linux-kernel - Re: [PATCH] fs: Clear a UBSAN shift-out-of-bounds warning

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Date:   Sat, 19 Nov 2022 11:39:43 -0800
From:   Eric Biggers <ebiggers@...nel.org>
To:     Zhen Lei <thunder.leizhen@...wei.com>
Cc:     Alexander Viro <viro@...iv.linux.org.uk>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] fs: Clear a UBSAN shift-out-of-bounds warning

On Thu, Nov 10, 2022 at 11:10:24AM +0800, Zhen Lei wrote:
> UBSAN: shift-out-of-bounds in fs/locks.c:2572:16
> left shift of 1 by 63 places cannot be represented in type 'long long int'
> 
> Switch the calculation method to ((quarter - 1) * 2 + 1) can help us
> eliminate this false positive.
> 
> On the other hand, the old implementation has problems with char and
> short types, although not currently involved.
> printf("%d: %x\n", sizeof(char),  INT_LIMIT(char));
> printf("%d: %x\n", sizeof(short), INT_LIMIT(short));
> 1: ffffff7f
> 2: ffff7fff
> 
> Signed-off-by: Zhen Lei <thunder.leizhen@...wei.com>
> ---
>  include/linux/fs.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index e654435f16512c1..88d42e2daed9f6c 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -1131,7 +1131,7 @@ struct file_lock_context {
>  
>  /* The following constant reflects the upper bound of the file/locking space */
>  #ifndef OFFSET_MAX
> -#define INT_LIMIT(x)	(~((x)1 << (sizeof(x)*8 - 1)))
> +#define INT_LIMIT(x)	((((x)1 << (sizeof(x) * 8 - 2)) - 1) * 2  + 1)
>  #define OFFSET_MAX	INT_LIMIT(loff_t)
>  #define OFFT_OFFSET_MAX	INT_LIMIT(off_t)
>  #endif

This problem has already been solved by type_max() in include/linux/overflow.h.
How about removing INT_LIMIT() and using type_max() instead?

- Eric