linux-kernel - Re: -Wswitch Clang warnings in drivers/scsi

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Date:   Thu, 4 Oct 2018 11:45:40 -0700
From:   Nathan Chancellor <natechancellor@...il.com>
To:     Bart Van Assche <bvanassche@....org>
Cc:     "James E.J. Bottomley" <jejb@...ux.vnet.ibm.com>,
        "Martin K. Petersen" <martin.petersen@...cle.com>,
        linux-scsi@...r.kernel.org, linux-kernel@...r.kernel.org,
        Nick Desaulniers <ndesaulniers@...gle.com>
Subject: Re: -Wswitch Clang warnings in drivers/scsi

On Thu, Oct 04, 2018 at 11:34:29AM -0700, Bart Van Assche wrote:
> On Thu, 2018-10-04 at 11:30 -0700, Nathan Chancellor wrote:
> > Hi SCSI folks,
> > 
> > In an effort to get the kernel building warning free with Clang, we've
> > come across an interesting occurrence in a few scsi drivers:
> > 
> > drivers/scsi/hpsa.c:6533:7: warning: overflow converting case value to switch condition type (2148024833 to 18446744071562609153) [-Wswitch]
> >         case CCISS_GETPCIINFO:
> >              ^
> > ./include/uapi/linux/cciss_ioctl.h:65:26: note: expanded from macro 'CCISS_GETPCIINFO'
> > #define CCISS_GETPCIINFO _IOR(CCISS_IOC_MAGIC, 1, cciss_pci_info_struct)
> >                          ^
> > ./include/uapi/asm-generic/ioctl.h:86:28: note: expanded from macro '_IOR'
> > #define _IOR(type,nr,size)      _IOC(_IOC_READ,(type),(nr),(_IOC_TYPECHECK(size)))
> >                                 ^
> > ./include/uapi/asm-generic/ioctl.h:70:2: note: expanded from macro '_IOC'
> >         (((dir)  << _IOC_DIRSHIFT) | \
> >         ^
> > 
> > I see this warning in drivers/scsi/hpsa.c and drivers/scsi/smartpqi/smartpqi_init.c
> > on an arm64 allyesconfig build and it has also been reported in a couple of files in
> > drivers/scsi/cxlflash.
> > 
> > As the warning states, there is an overflow because the switch statement's value is of
> > type int but the switch value is greater than INT_MAX. I did a brief sweep of the tree
> > and it seems that all uses of _IOC in switch statement values either are small enough
> > to fit into size int or the value is of size unsigned int.
> > 
> > I am unsure of the implications of using a smaller _IOC value or converting all ioctls
> > to expect a cmd of type unsigned int (especially since that has userspace implications)
> > but I didn't see any negative ioctl commands. Some clarity and insight would be
> > appreciated.
> 
> Have you verified how gcc compiles these switch statements? Maybe gcc supports
> switch / case statements on integral types that are larger than an int?
> 
> Bart.

Hi Bart,

That is entirely possible, I will do some research. I did build with GCC
to see if there was any warning and there isn't so I'll be curious to
see what is happening at a lower level.

Thanks for the comment!
Nathan