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

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Message-ID: <1538678069.230807.6.camel@acm.org>
Date:   Thu, 04 Oct 2018 11:34:29 -0700
From:   Bart Van Assche <bvanassche@....org>
To:     Nathan Chancellor <natechancellor@...il.com>,
        "James E.J. Bottomley" <jejb@...ux.vnet.ibm.com>,
        "Martin K. Petersen" <martin.petersen@...cle.com>
Cc:     linux-scsi@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: -Wswitch Clang warnings in drivers/scsi

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.