lib: introduce strncpy_from_unsafe()

generalize FETCH_FUNC_NAME(memory, string) into
strncpy_from_unsafe() and fix sparse warnings that were
present in original implementation.
Signed-off-by: Alexei Starovoitov <ast@plumgrid.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/uaccess.h

@@ -129,4 +129,6 @@ extern long __probe_kernel_read(void *dst, const void *src, size_t size);
 extern long notrace probe_kernel_write(void *dst, const void *src, size_t size);
 extern long notrace __probe_kernel_write(void *dst, const void *src, size_t size);
 
+extern long strncpy_from_unsafe(char *dst, const void *unsafe_addr, long count);
+
 #endif		/* __LINUX_UACCESS_H__ */

kernel/trace/trace_kprobe.c

@@ -165,11 +165,9 @@ DEFINE_BASIC_FETCH_FUNCS(memory)
 static void FETCH_FUNC_NAME(memory, string)(struct pt_regs *regs,
 					    void *addr, void *dest)
 {
-	long ret;
 	int maxlen = get_rloc_len(*(u32 *)dest);
 	u8 *dst = get_rloc_data(dest);
-	u8 *src = addr;
-	mm_segment_t old_fs = get_fs();
+	long ret;
 
 	if (!maxlen)
 		return;
@@ -178,23 +176,13 @@ static void FETCH_FUNC_NAME(memory, string)(struct pt_regs *regs,
 	 * Try to get string again, since the string can be changed while
 	 * probing.
 	 */
-	set_fs(KERNEL_DS);
-	pagefault_disable();
-
-	do
-		ret = __copy_from_user_inatomic(dst++, src++, 1);
-	while (dst[-1] && ret == 0 && src - (u8 *)addr < maxlen);
-
-	dst[-1] = '\0';
-	pagefault_enable();
-	set_fs(old_fs);
+	ret = strncpy_from_unsafe(dst, addr, maxlen);
 
 	if (ret < 0) {	/* Failed to fetch string */
-		((u8 *)get_rloc_data(dest))[0] = '\0';
+		dst[0] = '\0';
 		*(u32 *)dest = make_data_rloc(0, get_rloc_offs(*(u32 *)dest));
 	} else {
-		*(u32 *)dest = make_data_rloc(src - (u8 *)addr,
-					      get_rloc_offs(*(u32 *)dest));
+		*(u32 *)dest = make_data_rloc(ret, get_rloc_offs(*(u32 *)dest));
 	}
 }
 NOKPROBE_SYMBOL(FETCH_FUNC_NAME(memory, string));

lib/strncpy_from_user.c

@@ -112,3 +112,44 @@ long strncpy_from_user(char *dst, const char __user *src, long count)
 	return -EFAULT;
 }
 EXPORT_SYMBOL(strncpy_from_user);
+
+/**
+ * strncpy_from_unsafe: - Copy a NUL terminated string from unsafe address.
+ * @dst:   Destination address, in kernel space.  This buffer must be at
+ *         least @count bytes long.
+ * @src:   Unsafe address.
+ * @count: Maximum number of bytes to copy, including the trailing NUL.
+ *
+ * Copies a NUL-terminated string from unsafe address to kernel buffer.
+ *
+ * On success, returns the length of the string INCLUDING the trailing NUL.
+ *
+ * If access fails, returns -EFAULT (some data may have been copied
+ * and the trailing NUL added).
+ *
+ * If @count is smaller than the length of the string, copies @count-1 bytes,
+ * sets the last byte of @dst buffer to NUL and returns @count.
+ */
+long strncpy_from_unsafe(char *dst, const void *unsafe_addr, long count)
+{
+	mm_segment_t old_fs = get_fs();
+	const void *src = unsafe_addr;
+	long ret;
+
+	if (unlikely(count <= 0))
+		return 0;
+
+	set_fs(KERNEL_DS);
+	pagefault_disable();
+
+	do {
+		ret = __copy_from_user_inatomic(dst++,
+						(const void __user __force *)src++, 1);
+	} while (dst[-1] && ret == 0 && src - unsafe_addr < count);
+
+	dst[-1] = '\0';
+	pagefault_enable();
+	set_fs(old_fs);
+
+	return ret < 0 ? ret : src - unsafe_addr;
+}