In powerpc code, there are several places implementing safe
access to user data. This is sometimes implemented using
probe_kernel_address() with additional access_ok() verification,
sometimes with get_user() enclosed in a pagefault_disable()/enable()
pair, etc. :
    show_user_instructions()
    bad_stack_expansion()
    p9_hmi_special_emu()
    fsl_pci_mcheck_exception()
    read_user_stack_64()
    read_user_stack_32() on PPC64
    read_user_stack_32() on PPC32
    power_pmu_bhrb_to()

In the same spirit as probe_kernel_read(), this patch adds
probe_user_read().

probe_user_read() does the same as probe_kernel_read() but
first checks that it is really a user address.

The patch defines this function as a static inline so the "size"
variable can be examined for const-ness by the check_object_size()
in __copy_from_user_inatomic()

Signed-off-by: Christophe Leroy <redacted>
---
 v3: Moved 'Returns:" comment after description.
     Explained in the commit log why the function is defined static inline

 v2: Added "Returns:" comment and removed probe_user_address()

 include/linux/uaccess.h | 34 ++++++++++++++++++++++++++++++++++
 1 file changed, 34 insertions(+)

diff --git a/include/linux/uaccess.h b/include/linux/uaccess.h
index 37b226e8df13..ef99edd63da3 100644
--- a/include/linux/uaccess.h
+++ b/include/linux/uaccess.h

@@ -263,6 +263,40 @@ extern long strncpy_from_unsafe(char *dst, const void *unsafe_addr, long count);
 #define probe_kernel_address(addr, retval)		\
 	probe_kernel_read(&retval, addr, sizeof(retval))
 
+/**
+ * probe_user_read(): safely attempt to read from a user location
+ * @dst: pointer to the buffer that shall take the data
+ * @src: address to read from
+ * @size: size of the data chunk
+ *
+ * Safely read from address @src to the buffer at @dst.  If a kernel fault
+ * happens, handle that and return -EFAULT.
+ *
+ * We ensure that the copy_from_user is executed in atomic context so that
+ * do_page_fault() doesn't attempt to take mmap_sem.  This makes
+ * probe_user_read() suitable for use within regions where the caller
+ * already holds mmap_sem, or other locks which nest inside mmap_sem.
+ *
+ * Returns: 0 on success, -EFAULT on error.
+ */
+
+#ifndef probe_user_read
+static __always_inline long probe_user_read(void *dst, const void __user *src,
+					    size_t size)
+{
+	long ret;
+