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Date:   Sun, 19 Apr 2020 10:06:46 +0200
From:   Christoph Hellwig <hch@lst.de>
To:     Christophe Leroy <christophe.leroy@c-s.fr>
Cc:     Christoph Hellwig <hch@lst.de>,
        Andrew Morton <akpm@linux-foundation.org>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Arnd Bergmann <arnd@arndb.de>, linux-kernel@vger.kernel.org,
        Jeremy Kerr <jk@ozlabs.org>, linux-fsdevel@vger.kernel.org,
        linuxppc-dev@lists.ozlabs.org,
        "Eric W . Biederman" <ebiederm@xmission.com>
Subject: Re: [PATCH 8/8] exec: open code copy_string_kernel
Message-ID: <20200419080646.GE12222@lst.de>
References: <20200414070142.288696-1-hch@lst.de> <20200414070142.288696-9-hch@lst.de> <ffea91ee-f386-9d19-0bc9-ab59eb7b9a41@c-s.fr>
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On Sat, Apr 18, 2020 at 10:15:42AM +0200, Christophe Leroy wrote:
>
>
> Le 14/04/2020 à 09:01, Christoph Hellwig a écrit :
>> Currently copy_string_kernel is just a wrapper around copy_strings that
>> simplifies the calling conventions and uses set_fs to allow passing a
>> kernel pointer.  But due to the fact the we only need to handle a single
>> kernel argument pointer, the logic can be sigificantly simplified while
>> getting rid of the set_fs.
>
>
> Instead of duplicating almost identical code, can you write a function that 
> takes whether the source is from user or from kernel, then you just do 
> things like:
>
> 	if (from_user)
> 		len = strnlen_user(str, MAX_ARG_STRLEN);
> 	else
> 		len = strnlen(str, MAX_ARG_STRLEN);
>
>
> 	if (from_user)
> 		copy_from_user(kaddr+offset, str, bytes_to_copy);
> 	else
> 		memcpy(kaddr+offset, str, bytes_to_copy);

We'll need two different str variables then with and without __user
annotations to keep type safety.  And introduce a branch-y and unreadable
mess in the exec fast path instead of adding a simple and well understood
function for the kernel case that just deals with the much simpler case
of just copying a single arg vector from a kernel address.