linux-kernel - Re: [PATCH] selftests: kselftest.h: mark functions with 'noreturn'

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Message-ID: <CAKwvOdk8D5=AxzSpqjvXJc4XXL8CA7O=WY-LW0mZb3eQRK_EWg@mail.gmail.com>
Date:   Fri, 29 Oct 2021 15:08:48 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Shuah Khan <skhan@...uxfoundation.org>,
        Anders Roxell <anders.roxell@...aro.org>
Cc:     shuah@...nel.org, fenghua.yu@...el.com, reinette.chatre@...el.com,
        john.stultz@...aro.org, tglx@...utronix.de,
        akpm@...ux-foundation.org, nathan@...nel.org,
        linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org,
        linux-mm@...ck.org, llvm@...ts.linux.dev
Subject: Re: [PATCH] selftests: kselftest.h: mark functions with 'noreturn'

On Fri, Oct 29, 2021 at 11:19 AM Shuah Khan <skhan@...uxfoundation.org> wrote:
>
> On 10/29/21 5:43 AM, Anders Roxell wrote:
> > When building kselftests/capabilities the following warning shows up:
> >
> > clang -O2 -g -std=gnu99 -Wall    test_execve.c -lcap-ng -lrt -ldl -o test_execve
> > test_execve.c:121:13: warning: variable 'have_outer_privilege' is used uninitialized whenever 'if' condition is false [-Wsometimes-uninitialized]
> >          } else if (unshare(CLONE_NEWUSER | CLONE_NEWNS) == 0) {
> >                     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> > test_execve.c:136:9: note: uninitialized use occurs here
> >          return have_outer_privilege;
> >                 ^~~~~~~~~~~~~~~~~~~~
> > test_execve.c:121:9: note: remove the 'if' if its condition is always true
> >          } else if (unshare(CLONE_NEWUSER | CLONE_NEWNS) == 0) {
> >                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> > test_execve.c:94:27: note: initialize the variable 'have_outer_privilege' to silence this warning
> >          bool have_outer_privilege;
> >                                   ^
> >                                    = false
> >
> > Rework so all the ksft_exit_*() functions have attribue
> > '__attribute__((noreturn))' so the compiler knows that there wont be
> > any return from the function. That said, without
> > '__attribute__((noreturn))' the compiler warns about the above issue
> > since it thinks that it will get back from the ksft_exit_skip()
> > function, which it wont.
> > Cleaning up the callers that rely on ksft_exit_*() return code, since
> > the functions ksft_exit_*() have never returned anything.
> >
> > Signed-off-by: Anders Roxell <anders.roxell@...aro.org>
>
> Lot of changes to fix this warning. Is this necessary? I would
> like to explore if there is an easier and localized change that
> can fix the problem.

via `man 3 exit`:
```
The  exit() function causes normal process termination ...
...
RETURN VALUE
       The exit() function does not return.
```
so seeing `ksft_exit_pass`, `ksft_exit_fail`, `ksft_exit_fail_msg`,
`ksft_exit_xfail`, `ksft_exit_xpass`, and `ksft_exit_skip` all
unconditional call `exit` yet return an `int` looks wrong to me on
first glance. So on that point this patch and its resulting diffstat
LGTM.

That said, there are many changes that explicitly call `ksft_exit`
with an expression; are those setting the correct exit code? Note that
ksft_exit_pass is calling exit with KSFT_PASS which is 0.  So some of
the negations don't look quite correct to me.  For example:

-       return !ksft_get_fail_cnt() ? ksft_exit_pass() : ksft_exit_fail();
+       ksft_exit(!ksft_get_fail_cnt());

so if ksft_get_fail_cnt() returns 0, then we were calling
ksft_exit_pass() which exited with 0. Now we'd be exiting with 1?
-- 
Thanks,
~Nick Desaulniers