From: Andrew Morton Add a version of __get_user() which is safe to call inside mmap_sem. Signed-off-by: Andrew Morton --- include/linux/uaccess.h | 22 ++++++++++++++++++++++ 1 files changed, 22 insertions(+) diff -puN include/linux/uaccess.h~add-probe_kernel_address include/linux/uaccess.h --- 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__ */ _