linux-kernel - Re: [PATCH v2 RESEND] fs: cramfs_fs.h: Fix shifting signed 32-bit value by 31 bits problem

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Date:   Fri, 21 Jun 2019 11:40:38 -0400 (EDT)
From:   Nicolas Pitre <nico@...xnic.net>
To:     Puranjay Mohan <puranjay12@...il.com>
cc:     Shuah Khan <skhan@...uxfoundation.org>,
        linux-kernel-mentees@...ts.linuxfoundation.org,
        linux-kernel@...r.kernel.org, hch@...radead.org,
        Christoph Hellwig <hch@....de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [PATCH v2 RESEND] fs: cramfs_fs.h: Fix shifting signed 32-bit
 value by 31 bits problem

On Tue, 18 Jun 2019, Puranjay Mohan wrote:

> Fix CRAMFS_BLK_FLAG_UNCOMPRESSED to use "U" cast to avoid shifting signed
> 32-bit value by 31 bits problem. This isn't a problem for kernel builds
> with gcc.
> 
> This could be problem since this header is part of public API which
> could be included for builds using compilers that don't handle this
> condition safely resulting in undefined behavior.
> 
> Signed-off-by: Puranjay Mohan <puranjay12@...il.com>
> Reviewed-by: Christoph Hellwig <hch@....de>
> Reviewed-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> Reviewed-by: Shuah Khan <skhan@...uxfoundation.org>

Acked-by: Nicolas Pitre <nico@...xnic.net>



> ---
> V2 - use the unsigned constants for all flags, not just one
> RESEND - Added Nicolas Pitre to CC list, added reviewed by tags.
> 
>  include/uapi/linux/cramfs_fs.h | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/include/uapi/linux/cramfs_fs.h b/include/uapi/linux/cramfs_fs.h
> index 6713669aa2ed..71cb602d4198 100644
> --- a/include/uapi/linux/cramfs_fs.h
> +++ b/include/uapi/linux/cramfs_fs.h
> @@ -98,8 +98,8 @@ struct cramfs_super {
>   *
>   * That leaves room for 3 flag bits in the block pointer table.
>   */
> -#define CRAMFS_BLK_FLAG_UNCOMPRESSED	(1 << 31)
> -#define CRAMFS_BLK_FLAG_DIRECT_PTR	(1 << 30)
> +#define CRAMFS_BLK_FLAG_UNCOMPRESSED	(1U << 31)
> +#define CRAMFS_BLK_FLAG_DIRECT_PTR	(1U << 30)
>  
>  #define CRAMFS_BLK_FLAGS	( CRAMFS_BLK_FLAG_UNCOMPRESSED \
>  				| CRAMFS_BLK_FLAG_DIRECT_PTR )
> -- 
> 2.21.0
> 
>