linux-kernel - Re: [PATCH] x86: Fixes warning: cast removes address space '__user' of expression in uaccess

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Message-ID: <3ff3e05a-8377-4b38-84ae-be24b6cae6a4@intel.com>
Date:   Fri, 17 Nov 2023 07:19:12 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     Dipendra Khadka <kdipendra88@...il.com>, tglx@...utronix.de,
        mingo@...hat.com, bp@...en8.de, dave.hansen@...ux.intel.com,
        x86@...nel.org, hpa@...or.com
Cc:     mjguzik@...il.com, ira.weiny@...el.com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86: Fixes warning: cast removes address space '__user'
 of expression in uaccess_64.h

On 11/16/23 09:38, Dipendra Khadka wrote:
> Sparse has identified a warning as follows:
> 
> ./arch/x86/include/asm/uaccess_64.h:88:24: warning: cast removes address space '__user' of expression.
> 
> Since the valid_user_address(x) macro implicitly casts the argument
> to long and compares the converted value of x to zero, casting ptr
> to unsigned long has no functional impact and does not trigger a 
> Sparse warning either.

Why does sparse complain about a cast to 'long' but not 'unsigned long'?
 Both remove the '__user' address space from the expression.  Were there
just so many __user pointers being cast to 'unsigned long' that there's
an exception in sparse for 'void __user *' => 'unsigned long'?

Either way, if we're going to fix it it seems like it would be better to
valid_user_address() actually handle, well, __user addresses rather than
expecting callers to do it.