linux-kernel - Re: [PATCH v6 5/9] kernel: Implement selective syscall userspace redirection

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Date:   Sat, 5 Sep 2020 12:24:38 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Gabriel Krisman Bertazi <krisman@...labora.com>
Cc:     luto@...nel.org, tglx@...utronix.de, keescook@...omium.org,
        x86@...nel.org, linux-kernel@...r.kernel.org,
        linux-api@...r.kernel.org, linux-kselftest@...r.kernel.org,
        shuah@...nel.org, kernel@...labora.com,
        Paul Gofman <gofmanp@...il.com>
Subject: Re: [PATCH v6 5/9] kernel: Implement selective syscall userspace
 redirection

On Fri, Sep 04, 2020 at 04:31:43PM -0400, Gabriel Krisman Bertazi wrote:
> +int set_syscall_user_dispatch(int mode, unsigned long dispatcher_start,
> +			      unsigned long dispatcher_end, char __user *selector)
> +{
> +	switch (mode) {
> +	case PR_SYS_DISPATCH_OFF:
...
> +	case PR_SYS_DISPATCH_ON:
...
> +	default:
> +		return -EINVAL;
...
> +	case PR_SET_SYSCALL_USER_DISPATCH:
> +		error = set_syscall_user_dispatch((int) arg2, arg3, arg4,
> +						  (char __user *) arg5);

This makes aliases of DISPATCH_OFF and DISPATCH_ON every 4GB throughout
the 64-bit space of arg2.  I don't think that was intentional (nor
desirable).  I'd suggest just making 'mode' a long and dropping the cast.