Linus Torvalds <[email protected]> wrote:
>
> That said, I think it's wrong to use "__get_user()", which can hang on the
> MM semaphore if something is bogus. We should probably mark us as being
> "in_atomic()" to make sure that the page fault handler, if it is entered,
> will not try to get the semaphore or page anything in.
This?
--- a/include/linux/uaccess.h~add-probe_kernel_address
+++ a/include/linux/uaccess.h
@@ -19,4 +19,26 @@ static inline unsigned long __copy_from_
#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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