[PATCH 2/7] exec: Factor out alloc_bprm

[Christoph Hellwig]

On Tue, Jul 14, 2020 at 08:29:05AM -0500, Eric W. Biederman wrote:
> 
> Currently it is necessary for the usermode helper code and the code
> that launches init to use set_fs so that pages coming from the kernel
> look like they are coming from userspace.
> 
> To allow that usage of set_fs to be removed cleanly the argument
> copying from userspace needs to happen earlier.  Move the allocation
> of the bprm into it's own function (alloc_bprm) and move the call of
> alloc_bprm before unshare_files so that bprm can ultimately be
> allocated, the arguments can be placed on the new stack, and then the
> bprm can be passed into the core of exec.
> 
> Neither the allocation of struct binprm nor the unsharing depend upon each
> other so swapping the order in which they are called is trivially safe.
> 
> To keep things consistent the order of cleanup at the end of
> do_execve_common swapped to match the order of initialization.
> 
> Signed-off-by: "Eric W. Biederman" <ebiederm at xmission.com>
> ---
>  fs/exec.c | 29 +++++++++++++++++++----------
>  1 file changed, 19 insertions(+), 10 deletions(-)
> 
> diff --git a/fs/exec.c b/fs/exec.c
> index 23dfbb820626..526156d6461d 100644
> --- a/fs/exec.c
> +++ b/fs/exec.c
> @@ -1560,6 +1560,14 @@ static void free_bprm(struct linux_binprm *bprm)
>  	kfree(bprm);
>  }
>  
> +static struct linux_binprm *alloc_bprm(void)
> +{
> +	struct linux_binprm *bprm = kzalloc(sizeof(*bprm), GFP_KERNEL);
> +	if (!bprm)
> +		return ERR_PTR(-ENOMEM);
> +	return bprm;

Unless this helper grows later I really don't see the point of it.
Also a NULL return vs ERR_PTR would simplify this a bit (again unless
this grows more code later with different return codes, but then again
it might make sense to only add the helper once it becomes useful).

The actual allocation order move looks fine, though.