From 1b79e5513d52e8533a08af35a3595dad80c74d1f Mon Sep 17 00:00:00 2001
From: Andrew Morton <akpm@osdl.org>
Date: Wed, 27 Sep 2006 01:51:14 -0700
Subject: [PATCH] [PATCH] add probe_kernel_address()
Add a version of __get_user() which is safe to call inside mmap_sem.
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>
---
include/linux/uaccess.h | 22 ++++++++++++++++++++++
1 file changed, 22 insertions(+)
diff --git a/include/linux/uaccess.h b/include/linux/uaccess.h
index 391e7ed1eb3fc0..a48d7f11c7be06 100644
--- a/include/linux/uaccess.h
+++ b/include/linux/uaccess.h
@@ -19,4 +19,26 @@ static inline unsigned long __copy_from_user_nocache(void *to,
#endif /* ARCH_HAS_NOCACHE_UACCESS */
+/**
+ * probe_kernel_address(): safely attempt to read from a location
+ * @addr: address to read from - its type is type typeof(retval)*
+ * @retval: read into this variable
+ *
+ * Safely read from address @addr into variable @revtal. If a kernel fault
+ * happens, handle that and return -EFAULT.
+ * We ensure that the __get_user() is executed in atomic context so that
+ * do_page_fault() doesn't attempt to take mmap_sem. This makes
+ * probe_kernel_address() suitable for use within regions where the caller
+ * already holds mmap_sem, or other locks which nest inside mmap_sem.
+ */
+#define probe_kernel_address(addr, retval) \
+ ({ \
+ long ret; \
+ \
+ inc_preempt_count(); \
+ ret = __get_user(retval, addr); \
+ dec_preempt_count(); \
+ ret; \
+ })
+
#endif /* __LINUX_UACCESS_H__ */
--
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