Re: [PATCH 8/8] exec: open code copy_string_kernel

[Christoph Hellwig <hch@lst.de>]

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.