This routine acts almost as fault_in_pages_readable(), but returns
accessible amount of bytes instead of plain OK/FAIL, so callers
may know how many data can be proceeded without GPF.
Signed-off-by: Vitaly Mayatskikh <v.mayatskih@xxxxxxxxx>
---
include/linux/pagemap.h | 35 +++++++++++++++++++++++++++++++++++
1 files changed, 35 insertions(+), 0 deletions(-)
diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
index 34da523..f931308 100644
--- a/include/linux/pagemap.h
+++ b/include/linux/pagemap.h
@@ -439,6 +439,41 @@ static inline int fault_in_pages_readable(const char __user *uaddr, int size)
return ret;
}
+/*
+ * check_readable_bytes - check if given amount of bytes can be read
+ * at given address.
+ * @uaddr: address to check
+ * @size: size to check
+ *
+ * Returns 0 when @size is 0, -EFAULT when @uaddr points to
+ * unaccessible region, or count of accessible bytes.
+ */
+static inline int check_readable_bytes(const char __user *uaddr, int size)
+{
+ volatile char c;
+ int ret = 0;
+ long page_begin = (unsigned long)uaddr & PAGE_MASK;
+ long page_end = ((unsigned long)uaddr + size) & PAGE_MASK;
+ long ptr = page_begin;
+
+ if (unlikely(size == 0))
+ goto out;
+
+ while (!ret && ptr <= page_end) {
+ ret = __get_user(c, (const char __user*)ptr);
+ if (!ret)
+ ptr += PAGE_SIZE;
+ }
+
+ if (likely(!ret))
+ ret = size;
+ else
+ ret = (ptr == page_begin) ?
+ -EFAULT : (const char __user *)ptr - uaddr;
+out:
+ return ret;
+}
+
int add_to_page_cache_locked(struct page *page, struct address_space *mapping,
pgoff_t index, gfp_t gfp_mask);
int add_to_page_cache_lru(struct page *page, struct address_space *mapping,
--
1.6.2.2
--
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