[PATCH 8/8] exec: open code copy_string_kernel

[Christophe Leroy]

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.