linux-kernel - Re: [PATCH] kselftest: Mark functions that unconditionally call exit() as _

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Message-ID: <CAGG=3QXzs0VdaQjNtGD6aKEPXCy1dGQ06BSKww_HutB-x7BuMQ@mail.gmail.com>
Date: Fri, 12 Apr 2024 09:11:46 -0700
From: Bill Wendling <morbo@...gle.com>
To: Nathan Chancellor <nathan@...nel.org>
Cc: tglx@...utronix.de, shuah@...nel.org, oleg@...hat.com, 
	anna-maria@...utronix.de, frederic@...nel.org, ndesaulniers@...gle.com, 
	justinstitt@...gle.com, linux-kselftest@...r.kernel.org, 
	linux-kernel@...r.kernel.org, llvm@...ts.linux.dev, patches@...ts.linux.dev, 
	John Stultz <jstultz@...gle.com>
Subject: Re: [PATCH] kselftest: Mark functions that unconditionally call
 exit() as __noreturn

On Fri, Apr 12, 2024 at 9:08 AM Bill Wendling <morbo@...gle.com> wrote:
>
> On Thu, Apr 11, 2024 at 11:45 AM Nathan Chancellor <nathan@...nelorg> wrote:
> >
> > After commit 6d029c25b71f ("selftests/timers/posix_timers: Reimplement
> > check_timer_distribution()"), clang warns:
> >
> >   tools/testing/selftests/timers/../kselftest.h:398:6: warning: variable 'major' is used uninitialized whenever '||' condition is true [-Wsometimes-uninitialized]
> >     398 |         if (uname(&info) || sscanf(info.release, "%u.%u.", &major, &minor) != 2)
> >         |             ^~~~~~~~~~~~
> >   tools/testing/selftests/timers/../kselftest.h:401:9: note: uninitialized use occurs here
> >     401 |         return major > min_major || (major == min_major && minor >= min_minor);
> >         |                ^~~~~
> >   tools/testing/selftests/timers/../kselftest.h:398:6: note: remove the '||' if its condition is always false
> >     398 |         if (uname(&info) || sscanf(info.release, "%u.%u.", &major, &minor) != 2)
> >         |             ^~~~~~~~~~~~~~~
> >   tools/testing/selftests/timers/../kselftest.h:395:20: note: initialize the variable 'major' to silence this warning
> >     395 |         unsigned int major, minor;
> >         |                           ^
> >         |                            = 0
> >
> > This is a false positive because if uname() fails, ksft_exit_fail_msg()
> > will be called, which unconditionally calls exit(), a noreturn function.
> > However, clang does not know that ksft_exit_fail_msg() will call exit()
> > at the point in the pipeline that the warning is emitted because
> > inlining has not occurred, so it assumes control flow will resume
> > normally after ksft_exit_fail_msg() is called.
> >
> > Make it clear to clang that all of the functions that call exit()
> > unconditionally in kselftest.h are noreturn transitively by marking them
> > explicitly with '__attribute__((__noreturn__))', which clears up the
> > warning above and any future warnings that may appear for the same
> > reason.
> >
> > Fixes: 6d029c25b71f ("selftests/timers/posix_timers: Reimplement check_timer_distribution()")
> > Reported-by: John Stultz <jstultz@...gle.com>
> > Closes: https://lore.kernel.org/all/20240410232637.4135564-2-jstultz@google.com/
> > Signed-off-by: Nathan Chancellor <nathan@...nel.org>
> > ---
> > I have based this change on timers/urgent, as the commit that introduces
> > this particular warning is there and it is marked for stable, even
> > though this appears to be a generic kselftest issue. I think it makes
> > the most sense for this change to go via timers/urgent with Shuah's ack.
> > While __noreturn with a return type other than 'void' does not make much
> > sense semantically, there are many places that these functions are used
> > as the return value for other functions such as main(), so I did not
> > change the return type of these functions from 'int' to 'void' to
> > minimize the necessary changes for a backport (it is an existing issue
> > anyways).
> >
> > I see there is another instance of this problem that will need to be
> > addressed in -next, introduced by commit f07041728422 ("selftests: add
> > ksft_exit_fail_perror()").
> > ---
> >  tools/testing/selftests/kselftest.h | 15 +++++++++------
> >  1 file changed, 9 insertions(+), 6 deletions(-)
> >
> > diff --git a/tools/testing/selftests/kselftest.h b/tools/testing/selftests/kselftest.h
> > index 973b18e156b2..0591974b57e0 100644
> > --- a/tools/testing/selftests/kselftest.h
> > +++ b/tools/testing/selftests/kselftest.h
> > @@ -80,6 +80,9 @@
> >  #define KSFT_XPASS 3
> >  #define KSFT_SKIP  4
> >
> > +#ifndef __noreturn
> > +#define __noreturn       __attribute__((__noreturn__))
> > +#endif
> >  #define __printf(a, b)   __attribute__((format(printf, a, b)))
> >
> >  /* counters */
> > @@ -300,13 +303,13 @@ void ksft_test_result_code(int exit_code, const char *test_name,
> >         va_end(args);
> >  }
> >
> > -static inline int ksft_exit_pass(void)
> > +static inline __noreturn int ksft_exit_pass(void)
>
> Orthogonal to this fix, but why does a "no return" function have a
> non-void return type?
>
Oops...That's been asked. Sorry for the extra message.

-bw