linux-kernel - Re: drivers/pci/pcie/../pci.h:325:17: sparse: sparse: cast from restricted pci_channel_state

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Message-ID: <Y/7YN7U9Q2iqNDFo@kadam>
Date:   Wed, 1 Mar 2023 07:44:39 +0300
From:   Dan Carpenter <error27@...il.com>
To:     Lukas Wunner <lukas@...ner.de>
Cc:     oe-kbuild-all@...ts.linux.dev, linux-kernel@...r.kernel.org,
        Bjorn Helgaas <helgaas@...nel.org>,
        kernel test robot <lkp@...el.com>
Subject: Re: drivers/pci/pcie/../pci.h:325:17: sparse: sparse: cast from
 restricted pci_channel_state_t

Hi Lukas,

I do smatch, not sparse.  Sparse maintainers are on
linux-sparse@...r.kernel.org.

On Tue, Feb 28, 2023 at 10:33:25PM +0100, Lukas Wunner wrote:
> Hi Dan,
> 
> 0-day is reporting sparse warnings introduced by commit 74ff8864cc84
> ("PCI: hotplug: Allow marking devices as disconnected during
> bind/unbind"), which landed in Linus' tree this week.
> 
> The warnings are caused by invocations of xchg() and cmpxchg()
> on an enum type ("cast from restricted pci_channel_state_t").

pci_channel_state_t is not an enum.  The problem is the __bit_wise.

> 
> It seems they are only reported for architectures whose arch_xchg()
> and arch_cmpxchg() macros cast the argument to an unsigned long.
> Archictures such as x86 don't do that, but a number of others do.
> The 0-day report, reproduced below in full, is for loongarch.
> 
> I'm wondering why the cast is necessary at all.  Digging in the
> git history, I noticed that it has existed at least on arm since
> forever.  I suspect that its use on newer arches such as loongarch
> may be due to cargo-culting.
> 

Speaking as an absolutely newbie and ignoramous, I can't see any point
to the cast in arch_xchg().  But I am also surprised that silences the
warning.  I would have thought that removing the cast would change the
warning from "warning: cast from restricted my_type_t" to
"warning: incorrect type in argument 1 (different base types)".

> Please advise whether these sparse warnings are false positives which
> can be ignored and if they aren't, how to resolve them.  If you happen
> to know the rationale for the cast, I'd be grateful if you could shed
> some light on it.  Thanks a lot!

The question is more why is pci_channel_state_t declared as __bit_wise.
__bit_wise data can only be used through accessor functions, like user
pointers have to go through copy_from_user() and endian data has to go
through le32_to_cpu() etc.

I don't know the answer to that though.

regards,
dan carpenter

> 
> 
> On Wed, Mar 01, 2023 at 04:39:24AM +0800, kernel test robot wrote:
> > tree:   https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git master
> > head:   e492250d5252635b6c97d52eddf2792ec26f1ec1
> > commit: 74ff8864cc842be994853095dba6db48e716400a PCI: hotplug: Allow marking devices as disconnected during bind/unbind
> > date:   13 days ago
> > config: loongarch-randconfig-s042-20230226 (https://download.01.org/0day-ci/archive/20230301/202303010454.jI5Jg2sT-lkp@intel.com/config)
> > compiler: loongarch64-linux-gcc (GCC) 12.1.0
> > reproduce:
> >         wget https://raw.githubusercontent.com/intel/lkp-tests/master/sbin/make.cross -O ~/bin/make.cross
> >         chmod +x ~/bin/make.cross
> >         # apt-get install sparse
> >         # sparse version: v0.6.4-39-gce1a6720-dirty
> >         # https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=74ff8864cc842be994853095dba6db48e716400a
> >         git remote add linus https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
> >         git fetch --no-tags linus master
> >         git checkout 74ff8864cc842be994853095dba6db48e716400a
> >         # save the config file
> >         mkdir build_dir && cp config build_dir/.config
> >         COMPILER_INSTALL_PATH=$HOME/0day COMPILER=gcc-12.1.0 make.cross C=1 CF='-fdiagnostic-prefix -D__CHECK_ENDIAN__' O=build_dir ARCH=loongarch olddefconfig
> >         COMPILER_INSTALL_PATH=$HOME/0day COMPILER=gcc-12.1.0 make.cross C=1 CF='-fdiagnostic-prefix -D__CHECK_ENDIAN__' O=build_dir ARCH=loongarch SHELL=/bin/bash drivers/
> > 
> > If you fix the issue, kindly add following tag where applicable
> > | Reported-by: kernel test robot <lkp@...el.com>
> > | Link: https://lore.kernel.org/oe-kbuild-all/202303010454.jI5Jg2sT-lkp@intel.com/
> > 
> > sparse warnings: (new ones prefixed by >>)
> >    drivers/pci/pcie/err.c: note: in included file:
> > >> drivers/pci/pcie/../pci.h:325:17: sparse: sparse: cast from restricted pci_channel_state_t
> > >> drivers/pci/pcie/../pci.h:325:17: sparse: sparse: cast to restricted pci_channel_state_t
> >    drivers/pci/pcie/../pci.h:328:23: sparse: sparse: cast from restricted pci_channel_state_t
> >    drivers/pci/pcie/../pci.h:328:23: sparse: sparse: cast from restricted pci_channel_state_t
> >    drivers/pci/pcie/../pci.h:328:23: sparse: sparse: cast to restricted pci_channel_state_t
> >    drivers/pci/pcie/../pci.h:332:23: sparse: sparse: cast from restricted pci_channel_state_t
> >    drivers/pci/pcie/../pci.h:332:23: sparse: sparse: cast from restricted pci_channel_state_t
> >    drivers/pci/pcie/../pci.h:332:23: sparse: sparse: cast to restricted pci_channel_state_t
> > >> drivers/pci/pcie/../pci.h:325:17: sparse: sparse: cast to restricted pci_channel_state_t
> >    drivers/pci/pcie/../pci.h:328:23: sparse: sparse: cast to restricted pci_channel_state_t
> >    drivers/pci/pcie/../pci.h:332:23: sparse: sparse: cast to restricted pci_channel_state_t
> > 
> > vim +325 drivers/pci/pcie/../pci.h
> > 
> >    306	
> >    307	/**
> >    308	 * pci_dev_set_io_state - Set the new error state if possible.
> >    309	 *
> >    310	 * @dev: PCI device to set new error_state
> >    311	 * @new: the state we want dev to be in
> >    312	 *
> >    313	 * If the device is experiencing perm_failure, it has to remain in that state.
> >    314	 * Any other transition is allowed.
> >    315	 *
> >    316	 * Returns true if state has been changed to the requested state.
> >    317	 */
> >    318	static inline bool pci_dev_set_io_state(struct pci_dev *dev,
> >    319						pci_channel_state_t new)
> >    320	{
> >    321		pci_channel_state_t old;
> >    322	
> >    323		switch (new) {
> >    324		case pci_channel_io_perm_failure:
> >  > 325			xchg(&dev->error_state, pci_channel_io_perm_failure);
> >    326			return true;
> >    327		case pci_channel_io_frozen:
> >    328			old = cmpxchg(&dev->error_state, pci_channel_io_normal,
> >    329				      pci_channel_io_frozen);
> >    330			return old != pci_channel_io_perm_failure;
> >    331		case pci_channel_io_normal:
> >    332			old = cmpxchg(&dev->error_state, pci_channel_io_frozen,
> >    333				      pci_channel_io_normal);
> >    334			return old != pci_channel_io_perm_failure;
> >    335		default:
> >    336			return false;
> >    337		}
> >    338	}
> >    339	
> > 
> > -- 
> > 0-DAY CI Kernel Test Service
> > https://github.com/intel/lkp-tests