linux-kernel - Re: [PATCH v2 0/2] kdump: simplify code

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Date:   Sat, 11 Dec 2021 19:32:16 +0000
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     David Laight <David.Laight@...LAB.COM>,
        'Tiezhu Yang' <yangtiezhu@...ngson.cn>,
        Dave Young <dyoung@...hat.com>, Baoquan He <bhe@...hat.com>,
        Vivek Goyal <vgoyal@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>
CC:     "linux-ia64@...r.kernel.org" <linux-ia64@...r.kernel.org>,
        "linux-sh@...r.kernel.org" <linux-sh@...r.kernel.org>,
        Xuefeng Li <lixuefeng@...ngson.cn>,
        "x86@...nel.org" <x86@...nel.org>,
        "kexec@...ts.infradead.org" <kexec@...ts.infradead.org>,
        "linux-mips@...r.kernel.org" <linux-mips@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "linux-riscv@...ts.infradead.org" <linux-riscv@...ts.infradead.org>,
        "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH v2 0/2] kdump: simplify code



Le 11/12/2021 à 18:53, David Laight a écrit :
> From: Tiezhu Yang
>> Sent: 11 December 2021 03:33
>>
>> v2:
>>    -- add copy_to_user_or_kernel() in lib/usercopy.c
>>    -- define userbuf as bool type
> 
> Instead of having a flag to indicate whether the buffer is user or kernel,
> would it be better to have two separate buffer pointers.
> One for a user space buffer, the other for a kernel space buffer.
> Exactly one of the buffers should always be NULL.
> 
> That way the flag is never incorrectly set.
> 

It's a very good idea.

I was worried about the casts forcing the __user property away and back. 
With that approach we will preserve the __user tags on user buffers and 
enable sparse checking.

The only little drawback I see is that apparently GCC doesn't consider 
the NULL value as a constant and therefore doesn't perform constant 
folding on pointers. Not sure if this is a problem here.

Christophe