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Subject: Re: [PATCH 8/8] exec: open code copy_string_kernel
To:     Christoph Hellwig <hch@lst.de>
Cc:     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>
References: <20200414070142.288696-1-hch@lst.de>
 <20200414070142.288696-9-hch@lst.de>
 <ffea91ee-f386-9d19-0bc9-ab59eb7b9a41@c-s.fr> <20200419080646.GE12222@lst.de>
From:   Christophe Leroy <christophe.leroy@c-s.fr>
Message-ID: <5f96e86a-b084-4330-b7d1-08b78416994c@c-s.fr>
Date:   Sun, 19 Apr 2020 11:44:06 +0200
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Le 19/04/2020 à 10:06, Christoph Hellwig a écrit :
> 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.
> 

About the branch, I was expecting GCC to inline and eliminate the unused 
branch.