Re: [PATCH v2 1/5] powerpc/signal64: Access function descriptor with user access block

[Christoph Hellwig <hch@infradead.org>]

On Mon, Aug 23, 2021 at 03:35:51PM +0000, Christophe Leroy wrote:
> Access the function descriptor of the handler within a
> user access block.
> 
> Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
> ---
>  arch/powerpc/kernel/signal_64.c | 14 ++++++++++++--
>  1 file changed, 12 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/powerpc/kernel/signal_64.c b/arch/powerpc/kernel/signal_64.c
> index 1831bba0582e..790c450c2de8 100644
> --- a/arch/powerpc/kernel/signal_64.c
> +++ b/arch/powerpc/kernel/signal_64.c
> @@ -936,8 +936,18 @@ int handle_rt_signal64(struct ksignal *ksig, sigset_t *set,
>  		func_descr_t __user *funct_desc_ptr =
>  			(func_descr_t __user *) ksig->ka.sa.sa_handler;
>  
> -		err |= get_user(regs->ctr, &funct_desc_ptr->entry);
> -		err |= get_user(regs->gpr[2], &funct_desc_ptr->toc);
> +		if (user_read_access_begin(funct_desc_ptr, sizeof(func_descr_t))) {
> +			unsafe_get_user(regs->ctr, &funct_desc_ptr->entry, bad_funct_desc_block);
> +			unsafe_get_user(regs->gpr[2], &funct_desc_ptr->toc, bad_funct_desc_block);
> +		} else {
> +			goto bad_funct_desc;
> +bad_funct_desc_block:
> +			user_read_access_end();
> +bad_funct_desc:
> +			signal_fault(current, regs, __func__, funct_desc_ptr);
> +			return 1;
> +		}
> +		user_read_access_end();

Having a goto after an ele block, an then labels jumping into it is just
weird.

To make this somewhat readable just split it into a helper function:

static inline bool put_func_descr(func_descr_t __user *funct_desc_ptr,
		struct pt_regs *regs)
{
	if (!user_read_access_begin(funct_desc_ptr, sizeof(func_descr_t)))
		goto fault;
	unsafe_get_user(regs->ctr, &funct_desc_ptr->entry, uaccess_end);
	unsafe_get_user(regs->gpr[2], &funct_desc_ptr->toc, uaccess_end);
	user_read_access_end();
	return false;

uaccess_end:
	user_read_access_end();
fault:
	signal_fault(current, regs, __func__, funct_desc_ptr);
	return true;
}