[PATCH] powerpc/uaccess: Enable get_user(u64, *p) on 32-bit

[Michael Ellerman]

Currently if you build a 32-bit powerpc kernel and use get_user() to
load a u64 value it will fail to build with eg:

  kernel/rseq.o: In function `rseq_get_rseq_cs':
  kernel/rseq.c:123: undefined reference to `__get_user_bad'

This is hitting the check in __get_user_size() that makes sure the
size we're copying doesn't exceed the size of the destination:

  #define __get_user_size(x, ptr, size, retval)
  do {
        retval = 0;
        __chk_user_ptr(ptr);
        if (size > sizeof(x))
                (x) = __get_user_bad();

Which doesn't immediately make sense because the size of the
destination is u64, but it's not really, because __get_user_check()
etc. internally create an unsigned long and copy into that:

  #define __get_user_check(x, ptr, size)
  ({
        long __gu_err = -EFAULT;
        unsigned long  __gu_val = 0;

The problem being that on 32-bit unsigned long is not big enough to
hold a u64. We can fix this with a trick from hpa in the x86 code, we
statically check the type of x and set the type of __gu_val to either
unsigned long or unsigned long long.

Signed-off-by: Michael Ellerman <m...@ellerman.id.au>
---
 arch/powerpc/include/asm/uaccess.h | 13 ++++++++++---
 1 file changed, 10 insertions(+), 3 deletions(-)

diff --git a/arch/powerpc/include/asm/uaccess.h 
b/arch/powerpc/include/asm/uaccess.h
index 643cfbd5bcb5..bac225bb7f64 100644
--- a/arch/powerpc/include/asm/uaccess.h
+++ b/arch/powerpc/include/asm/uaccess.h
@@ -249,10 +249,17 @@ do {                                                      
        \
        }                                                       \
 } while (0)
 
+/*
+ * This is a type: either unsigned long, if the argument fits into
+ * that type, or otherwise unsigned long long.
+ */
+#define __long_type(x) \
+       __typeof__(__builtin_choose_expr(sizeof(x) > sizeof(0UL), 0ULL, 0UL))
+
 #define __get_user_nocheck(x, ptr, size)                       \
 ({                                                             \
        long __gu_err;                                          \
-       unsigned long __gu_val;                                 \
+       __long_type(*(ptr)) __gu_val;                           \
        const __typeof__(*(ptr)) __user *__gu_addr = (ptr);     \
        __chk_user_ptr(ptr);                                    \
        if (!is_kernel_addr((unsigned long)__gu_addr))          \
@@ -266,7 +273,7 @@ do {                                                        
        \
 #define __get_user_check(x, ptr, size)                                 \
 ({                                                                     \
        long __gu_err = -EFAULT;                                        \
-       unsigned long  __gu_val = 0;                                    \
+       __long_type(*(ptr)) __gu_val = 0;                               \
        const __typeof__(*(ptr)) __user *__gu_addr = (ptr);             \
        might_fault();                                                  \
        if (access_ok(VERIFY_READ, __gu_addr, (size))) {                \
@@ -280,7 +287,7 @@ do {                                                        
        \
 #define __get_user_nosleep(x, ptr, size)                       \
 ({                                                             \
        long __gu_err;                                          \
-       unsigned long __gu_val;                                 \
+       __long_type(*(ptr)) __gu_val;                           \
        const __typeof__(*(ptr)) __user *__gu_addr = (ptr);     \
        __chk_user_ptr(ptr);                                    \
        barrier_nospec();                                       \
-- 
2.14.1