linux-kernel - Re: [tip:x86/mm 5/16] sound/core/hwdep.c:243:24: sparse: sparse: incorrect type in assignment (different address spaces)

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Date:   Mon, 14 Nov 2022 14:55:46 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     kernel test robot <lkp@...el.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Cc:     oe-kbuild-all@...ts.linux.dev, linux-kernel@...r.kernel.org,
        x86@...nel.org, Dave Hansen <dave.hansen@...ux.intel.com>
Subject: Re: [tip:x86/mm 5/16] sound/core/hwdep.c:243:24: sparse: sparse:
 incorrect type in assignment (different address spaces)

On 11/14/22 13:41, kernel test robot wrote:
>    sound/core/hwdep.c:243:24: sparse:     expected int [noderef] __user *__ptr_clean
>    sound/core/hwdep.c:243:24: sparse:     got int *
>    sound/core/hwdep.c:273:29: sparse: sparse: incorrect type in assignment (different address spaces) @@     expected int [noderef] __user *__ptr_clean @@     got int * @@
>    sound/core/hwdep.c:273:29: sparse:     expected int [noderef] __user *__ptr_clean
>    sound/core/hwdep.c:273:29: sparse:     got int *
>    sound/core/hwdep.c:292:29: sparse: sparse: incorrect type in assignment (different address spaces) @@     expected int [noderef] __user *__ptr_clean @@     got int * @@
>    sound/core/hwdep.c:292:29: sparse:     expected int [noderef] __user *__ptr_clean
>    sound/core/hwdep.c:292:29: sparse:     got int *

I think the sparse ends up throwing away all of its annotations once it
dereferences a pointer.  So, '*(int __user *)' boils down to a plain
'int'.  Confusingly, a '*(int __user *) *' boils down to an 'int *'.

That's what happened here.  A __user-annotated point got dereferenced
down to an 'int' and then turned into a pointer again.

I think the trick in this case is to avoid dereferencing the pointer too
early by just moving the dereference outside of the casting, like the
attached patch.  But, it also feels kinda wrong.  I'd love a second
opinion on this one.
View attachment "lam-sparse.patch" of type "text/x-patch" (528 bytes)