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

[Christophe Leroy]

Le 19/04/2020 Ã 10:06, Christoph Hellwig a ÃcritÂ:
> On Sat, Apr 18, 2020 at 10:15:42AM +0200, Christophe Leroy wrote:
> > Le 14/04/2020 Ã 09:01, Christoph Hellwig a ÃcritÂ:
> > > 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.