Re: [tip:x86/mm 1/23] arch/x86/mm/init.c:75:6: warning: no previous prototype for function 'x86_has_pat_wp'

[Philip Li]

On Tue, May 19, 2020 at 05:26:18PM -0700, Nick Desaulniers wrote:
> On Tue, May 19, 2020 at 4:49 PM Li, Philip <philip.li@xxxxxxxxx> wrote:
> >
> > > Subject: Re: [tip:x86/mm 1/23] arch/x86/mm/init.c:75:6: warning: no previous
> > > prototype for function 'x86_has_pat_wp'
> > >
> > > Masahiro and Philip,
> > > We're seeing a bunch of -Wmissing-prototypes warnings from 0day bot
> > > suddenly today.  Did something change on the Kbuild or 0day bot side?
> > Hi Nick, W=1 was added to make. This leads to extra warnings.
> 
> Ah, ok.  Good to know the change is in 0day bot which rules out
> changes to Kbuild and Clang.
> 
> It might be helpful to note in the email that this is a W=1 build,
> since it's non obvious and cannot be set via config.
thanks for reminding this, we will update the report info to reflect
this, e.g. part of reproduce step.

> 
> Generally, the kernel is not W=1 clean.  I realize that 0day bot can
got it, we have logic to limit that not to report too old error, so
the idea is to scan recent submitted patches.

> filter old vs new.  Why -Wmissing-prototypes is a problem is that:
> 1. there are already many instances of it throughout the kernel;
> references without forward declarations or correct includes.
> 2. it's easy for someone to call a function like code that already
> exists in the translation unit they're touching.
> 
> So by adding another implicit call, they get warned for the whole
> file.  Generally, the fix is trivial; just include the correct header.
> If that leads to tangly-sphagetti-limbo/circular includes, then raw
> forward declarations can be used (though it's easy for those to get
> out of sync, which can lead to spooky bugs).
> 
> I think having in the top of the warning that this is a W=1 build will
> make it more obvious.
> 
> I get that -Wmissing-prototypes can be noisy, but it's trivial to fix.
> I do worry what other warnings lurk in W=1 though...
with some monitoring, so far, issue like unused-but-set-variable is quite
helpful. We will keep monitor for other issues and feedbacks.

> 
> >
> > >
> > > On Tue, May 19, 2020 at 2:25 PM Nathan Chancellor
> > > <natechancellor@xxxxxxxxx> wrote:
> > > >
> > > > On Tue, May 19, 2020 at 10:55:05PM +0200, Borislav Petkov wrote:
> > > > > On Wed, May 20, 2020 at 01:51:25AM +0800, kbuild test robot wrote:
> > > > > > tree:   https://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git x86/mm
> > > > > > head:   bd1de2a7aace4d1d312fb1be264b8fafdb706208
> > > > > > commit: 1f6f655e01adebf5bd5e6c3da2e843c104ded051 [1/23] x86/mm: Add
> > > a x86_has_pat_wp() helper
> > > > > > config: x86_64-randconfig-r012-20200519 (attached as .config)
> > > > > > compiler: clang version 11.0.0 (https://github.com/llvm/llvm-project
> > > 135b877874fae96b4372c8a3fbfaa8ff44ff86e3)
> > > > > > reproduce:
> > > > > >         wget https://raw.githubusercontent.com/intel/lkp-
> > > tests/master/sbin/make.cross -O ~/bin/make.cross
> > > > > >         chmod +x ~/bin/make.cross
> > > > > >         # install x86_64 cross compiling tool for clang build
> > > > > >         # apt-get install binutils-x86-64-linux-gnu
> > > > > >         git checkout 1f6f655e01adebf5bd5e6c3da2e843c104ded051
> > > > > >         # save the attached .config to linux build tree
> > > > > >         COMPILER_INSTALL_PATH=$HOME/0day COMPILER=clang
> > > make.cross ARCH=x86_64
> > > > > >
> > > > > > If you fix the issue, kindly add following tag as appropriate
> > > > > > Reported-by: kbuild test robot <lkp@xxxxxxxxx>
> > > > > >
> > > > > > All warnings (new ones prefixed by >>, old ones prefixed by <<):
> > > > > >
> > > > > > >> arch/x86/mm/init.c:75:6: warning: no previous prototype for function
> > > 'x86_has_pat_wp' [-Wmissing-prototypes]
> > > > > > bool x86_has_pat_wp(void)
> > > > > > ^
> > > > >
> > > > > Triggers with gcc too:
> > > > >
> > > > > make W=1 arch/x86/mm/init.o
> > > > >
> > > > > ...
> > > > >
> > > > > arch/x86/mm/init.c:81:6: warning: no previous prototype for âx86_has_pat_wpâ
> > > [-Wmissing-prototypes]
> > > > >    81 | bool x86_has_pat_wp(void)
> > > > >       |      ^~~~~~~~~~~~~~
> > > > >
> > > > >
> > > > > -Wmissing-prototypes is default off, though, dunno why clang 11 has it
> > > > > on.
> > > >
> > > > It doesn't: https://godbolt.org/z/rU5_5H
> > > >
> > > > I assume this is caused by some sort of internal change to the 0day
> > > > infrastructure (testing W=1?)
> > > >
> > > > I cannot see this without adding W=1 to make at the listed commit with
> > > > the provided config.
> > > >
> > > > Cheers,
> > > > Nathan
> > > >
> > > > > Anyway, something for Benni to fix. CCed and leaving the rest for him as
> > > > > reference.
> > > > >
> > > > > > arch/x86/mm/init.c:75:1: note: declare 'static' if the function is not intended
> > > to be used outside of this translation unit
> > > > > > bool x86_has_pat_wp(void)
> > > > > > ^
> > > > > > static
> > > > > > arch/x86/mm/init.c:866:13: warning: no previous prototype for function
> > > 'mem_encrypt_free_decrypted_mem' [-Wmissing-prototypes]
> > > > > > void __weak mem_encrypt_free_decrypted_mem(void) { }
> > > > > > ^
> > > > > > arch/x86/mm/init.c:866:1: note: declare 'static' if the function is not intended
> > > to be used outside of this translation unit
> > > > > > void __weak mem_encrypt_free_decrypted_mem(void) { }
> > > > > > ^
> > > > > > static
> > > > > > 2 warnings generated.
> > > > > >
> > > > > > vim +/x86_has_pat_wp +75 arch/x86/mm/init.c
> > > > > >
> > > > > >     73
> > > > > >     74      /* Check that the write-protect PAT entry is set for write-protect */
> > > > > >   > 75      bool x86_has_pat_wp(void)
> > > > > >     76      {
> > > > > >     77              return __pte2cachemode_tbl[_PAGE_CACHE_MODE_WP] ==
> > > _PAGE_CACHE_MODE_WP;
> > > > > >     78      }
> > > > > >     79
> > > > > >
> > > > > > ---
> > > > > > 0-DAY CI Kernel Test Service, Intel Corporation
> > > > > > https://lists.01.org/hyperkitty/list/kbuild-all@xxxxxxxxxxxx
> > > > >
> > > > >
> > > > >
> > > > > --
> > > > > Regards/Gruss,
> > > > >     Boris.
> > > > >
> > > > > https://people.kernel.org/tglx/notes-about-netiquette
> > > > >
> > > >
> > > > --
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> > > linux/20200519212541.GA3580016%40ubuntu-s3-xlarge-x86.
> > >
> > >
> > >
> > > --
> > > Thanks,
> > > ~Nick Desaulniers
> 
> 
> 
> -- 
> Thanks,
> ~Nick Desaulniers