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SubjectRe: [PATCH v2 20/31] arm64: User access library function
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On Tuesday 14 August 2012, Catalin Marinas wrote:

> +/*
> + * Single-value transfer routines. They automatically use the right
> + * size if we just have the right pointer type. Note that the functions
> + * which read from user space (*get_*) need to take care not to leak
> + * kernel data even if the calling code is buggy and fails to check
> + * the return value. This means zeroing out the destination variable
> + * or buffer on error. Normally this is done out of line by the
> + * fixup code, but there are a few places where it intrudes on the
> + * main code path. When we only write to user space, there is no
> + * problem.
> + */
> +extern long __get_user_1(void *);
> +extern long __get_user_2(void *);
> +extern long __get_user_4(void *);
> +extern long __get_user_8(void *);
> +
> +#define __get_user_x(__r2,__p,__e,__s,__i...) \
> + asm volatile( \
> + __asmeq("%0", "x0") __asmeq("%1", "x2") \
> + "bl __get_user_" #__s \
> + : "=&r" (__e), "=r" (__r2) \
> + : "0" (__p) \
> + : __i, "cc")
> +
> +#define get_user(x,p) \
> + ({ \
> + register const typeof(*(p)) __user *__p asm("x0") = (p);\
> + register unsigned long __r2 asm("x2"); \
> + register long __e asm("x0"); \
> + switch (sizeof(*(__p))) { \
> + case 1: \
> + __get_user_x(__r2, __p, __e, 1, "x30"); \
> + break; \
> + case 2: \
> + __get_user_x(__r2, __p, __e, 2, "x3", "x30"); \
> + break; \
> + case 4: \
> + __get_user_x(__r2, __p, __e, 4, "x30"); \
> + break; \
> + case 8: \
> + __get_user_x(__r2, __p, __e, 8, "x30"); \
> + break; \
> + default: __e = __get_user_bad(); break; \
> + } \
> + x = (typeof(*(p))) __r2; \
> + __e; \
> + })

It's fairly unusual to have out of line get_user/put_user functions.
What is the reason for this, other than copying from ARM?

> +
> +__get_user_bad:
> + mov x2, #0
> + mov x0, #-EFAULT
> + ret
> +ENDPROC(__get_user_bad)

> +__put_user_bad:
> + mov x0, #-EFAULT
> + ret
> +ENDPROC(__put_user_bad)
> +

The purpose of these symbols is to provoke a link error when you
pass the wrong data into get_user/put_user. Actually defining them
completely breaks this logic, so you should remove these!

Arnd


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