Re: [PATCH v2 1/3] base: mark 'no_warn' as unused

Re: [PATCH v2 1/3] base: mark 'no_warn' as unused

[Nick Desaulniers]

On Mon, Jul 26, 2021 at 11:41 PM Greg Kroah-Hartman
<gregkh@linuxfoundation.org> wrote:
>
> On Mon, Jul 26, 2021 at 11:15:52PM -0700, Bill Wendling wrote:
> > On Mon, Jul 26, 2021 at 10:27 PM Greg Kroah-Hartman
> > <gregkh@linuxfoundation.org> wrote:
> > > On Mon, Jul 26, 2021 at 01:47:33PM -0700, Nathan Chancellor wrote:
> > > > + Greg and Rafael as the maintainer and reviewer of drivers/base/module.c
> > > > respectively, drop everyone else.
> > >
> > > Odd no one cc:ed us originally, I guess they didn't want the patch ever
> > > merged?  :(

Bill,
$ wget https://lore.kernel.org/lkml/20210726201924.3202278-2-morbo@google.com/raw
-O bill.patch
$ ./scripts/get_maintainer.pl bill.patch
Greg Kroah-Hartman <gregkh@linuxfoundation.org> (supporter:DRIVER
CORE, KOBJECTS, DEBUGFS AND SYSFS)
"Rafael J. Wysocki" <rafael@kernel.org> (reviewer:DRIVER CORE,
KOBJECTS, DEBUGFS AND SYSFS)
Nathan Chancellor <nathan@kernel.org> (supporter:CLANG/LLVM BUILD SUPPORT)
Nick Desaulniers <ndesaulniers@google.com> (supporter:CLANG/LLVM BUILD SUPPORT)
linux-kernel@vger.kernel.org (open list)
clang-built-linux@googlegroups.com (open list:CLANG/LLVM BUILD SUPPORT)

You can use something like this shell function for small patches:

$ which kpatch
kpatch () {
        patch=$1
        shift
        if [[ -d $patch ]]
        then
                echo "Directory given"
        else
                git send-email --cc-cmd="./scripts/get_maintainer.pl
--norolestats $patch" $@ $patch
        fi
}
$ kpatch bill.patch --to "Greg Kroah-Hartman
<gregkh@linuxfoundation.org>" --to "Rafael J. Wysocki
<rafael@kernel.org>"

(I recommend always putting the maintainers or people you'd expect to
pick up the patches in To:.)

> > >
> > > >
> > > > Original post:
> > > >
> > > > https://lore.kernel.org/r/20210726201924.3202278-2-morbo@google.com/
> > > >
> > > > On 7/26/2021 1:19 PM, 'Bill Wendling' via Clang Built Linux wrote:
> > > > > Fix the following build warning:
> > > > >
> > > > >    drivers/base/module.c:36:6: error: variable 'no_warn' set but not used [-Werror,-Wunused-but-set-variable]
> > > > >          int no_warn;
> > >
> > > That's not going to be a good warning to ever have the kernel use due to
> > > how lots of hardware works (i.e. we need to do a read after a write but
> > > we can throw the read away as it does not matter).
> > >
> > >
> > > > >
> > > > > This variable is used to remove another warning, but causes a warning
> > > > > itself. Mark it as 'unused' to avoid that.
> > > > >
> > > > > Signed-off-by: Bill Wendling <morbo@google.com>
> > > >
> > > > Even though they evaluate to the same thing, it might be worth using
> > > > "__always_unused" here because it is :)
> > >
> > > But it is not unused, the value is written into it.
> > >
> > I believe that only matters if the variable is marked "volatile".
>
> "volatile" means nothing anymore, never use it or even think about it
> again please :)

What Greg is getting at is that the use of the volatile keyword in
variable declarations is slightly frowned on by the kernel community.
It's less flexible than making accesses volatile qualified via casts.
Then you have flexibility for some accesses to be volatile (ie. not
CSE'd), and some not (ie. CSE'd), if needed.

Though just because you assign to a variable doesn't mean that the
compiler generates an access, especially if the result is unused.
This warning is all about dead stores.  The cast to a volatile
qualified pointer then dereference is what guarantees the access.

https://godbolt.org/z/7K7369bGG

(To be explicit, IMO Greg's point about volatile stores is orthogonal
to discussions about dead stores.)

>
> > Otherwise, the variable itself is never used. A "variable that's
> > written to but not read from," in fact, is the whole reason for the
> > warning.
>
> But that is ok!  Sometimes you need to do this with hardware (like all
> PCI devices).  This is a legitimate code flow for many hardware types
> and if a C compiler thinks that this is not ok, then it is broken.
>
> So be VERY careful when changing drivers based on this warning.  Because
> of this, I do not think you can enable it over the whole kernel without
> causing major problems in some areas.
>
> But that is independent of this specific issue you are trying to patch
> here, I say this to warn you of a number of stupid places where people
> have tried to "optimize away" reads based on this compiler warning in
> drivers, and we have had to add them back because it broke
> functionality.
>
> > > So this isn't ok, sometimes we want to write to variables but never care
> > > about the value, that does not mean the compiler should complain about
> > > it.
> > >
> > Typically, if you don't care about the return value, you simply don't
> > assign it to a variable (cf. printf). However, the functions that
> > assign to "no_warn" have the "warn_unused_result" attribute. The fact
> > that the variable is named "no_warn" seems to indicate that it's meant
> > to remain unused, even if it probably should be checked.
>
> These functions have warn_unused_result set on them because for 99% of
> the time, I want the value to be checked.  But as you can see in this
> use, as per the comments in the code, we do not care about the result
> for a very good reason.  So we just assign it to a variable to make the
> compiler quiet.

I think warn_unused_result should only really be used for functions
where the return value should be used 100% of the time.  If there are
cases where it's ok to not check the return value, consider not using
warn_unused_result on function declarations.

That said, we have a very similar issue throughout LLVM that Bill
should recognize. In LLVM, we have pretty aggressive usage of
assertions.  Rather than:

assert(someReallyLongExpression && "error message");

where that statement might wrap across multiple lines, instead it
might be clearer to write:

bool IsOk = someReallyLongExpression;
assert(IsOk && "error message");

which looks nicer but now produces -Wunused-but-set-variable on IsOk
for release builds where assertions are disabled.  The common fix in
LLVM is to write:

bool IsOk = someReallyLongExpression;
assert(IsOk && "error message");
(void)IsOk;

The cast to void is technically a use that doesn't result in a dead
store.  That pattern could be used in the kernel rather than

int no_warn;
no_warn = warn_unused_result_fn();

at least to avoid -Wunused-but-set-variable.  Oh, looks like a curious
difference between compilers:
https://godbolt.org/z/GvznMM6o1
Filed https://bugs.llvm.org/show_bug.cgi?id=51228.  So I guess we
can't use the cast-to-void to avoid -Wunused-but-set-variable, since
that triggers -Wunused-result, at least with GCC.  :( Nevermind...

Though I still think the use of warn_unused_result on
sysfs_create_link() is worth revisiting.

(Orthogonally, I wonder if C would have been more ergonomic or less to
have all functions implicitly warn_unused_result then have callers be
explicit when they didn't want a result, rather than what we have
today which is the opposite.  Maybe that's a terrible idea, but
sometimes you can't tell until some volume of code has been written in
such a language.  Maybe it makes hello world less pretty, but maybe it
avoids more bugs in real code.)

>
> > Would you rather the warning be turned off on some level?
>
> Which warning?
>
> The code here, as-is, is correct.  We already have 1 compiler warning
> work around in place, do you want to add another one?  How many can we
> stack on top of each other?

Isn't -Wunused-but-set-variable enabled only for W=1 builds?

>
> And again, why did you not cc: the maintainers of this code for this
> change?  That's not good...
>
> thanks,
>
> greg k-h



-- 
Thanks,
~Nick Desaulniers

Re: [PATCH v2 1/3] base: mark 'no_warn' as unused

[Greg Kroah-Hartman]

On Tue, Jul 27, 2021 at 10:39:49AM -0700, Nick Desaulniers wrote:
> > > > > Original post:
> > > > >
> > > > > https://lore.kernel.org/r/20210726201924.3202278-2-morbo@google.com/
> > > > >
> > > > > On 7/26/2021 1:19 PM, 'Bill Wendling' via Clang Built Linux wrote:
> > > > > > Fix the following build warning:
> > > > > >
> > > > > >    drivers/base/module.c:36:6: error: variable 'no_warn' set but not used [-Werror,-Wunused-but-set-variable]
> > > > > >          int no_warn;
> > > >
> > > > That's not going to be a good warning to ever have the kernel use due to
> > > > how lots of hardware works (i.e. we need to do a read after a write but
> > > > we can throw the read away as it does not matter).
> > > >
> > > >
> > > > > >
> > > > > > This variable is used to remove another warning, but causes a warning
> > > > > > itself. Mark it as 'unused' to avoid that.
> > > > > >
> > > > > > Signed-off-by: Bill Wendling <morbo@google.com>
> > > > >
> > > > > Even though they evaluate to the same thing, it might be worth using
> > > > > "__always_unused" here because it is :)
> > > >
> > > > But it is not unused, the value is written into it.
> > > >
> > > I believe that only matters if the variable is marked "volatile".
> >
> > "volatile" means nothing anymore, never use it or even think about it
> > again please :)
> 
> What Greg is getting at is that the use of the volatile keyword in
> variable declarations is slightly frowned on by the kernel community.
> It's less flexible than making accesses volatile qualified via casts.
> Then you have flexibility for some accesses to be volatile (ie. not
> CSE'd), and some not (ie. CSE'd), if needed.
> 
> Though just because you assign to a variable doesn't mean that the
> compiler generates an access, especially if the result is unused.
> This warning is all about dead stores.  The cast to a volatile
> qualified pointer then dereference is what guarantees the access.
> 
> https://godbolt.org/z/7K7369bGG
> 
> (To be explicit, IMO Greg's point about volatile stores is orthogonal
> to discussions about dead stores.)

I didn't bring up that dirty word, Bill did :)

> > > Otherwise, the variable itself is never used. A "variable that's
> > > written to but not read from," in fact, is the whole reason for the
> > > warning.
> >
> > But that is ok!  Sometimes you need to do this with hardware (like all
> > PCI devices).  This is a legitimate code flow for many hardware types
> > and if a C compiler thinks that this is not ok, then it is broken.
> >
> > So be VERY careful when changing drivers based on this warning.  Because
> > of this, I do not think you can enable it over the whole kernel without
> > causing major problems in some areas.
> >
> > But that is independent of this specific issue you are trying to patch
> > here, I say this to warn you of a number of stupid places where people
> > have tried to "optimize away" reads based on this compiler warning in
> > drivers, and we have had to add them back because it broke
> > functionality.
> >
> > > > So this isn't ok, sometimes we want to write to variables but never care
> > > > about the value, that does not mean the compiler should complain about
> > > > it.
> > > >
> > > Typically, if you don't care about the return value, you simply don't
> > > assign it to a variable (cf. printf). However, the functions that
> > > assign to "no_warn" have the "warn_unused_result" attribute. The fact
> > > that the variable is named "no_warn" seems to indicate that it's meant
> > > to remain unused, even if it probably should be checked.
> >
> > These functions have warn_unused_result set on them because for 99% of
> > the time, I want the value to be checked.  But as you can see in this
> > use, as per the comments in the code, we do not care about the result
> > for a very good reason.  So we just assign it to a variable to make the
> > compiler quiet.
> 
> I think warn_unused_result should only really be used for functions
> where the return value should be used 100% of the time.

I too want a shiny new pony.

But here in the real world, sometimes you have functions that for 99% of
the users, you do want them to check the return value, but when you use
them in core code or startup code, you "know" you are safe to ignore the
return value.

That is the case here.  We have other fun examples of where people have
tried to add error handling to code that runs at boot that have actually
introduced security errors and they justify it with "but you have to
check error values!"

> If there are
> cases where it's ok to not check the return value, consider not using
> warn_unused_result on function declarations.

Ok, so what do you do when you have a function like this where 99.9% of
the users need to check this?  Do I really need to write a wrapper
function just for it so that I can use it "safely" in the core code
instead?

Something like:

void do_safe_thing_and_ignore_the_world(...)
{
	__unused int error;

	error = do_thing(...);
}

Or something else to get the compiler to be quiet about error being set
and never used?  There HAS to be that option somewhere anyway as we need
it for other parts of the kernel where we do:
	write_bus(device, &value);
	value = read_bus(device);
and then we ignore value as it is not needed, but yet we still HAVE to
call read_bus() here, yet read_bus() is set as warn_unused_result()
because, well, it is a read function :)

> That said, we have a very similar issue throughout LLVM that Bill
> should recognize. In LLVM, we have pretty aggressive usage of
> assertions.  Rather than:
> 
> assert(someReallyLongExpression && "error message");
> 
> where that statement might wrap across multiple lines, instead it
> might be clearer to write:
> 
> bool IsOk = someReallyLongExpression;
> assert(IsOk && "error message");
> 
> which looks nicer but now produces -Wunused-but-set-variable on IsOk
> for release builds where assertions are disabled.  The common fix in
> LLVM is to write:
> 
> bool IsOk = someReallyLongExpression;
> assert(IsOk && "error message");
> (void)IsOk;
> 
> The cast to void is technically a use that doesn't result in a dead
> store.  That pattern could be used in the kernel rather than
> 
> int no_warn;
> no_warn = warn_unused_result_fn();
> 
> at least to avoid -Wunused-but-set-variable.  Oh, looks like a curious
> difference between compilers:
> https://godbolt.org/z/GvznMM6o1
> Filed https://bugs.llvm.org/show_bug.cgi?id=51228.  So I guess we
> can't use the cast-to-void to avoid -Wunused-but-set-variable, since
> that triggers -Wunused-result, at least with GCC.  :( Nevermind...
> 
> Though I still think the use of warn_unused_result on
> sysfs_create_link() is worth revisiting.

Nope, not at all, I WANT users to check this as it is something that has
caused problems in drivers and subsystems in the past.

And doing the (void)sysfs_create_link(); hack is horrid, I thought we
were better than that.

Surely there is a "this variable is going to be assigned something but
never used" option somewhere?  This can't be the first time it has come
up, right?

> > > Would you rather the warning be turned off on some level?
> >
> > Which warning?
> >
> > The code here, as-is, is correct.  We already have 1 compiler warning
> > work around in place, do you want to add another one?  How many can we
> > stack on top of each other?
> 
> Isn't -Wunused-but-set-variable enabled only for W=1 builds?

No idea, as long as it is not a normal build option, that's fine.  "W=1"
is for kernel newbies wanting to clean up subsystems and get some patch
counts merged :)

thanks,

greg k-h

Re: [PATCH v2 1/3] base: mark 'no_warn' as unused

[Nick Desaulniers]

On Tue, Jul 27, 2021 at 10:59 AM Greg Kroah-Hartman
<gregkh@linuxfoundation.org> wrote:
>
> On Tue, Jul 27, 2021 at 10:39:49AM -0700, Nick Desaulniers wrote:
> > If there are
> > cases where it's ok to not check the return value, consider not using
> > warn_unused_result on function declarations.
>
> Ok, so what do you do when you have a function like this where 99.9% of
> the users need to check this?  Do I really need to write a wrapper
> function just for it so that I can use it "safely" in the core code
> instead?
>
> Something like:
>
> void do_safe_thing_and_ignore_the_world(...)
> {
>         __unused int error;
>
>         error = do_thing(...);
> }
>
> Or something else to get the compiler to be quiet about error being set
> and never used?  There HAS to be that option somewhere anyway as we need
> it for other parts of the kernel where we do:
>         write_bus(device, &value);
>         value = read_bus(device);
> and then we ignore value as it is not needed, but yet we still HAVE to
> call read_bus() here, yet read_bus() is set as warn_unused_result()
> because, well, it is a read function :)

Such wrappers are trivial with __attribute__((alias(""))):
https://godbolt.org/z/j5afPbGcM

At least then it's very obvious if someone adds more call sites to
such an alias. Then that calls for closer inspection in code review
that yes, this is one of those 0.01% of cases.  Since they occur 0.01%
of the time, I don't expect such aliases to occur too frequently.
-- 
Thanks,
~Nick Desaulniers

Re: [PATCH v2 1/3] base: mark 'no_warn' as unused

[Nathan Chancellor]

On 7/27/2021 10:39 AM, 'Nick Desaulniers' via Clang Built Linux wrote:
> Isn't -Wunused-but-set-variable enabled only for W=1 builds?

Maybe Bill's tree does not have commit 885480b08469 ("Makefile: Move 
-Wno-unused-but-set-variable out of GCC only block"), which disables the 
warning for clang just like GCC for regular builds?

Cheers,
Nathan

Re: [PATCH v2 1/3] base: mark 'no_warn' as unused

[Greg Kroah-Hartman]

On Tue, Jul 27, 2021 at 11:31:38AM -0700, Nick Desaulniers wrote:
> On Tue, Jul 27, 2021 at 10:59 AM Greg Kroah-Hartman
> <gregkh@linuxfoundation.org> wrote:
> >
> > On Tue, Jul 27, 2021 at 10:39:49AM -0700, Nick Desaulniers wrote:
> > > If there are
> > > cases where it's ok to not check the return value, consider not using
> > > warn_unused_result on function declarations.
> >
> > Ok, so what do you do when you have a function like this where 99.9% of
> > the users need to check this?  Do I really need to write a wrapper
> > function just for it so that I can use it "safely" in the core code
> > instead?
> >
> > Something like:
> >
> > void do_safe_thing_and_ignore_the_world(...)
> > {
> >         __unused int error;
> >
> >         error = do_thing(...);
> > }
> >
> > Or something else to get the compiler to be quiet about error being set
> > and never used?  There HAS to be that option somewhere anyway as we need
> > it for other parts of the kernel where we do:
> >         write_bus(device, &value);
> >         value = read_bus(device);
> > and then we ignore value as it is not needed, but yet we still HAVE to
> > call read_bus() here, yet read_bus() is set as warn_unused_result()
> > because, well, it is a read function :)
> 
> Such wrappers are trivial with __attribute__((alias(""))):
> https://godbolt.org/z/j5afPbGcM
> 
> At least then it's very obvious if someone adds more call sites to
> such an alias. Then that calls for closer inspection in code review
> that yes, this is one of those 0.01% of cases.  Since they occur 0.01%
> of the time, I don't expect such aliases to occur too frequently.

That is just, well, horrible.  Seriously horrible.  Wow.

And that is the "documented" way to do this?  That feels like an abuse
of the already-horrible-why-do-they-do-that-for-variables use of the
alias attribute.

How badly are compiler people going to complain to me about this if
it's in this file?

I can take a patch for that, but I feel the comments involved will make
people, including myself when I have to look a the code again in 5
years, even more confused...

ick, I feel dirty...

greg k-h

Re: [PATCH v2 1/3] base: mark 'no_warn' as unused

[Nick Desaulniers]

On Tue, Jul 27, 2021 at 11:45 AM Greg Kroah-Hartman
<gregkh@linuxfoundation.org> wrote:
>
> On Tue, Jul 27, 2021 at 11:31:38AM -0700, Nick Desaulniers wrote:
> > On Tue, Jul 27, 2021 at 10:59 AM Greg Kroah-Hartman
> > <gregkh@linuxfoundation.org> wrote:
> > >
> > > On Tue, Jul 27, 2021 at 10:39:49AM -0700, Nick Desaulniers wrote:
> > > > If there are
> > > > cases where it's ok to not check the return value, consider not using
> > > > warn_unused_result on function declarations.
> > >
> > > Ok, so what do you do when you have a function like this where 99.9% of
> > > the users need to check this?  Do I really need to write a wrapper
> > > function just for it so that I can use it "safely" in the core code
> > > instead?
> > >
> > > Something like:
> > >
> > > void do_safe_thing_and_ignore_the_world(...)
> > > {
> > >         __unused int error;
> > >
> > >         error = do_thing(...);
> > > }
> > >
> > > Or something else to get the compiler to be quiet about error being set
> > > and never used?  There HAS to be that option somewhere anyway as we need
> > > it for other parts of the kernel where we do:
> > >         write_bus(device, &value);
> > >         value = read_bus(device);
> > > and then we ignore value as it is not needed, but yet we still HAVE to
> > > call read_bus() here, yet read_bus() is set as warn_unused_result()
> > > because, well, it is a read function :)
> >
> > Such wrappers are trivial with __attribute__((alias(""))):
> > https://godbolt.org/z/j5afPbGcM
> >
> > At least then it's very obvious if someone adds more call sites to
> > such an alias. Then that calls for closer inspection in code review
> > that yes, this is one of those 0.01% of cases.  Since they occur 0.01%
> > of the time, I don't expect such aliases to occur too frequently.
>
> That is just, well, horrible.  Seriously horrible.  Wow.

Yeah, well, that's how I feel about warn_unused_result_except_I_didn't_mean_it.

> And that is the "documented" way to do this?  That feels like an abuse
> of the already-horrible-why-do-they-do-that-for-variables use of the
> alias attribute.

You could also use #pragma's to disable the warning locally, with a
good comment about why it's ok to ignore the return code.

> How badly are compiler people going to complain to me about this if
> it's in this file?
> I can take a patch for that, but I feel the comments involved will make
> people, including myself when I have to look a the code again in 5
> years, even more confused...
>
> ick, I feel dirty...
>
> greg k-h

-- 
Thanks,
~Nick Desaulniers

Re: [PATCH v2 1/3] base: mark 'no_warn' as unused

[Nick Desaulniers]

On Tue, Jul 27, 2021 at 11:32 AM Nathan Chancellor <nathan@kernel.org> wrote:
>
> On 7/27/2021 10:39 AM, 'Nick Desaulniers' via Clang Built Linux wrote:
> > Isn't -Wunused-but-set-variable enabled only for W=1 builds?
>
> Maybe Bill's tree does not have commit 885480b08469 ("Makefile: Move
> -Wno-unused-but-set-variable out of GCC only block"), which disables the
> warning for clang just like GCC for regular builds?

Looks like 885480b08469, which landed in v5.13-rc1, so that's a
possibility.  Should that be sent to stable@ so that we don't observe
these warnings for non-W=1 builds of stable branches with newer
versions of clang?
-- 
Thanks,
~Nick Desaulniers

Re: [PATCH v2 1/3] base: mark 'no_warn' as unused

[Nathan Chancellor]

On 7/27/2021 12:04 PM, 'Nick Desaulniers' via Clang Built Linux wrote:
> On Tue, Jul 27, 2021 at 11:32 AM Nathan Chancellor <nathan@kernel.org> wrote:
>>
>> On 7/27/2021 10:39 AM, 'Nick Desaulniers' via Clang Built Linux wrote:
>>> Isn't -Wunused-but-set-variable enabled only for W=1 builds?
>>
>> Maybe Bill's tree does not have commit 885480b08469 ("Makefile: Move
>> -Wno-unused-but-set-variable out of GCC only block"), which disables the
>> warning for clang just like GCC for regular builds?
> 
> Looks like 885480b08469, which landed in v5.13-rc1, so that's a
> possibility.  Should that be sent to stable@ so that we don't observe
> these warnings for non-W=1 builds of stable branches with newer
> versions of clang?

It is already in all supported stable versions.

Cheers,
Nathan

Re: [PATCH v2 1/3] base: mark 'no_warn' as unused

[Bill Wendling]

On Tue, Jul 27, 2021 at 11:32 AM Nathan Chancellor <nathan@kernel.org> wrote:
>
> On 7/27/2021 10:39 AM, 'Nick Desaulniers' via Clang Built Linux wrote:
> > Isn't -Wunused-but-set-variable enabled only for W=1 builds?
>
> Maybe Bill's tree does not have commit 885480b08469 ("Makefile: Move
> -Wno-unused-but-set-variable out of GCC only block"), which disables the
> warning for clang just like GCC for regular builds?
>
Ah! I don't have that. It would technically make this patch
unnecessary. Use that information as you see fit.

-bw

Re: [PATCH v2 1/3] base: mark 'no_warn' as unused

[Bill Wendling]

On Tue, Jul 27, 2021 at 10:59 AM Greg Kroah-Hartman
<gregkh@linuxfoundation.org> wrote:
>
> On Tue, Jul 27, 2021 at 10:39:49AM -0700, Nick Desaulniers wrote:
> > > > > > Original post:
> > > > > >
> > > > > > https://lore.kernel.org/r/20210726201924.3202278-2-morbo@google.com/
> > > > > >
> > > > > > On 7/26/2021 1:19 PM, 'Bill Wendling' via Clang Built Linux wrote:
> > > > > > > Fix the following build warning:
> > > > > > >
> > > > > > >    drivers/base/module.c:36:6: error: variable 'no_warn' set but not used [-Werror,-Wunused-but-set-variable]
> > > > > > >          int no_warn;
> > > > >
> > > > > That's not going to be a good warning to ever have the kernel use due to
> > > > > how lots of hardware works (i.e. we need to do a read after a write but
> > > > > we can throw the read away as it does not matter).
> > > > >
> > > > >
> > > > > > >
> > > > > > > This variable is used to remove another warning, but causes a warning
> > > > > > > itself. Mark it as 'unused' to avoid that.
> > > > > > >
> > > > > > > Signed-off-by: Bill Wendling <morbo@google.com>
> > > > > >
> > > > > > Even though they evaluate to the same thing, it might be worth using
> > > > > > "__always_unused" here because it is :)
> > > > >
> > > > > But it is not unused, the value is written into it.
> > > > >
> > > > I believe that only matters if the variable is marked "volatile".
> > >
> > > "volatile" means nothing anymore, never use it or even think about it
> > > again please :)
> >
> > What Greg is getting at is that the use of the volatile keyword in
> > variable declarations is slightly frowned on by the kernel community.
> > It's less flexible than making accesses volatile qualified via casts.
> > Then you have flexibility for some accesses to be volatile (ie. not
> > CSE'd), and some not (ie. CSE'd), if needed.
> >
> > Though just because you assign to a variable doesn't mean that the
> > compiler generates an access, especially if the result is unused.
> > This warning is all about dead stores.  The cast to a volatile
> > qualified pointer then dereference is what guarantees the access.
> >
> > https://godbolt.org/z/7K7369bGG
> >
> > (To be explicit, IMO Greg's point about volatile stores is orthogonal
> > to discussions about dead stores.)
>
> I didn't bring up that dirty word, Bill did :)
>
I brought it up only as a potential reason for the compiler *not* to
emit the warning. We really shouldn't be spending this much time on
it...

> > > > Otherwise, the variable itself is never used. A "variable that's
> > > > written to but not read from," in fact, is the whole reason for the
> > > > warning.
> > >
> > > But that is ok!  Sometimes you need to do this with hardware (like all
> > > PCI devices).  This is a legitimate code flow for many hardware types
> > > and if a C compiler thinks that this is not ok, then it is broken.
> > >
> > > So be VERY careful when changing drivers based on this warning.  Because
> > > of this, I do not think you can enable it over the whole kernel without
> > > causing major problems in some areas.
> > >
> > > But that is independent of this specific issue you are trying to patch
> > > here, I say this to warn you of a number of stupid places where people
> > > have tried to "optimize away" reads based on this compiler warning in
> > > drivers, and we have had to add them back because it broke
> > > functionality.
> > >
> > > > > So this isn't ok, sometimes we want to write to variables but never care
> > > > > about the value, that does not mean the compiler should complain about
> > > > > it.
> > > > >
> > > > Typically, if you don't care about the return value, you simply don't
> > > > assign it to a variable (cf. printf). However, the functions that
> > > > assign to "no_warn" have the "warn_unused_result" attribute. The fact
> > > > that the variable is named "no_warn" seems to indicate that it's meant
> > > > to remain unused, even if it probably should be checked.
> > >
> > > These functions have warn_unused_result set on them because for 99% of
> > > the time, I want the value to be checked.  But as you can see in this
> > > use, as per the comments in the code, we do not care about the result
> > > for a very good reason.  So we just assign it to a variable to make the
> > > compiler quiet.
> >
> > I think warn_unused_result should only really be used for functions
> > where the return value should be used 100% of the time.
>
> I too want a shiny new pony.
>
You do? Ponies cost a lot of money and need ranches to live on and
constant care...a lot of work. Cats are better.

> But here in the real world, sometimes you have functions that for 99% of
> the users, you do want them to check the return value, but when you use
> them in core code or startup code, you "know" you are safe to ignore the
> return value.
>
> That is the case here.  We have other fun examples of where people have
> tried to add error handling to code that runs at boot that have actually
> introduced security errors and they justify it with "but you have to
> check error values!"
>
That's fine, and I fully support this. But when you mark a function
whose return value is 99.999999% checked except for the
I-definitely-know-what-I'm-doing-you-stupid-compiler times, then
you're going to get a warning from the compiler, because you've *told*
the compiler that the return value needs to be checked, but the code
doesn't check it. Compilers aren't mind readers.

The option then is to tell the compiler that "Yes, I know what I'm
doing, stop telling me otherwise" or disable the warning. As Nathan
pointed out, the warning was disabled in an April commit I guess.

> > If there are
> > cases where it's ok to not check the return value, consider not using
> > warn_unused_result on function declarations.
>
> Ok, so what do you do when you have a function like this where 99.9% of
> the users need to check this?  Do I really need to write a wrapper
> function just for it so that I can use it "safely" in the core code
> instead?
>
> Something like:
>
> void do_safe_thing_and_ignore_the_world(...)
> {
>         __unused int error;
>
>         error = do_thing(...);
> }
>
> Or something else to get the compiler to be quiet about error being set
> and never used?  There HAS to be that option somewhere anyway as we need
> it for other parts of the kernel where we do:
>         write_bus(device, &value);
>         value = read_bus(device);
> and then we ignore value as it is not needed, but yet we still HAVE to
> call read_bus() here, yet read_bus() is set as warn_unused_result()
> because, well, it is a read function :)
>
We have a perfectly fine way of doing this, by marking the variable as
"__maybe_unused". There's no need to come up with a convoluted
workaround. Since we don't want to check the return value in roughly
0.1% of the use cases, adding the __maybe_unused attribute isn't a
major headache. And it will prompt someone to really check whether
it's the "right thing" to do or not, which is what warnings are meant
for...

> > That said, we have a very similar issue throughout LLVM that Bill
> > should recognize. In LLVM, we have pretty aggressive usage of
> > assertions.  Rather than:
> >
> > assert(someReallyLongExpression && "error message");
> >
> > where that statement might wrap across multiple lines, instead it
> > might be clearer to write:
> >
> > bool IsOk = someReallyLongExpression;
> > assert(IsOk && "error message");
> >
> > which looks nicer but now produces -Wunused-but-set-variable on IsOk
> > for release builds where assertions are disabled.  The common fix in
> > LLVM is to write:
> >
> > bool IsOk = someReallyLongExpression;
> > assert(IsOk && "error message");
> > (void)IsOk;
> >
> > The cast to void is technically a use that doesn't result in a dead
> > store.  That pattern could be used in the kernel rather than
> >
> > int no_warn;
> > no_warn = warn_unused_result_fn();
> >
> > at least to avoid -Wunused-but-set-variable.  Oh, looks like a curious
> > difference between compilers:
> > https://godbolt.org/z/GvznMM6o1
> > Filed https://bugs.llvm.org/show_bug.cgi?id=51228.  So I guess we
> > can't use the cast-to-void to avoid -Wunused-but-set-variable, since
> > that triggers -Wunused-result, at least with GCC.  :( Nevermind...
> >
> > Though I still think the use of warn_unused_result on
> > sysfs_create_link() is worth revisiting.
>
> Nope, not at all, I WANT users to check this as it is something that has
> caused problems in drivers and subsystems in the past.
>
> And doing the (void)sysfs_create_link(); hack is horrid, I thought we
> were better than that.
>
> Surely there is a "this variable is going to be assigned something but
> never used" option somewhere?  This can't be the first time it has come
> up, right?
>
> > > > Would you rather the warning be turned off on some level?
> > >
> > > Which warning?
> > >
> > > The code here, as-is, is correct.  We already have 1 compiler warning
> > > work around in place, do you want to add another one?  How many can we
> > > stack on top of each other?
> >
> > Isn't -Wunused-but-set-variable enabled only for W=1 builds?
>
> No idea, as long as it is not a normal build option, that's fine.  "W=1"
> is for kernel newbies wanting to clean up subsystems and get some patch
> counts merged :)
>
> thanks,
>
> greg k-h

Re: [PATCH v2 1/3] base: mark 'no_warn' as unused

[Segher Boessenkool]

On Tue, Jul 27, 2021 at 07:59:24PM +0200, Greg Kroah-Hartman wrote:
> On Tue, Jul 27, 2021 at 10:39:49AM -0700, Nick Desaulniers wrote:
> > I think warn_unused_result should only really be used for functions
> > where the return value should be used 100% of the time.
> 
> I too want a shiny new pony.
> 
> But here in the real world, sometimes you have functions that for 99% of
> the users, you do want them to check the return value, but when you use
> them in core code or startup code, you "know" you are safe to ignore the
> return value.
> 
> That is the case here.  We have other fun examples of where people have
> tried to add error handling to code that runs at boot that have actually
> introduced security errors and they justify it with "but you have to
> check error values!"
> 
> > If there are
> > cases where it's ok to not check the return value, consider not using
> > warn_unused_result on function declarations.
> 
> Ok, so what do you do when you have a function like this where 99.9% of
> the users need to check this?  Do I really need to write a wrapper
> function just for it so that I can use it "safely" in the core code
> instead?
> 
> Something like:
> 
> void do_safe_thing_and_ignore_the_world(...)
> {
> 	__unused int error;
> 
> 	error = do_thing(...);
> }
> 
> Or something else to get the compiler to be quiet about error being set
> and never used?

The simplest is to write
	if (do_thing()) {
		/* Nothing here, we can safely ignore the return value
		 * here, because of X and Y and I don't know, I have no
		 * idea actually why we can in this example.  Hopefully
		 * in real code people do have a good reason :-)
		 */
	}

which should work in *any* compiler, doesn't need any extension, is
quite elegant, and encourages documenting why we ignore the return
value here.


Segher

Re: [PATCH v2 1/3] base: mark 'no_warn' as unused

[Bill Wendling]

On Tue, Jul 27, 2021 at 1:17 PM Segher Boessenkool
<segher@kernel.crashing.org> wrote:
>
> On Tue, Jul 27, 2021 at 07:59:24PM +0200, Greg Kroah-Hartman wrote:
> > On Tue, Jul 27, 2021 at 10:39:49AM -0700, Nick Desaulniers wrote:
> > > I think warn_unused_result should only really be used for functions
> > > where the return value should be used 100% of the time.
> >
> > I too want a shiny new pony.
> >
> > But here in the real world, sometimes you have functions that for 99% of
> > the users, you do want them to check the return value, but when you use
> > them in core code or startup code, you "know" you are safe to ignore the
> > return value.
> >
> > That is the case here.  We have other fun examples of where people have
> > tried to add error handling to code that runs at boot that have actually
> > introduced security errors and they justify it with "but you have to
> > check error values!"
> >
> > > If there are
> > > cases where it's ok to not check the return value, consider not using
> > > warn_unused_result on function declarations.
> >
> > Ok, so what do you do when you have a function like this where 99.9% of
> > the users need to check this?  Do I really need to write a wrapper
> > function just for it so that I can use it "safely" in the core code
> > instead?
> >
> > Something like:
> >
> > void do_safe_thing_and_ignore_the_world(...)
> > {
> >       __unused int error;
> >
> >       error = do_thing(...);
> > }
> >
> > Or something else to get the compiler to be quiet about error being set
> > and never used?
>
> The simplest is to write
>         if (do_thing()) {
>                 /* Nothing here, we can safely ignore the return value
>                  * here, because of X and Y and I don't know, I have no
>                  * idea actually why we can in this example.  Hopefully
>                  * in real code people do have a good reason :-)
>                  */
>         }
>
> which should work in *any* compiler, doesn't need any extension, is
> quite elegant, and encourages documenting why we ignore the return
> value here.
>
Or better still, use sysfs_create_link_nowarn() instead of
sysfs_create_link(). We'll just have to take the "__must_check"
attribute off the sysfs_create_link_nowarn() declaration.

-bw

Re: [PATCH v2 1/3] base: mark 'no_warn' as unused

[Bill Wendling]

On Tue, Jul 27, 2021 at 1:22 PM Bill Wendling <morbo@google.com> wrote:
>
> On Tue, Jul 27, 2021 at 1:17 PM Segher Boessenkool
> <segher@kernel.crashing.org> wrote:
> >
> > On Tue, Jul 27, 2021 at 07:59:24PM +0200, Greg Kroah-Hartman wrote:
> > > On Tue, Jul 27, 2021 at 10:39:49AM -0700, Nick Desaulniers wrote:
> > > > I think warn_unused_result should only really be used for functions
> > > > where the return value should be used 100% of the time.
> > >
> > > I too want a shiny new pony.
> > >
> > > But here in the real world, sometimes you have functions that for 99% of
> > > the users, you do want them to check the return value, but when you use
> > > them in core code or startup code, you "know" you are safe to ignore the
> > > return value.
> > >
> > > That is the case here.  We have other fun examples of where people have
> > > tried to add error handling to code that runs at boot that have actually
> > > introduced security errors and they justify it with "but you have to
> > > check error values!"
> > >
> > > > If there are
> > > > cases where it's ok to not check the return value, consider not using
> > > > warn_unused_result on function declarations.
> > >
> > > Ok, so what do you do when you have a function like this where 99.9% of
> > > the users need to check this?  Do I really need to write a wrapper
> > > function just for it so that I can use it "safely" in the core code
> > > instead?
> > >
> > > Something like:
> > >
> > > void do_safe_thing_and_ignore_the_world(...)
> > > {
> > >       __unused int error;
> > >
> > >       error = do_thing(...);
> > > }
> > >
> > > Or something else to get the compiler to be quiet about error being set
> > > and never used?
> >
> > The simplest is to write
> >         if (do_thing()) {
> >                 /* Nothing here, we can safely ignore the return value
> >                  * here, because of X and Y and I don't know, I have no
> >                  * idea actually why we can in this example.  Hopefully
> >                  * in real code people do have a good reason :-)
> >                  */
> >         }
> >
> > which should work in *any* compiler, doesn't need any extension, is
> > quite elegant, and encourages documenting why we ignore the return
> > value here.
> >
> Or better still, use sysfs_create_link_nowarn() instead of
> sysfs_create_link(). We'll just have to take the "__must_check"
> attribute off the sysfs_create_link_nowarn() declaration.
>
Strike that. I mistook the nowarn.
-bw