > > 
> > > > > Something important to mention is that there is currently a discrepancy
> > > > > between GCC and Clang when dealing with switch fall-through to empty case
> > > > > statements or to cases that only contain a break/continue/return
> > > > > statement[2][3][4].  
> > > > 
> > > > Are we sure we want to make this change? Was it discussed before?
> > > > 
> > > > Are there any bugs Clangs puritanical definition of fallthrough helped
> > > > find?
> > > > 
> > > > IMVHO compiler warnings are supposed to warn about issues that could
> > > > be bugs. Falling through to default: break; can hardly be a bug?!  
> > > 
> > > It's certainly a place where the intent is not always clear. I think
> > > this makes all the cases unambiguous, and doesn't impact the machine
> > > code, since the compiler will happily optimize away any behavioral
> > > redundancy.
> > 
> > If none of the 140 patches here fix a real bug, and there is no change
> > to machine code then it sounds to me like a W=2 kind of a warning.
> 
> FWIW, this series has found at least one bug so far:
> https://lore.kernel.org/lkml/CAFCwf11izHF=g1mGry1fE5kvFFFrxzhPSM6qKAO8gxSp=Kr_CQ@mail.gmail.com/

This is a fallthrough to a return and not to a break.  That should
trigger a warning.  The fallthrough to a break should not generate a
warning.

The bug we're trying to fix is "missing break statement" but if the
result of the bug is "we hit a break statement" then now we're just
talking about style.  GCC should limit itself to warning about
potentially buggy code.

regards,
dan carpenter
_______________________________________________
Virtualization mailing list
Virtualization@lists.linux-foundation.org
https://lists.linuxfoundation.org/mailman/listinfo/virtualization

Re: [PATCH 000/141] Fix fall-through warnings for Clang

[Miguel Ojeda]

Hi Gustavo,

On Fri, Nov 20, 2020 at 7:21 PM Gustavo A. R. Silva
<gustavoars@kernel.org> wrote:
>
> Hi all,
>
> This series aims to fix almost all remaining fall-through warnings in
> order to enable -Wimplicit-fallthrough for Clang.

Thanks for this.

Since this warning is reliable in both/all compilers and we are
eventually getting rid of all the cases, what about going even further
and making it an error right after?

Cheers,
Miguel
_______________________________________________
Virtualization mailing list
Virtualization@lists.linux-foundation.org
https://lists.linuxfoundation.org/mailman/listinfo/virtualization

Re: [PATCH 000/141] Fix fall-through warnings for Clang

[Mark Brown]

On Fri, 20 Nov 2020 12:21:39 -0600, Gustavo A. R. Silva wrote:
> This series aims to fix almost all remaining fall-through warnings in
> order to enable -Wimplicit-fallthrough for Clang.
> 
> In preparation to enable -Wimplicit-fallthrough for Clang, explicitly
> add multiple break/goto/return/fallthrough statements instead of just
> letting the code fall through to the next case.
> 
> [...]

Applied to

   https://git.kernel.org/pub/scm/linux/kernel/git/broonie/regulator.git for-next

Thanks!

[1/1] regulator: as3722: Fix fall-through warnings for Clang
      commit: b52b417ccac4fae5b1f2ec4f1d46eb91e4493dc5

All being well this means that it will be integrated into the linux-next
tree (usually sometime in the next 24 hours) and sent to Linus during
the next merge window (or sooner if it is a bug fix), however if
problems are discovered then the patch may be dropped or reverted.

You may get further e-mails resulting from automated or manual testing
and review of the tree, please engage with people reporting problems and
send followup patches addressing any issues that are reported if needed.

If any updates are required or you are submitting further changes they
should be sent as incremental updates against current git, existing
patches will not be replaced.

Please add any relevant lists and maintainers to the CCs when replying
to this mail.

Thanks,
Mark
_______________________________________________
Virtualization mailing list
Virtualization@lists.linux-foundation.org
https://lists.linuxfoundation.org/mailman/listinfo/virtualization

Re: [PATCH 000/141] Fix fall-through warnings for Clang

[Martin K. Petersen]

Gustavo,

> This series aims to fix almost all remaining fall-through warnings in
> order to enable -Wimplicit-fallthrough for Clang.

Applied 20-22,54,120-124 to 5.11/scsi-staging, thanks.

-- 
Martin K. Petersen	Oracle Linux Engineering
_______________________________________________
Virtualization mailing list
Virtualization@lists.linux-foundation.org
https://lists.linuxfoundation.org/mailman/listinfo/virtualization

Re: (subset) [PATCH 000/141] Fix fall-through warnings for Clang

[Martin K. Petersen]

On Fri, 20 Nov 2020 12:21:39 -0600, Gustavo A. R. Silva wrote:

> This series aims to fix almost all remaining fall-through warnings in
> order to enable -Wimplicit-fallthrough for Clang.
> 
> In preparation to enable -Wimplicit-fallthrough for Clang, explicitly
> add multiple break/goto/return/fallthrough statements instead of just
> letting the code fall through to the next case.
> 
> [...]

Applied to 5.11/scsi-queue, thanks!

[054/141] target: Fix fall-through warnings for Clang
          https://git.kernel.org/mkp/scsi/c/492096ecfa39

-- 
Martin K. Petersen	Oracle Linux Engineering
_______________________________________________
Virtualization mailing list
Virtualization@lists.linux-foundation.org
https://lists.linuxfoundation.org/mailman/listinfo/virtualization

Re: [PATCH 136/141] virtio_net: Fix fall-through warnings for Clang

[Gustavo A. R. Silva]

Hi all,

Who can take this? :)

Thanks
--
Gustavo

On 11/20/20 12:40, Gustavo A. R. Silva wrote:
> In preparation to enable -Wimplicit-fallthrough for Clang, fix a warning
> by explicitly adding a goto statement instead of letting the code fall
> through to the next case.
> 
> Link: https://github.com/KSPP/linux/issues/115
> Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
> ---
>  drivers/net/virtio_net.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index 21b71148c532..fd326dc586aa 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -732,6 +732,7 @@ static struct sk_buff *receive_small(struct net_device *dev,
>  			fallthrough;
>  		case XDP_ABORTED:
>  			trace_xdp_exception(vi->dev, xdp_prog, act);
> +			goto err_xdp;
>  		case XDP_DROP:
>  			goto err_xdp;
>  		}
> 
_______________________________________________
Virtualization mailing list
Virtualization@lists.linux-foundation.org
https://lists.linuxfoundation.org/mailman/listinfo/virtualization