Re: [patch V2 1/6] ARM: uaccess: Implement missing __get_user_asm_dword()

[Thomas Gleixner <tglx@linutronix.de>]

On Tue, Sep 16 2025 at 22:26, Russell King wrote:
> On Tue, Sep 16, 2025 at 06:33:09PM +0200, Thomas Gleixner wrote:
>> When CONFIG_CPU_SPECTRE=n then get_user() is missing the 8 byte ASM variant
>> for no real good reason. This prevents using get_user(u64) in generic code.
>
> I'm sure you will eventually discover the reason when you start getting
> all the kernel build bot warnings that will result from a cast from a
> u64 to a pointer.

I really don't know which cast you are talking about.

  	u64 __user *uaddr =  ...;
	u64 val;

        ....
        unsafe_get_user(val, uaddr, fault);

The only casts in this macro maze are in __get_user_err():

 1) Casting the uaddr pointer to unsigned long:

     unsigned long __gu_addr = (unsigned long)(ptr);

    which is correct because a *u64 pointer is still only 32bit wide on a
    32bit machine, no?

 2) Casting the result:

    (x) = (__typeof__(*(ptr)))__gu_val;

    which is casting to the type to which the pointer points to,
    i.e. u64 in this case.

I definitely checked the ASM result after I successfully compiled the
above w/o warnings. It compiles to:

 ad0:	ee032f10 	mcr	15, 0, r2, cr3, cr0, {0}
 ad4:	e3a00000 	mov	r0, #0
 ad8:	e4b3e000 	ldrt	lr, [r3], #0
 adc:	e2833004 	add	r3, r3, #4
 ae0:	e4b32000 	ldrt	r2, [r3], #0
 ae4:	ee03cf10 	mcr	15, 0, ip, cr3, cr0, {0}
 ae8:	e16f0f10 	clz	r0, r0
 aec:	e581e000 	str	lr, [r1]
 af0:	e5812004 	str	r2, [r1, #4]

which is magically correct despite the fact that I missed to change the
type of __gu_val to 'unsigned long long'. I just noticed when I tried to
figure out which cast you were referring to.

The wonderful and surprising world of macro preprocessing. :)

That unsigned long long is not hurtful as the compiler is smart enough
to optimize it away when __get_user_err() is invoked to read an u8 from
user:

 b18:	ee033f10 	mcr	15, 0, r3, cr3, cr0, {0}
 b1c:	e3a03000 	mov	r3, #0
 b20:	e4f04000 	ldrbt	r4, [r0], #0
 b24:	ee032f10 	mcr	15, 0, r2, cr3, cr0, {0}
 b28:	e16f0f13 	clz	r0, r3
 b2c:	e5c14000 	strb	r4, [r1]

which is exactly the same result as before this change.

Thanks,

        tglx

--- a/arch/arm/include/asm/uaccess.h
+++ b/arch/arm/include/asm/uaccess.h
@@ -286,7 +286,7 @@ extern int __put_user_8(void *, unsigned
 #define __get_user_err(x, ptr, err, __t)				\
 do {									\
 	unsigned long __gu_addr = (unsigned long)(ptr);			\
-	unsigned long __gu_val;						\
+	unsigned long long __gu_val;					\
 	unsigned int __ua_flags;					\
 	__chk_user_ptr(ptr);						\
 	might_fault();							\