linux-kernel - Re: [PATCH v2 3/4] MIPS: uaccess: Remove get_fs/set

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Date:   Thu, 1 Apr 2021 08:30:55 +0200
From:   Christoph Hellwig <hch@....de>
To:     Thomas Bogendoerfer <tsbogend@...ha.franken.de>
Cc:     linux-mips@...r.kernel.org, linux-kernel@...r.kernel.org,
        hch@....de
Subject: Re: [PATCH v2 3/4] MIPS: uaccess: Remove get_fs/set_fs call sites

On Wed, Mar 31, 2021 at 01:56:00PM +0200, Thomas Bogendoerfer wrote:
> +#define __get_user_nofault(dst, src, type, err_label)			\
> +do {									\
> +	int __gu_err;							\
> +									\
> +	__get_user_common(*((type *)(dst)), sizeof(type),		\
> +			  (__force type *)(src));			\
> +	if (unlikely(__gu_err))						\
> +		goto err_label;						\
> +} while (0)
> +
> +
> +static inline int __get_addr(unsigned long *a, unsigned long *p, bool user)
> +{
> +	if (user)
> +		__get_user_nofault(a, p, unsigned long, fault);
> +	else
> +		__get_kernel_nofault(a, p, unsigned long, fault);
> +
> +	return 0;
> +
> +fault:
> +	return -EFAULT;
> +}

Why can't these use plain old get_user and get_kernel_nofault?
You "optimize" away the access_ok / get_kernel_nofaul_allowed checks
here, but now use totally non-standard and possibly dangerous APIs.