On Thu, Sep 05, 2019 at 06:19:22AM +1000, Aleksa Sarai wrote:

+/**
+ * copy_struct_to_user: copy a struct to user space
+ * @dst:   Destination address, in user space.
+ * @usize: Size of @dst struct.
+ * @src:   Source address, in kernel space.
+ * @ksize: Size of @src struct.
+ *
+ * Copies a struct from kernel space to user space, in a way that guarantees
+ * backwards-compatibility for struct syscall arguments (as long as future
+ * struct extensions are made such that all new fields are *appended* to the
+ * old struct, and zeroed-out new fields have the same meaning as the old
+ * struct).
+ *
+ * @ksize is just sizeof(*dst), and @usize should've been passed by user space.
+ * The recommended usage is something like the following:
+ *
+ *   SYSCALL_DEFINE2(foobar, struct foo __user *, uarg, size_t, usize)
+ *   {
+ *      int err;
+ *      struct foo karg = {};
+ *
+ *      // do something with karg
+ *
+ *      err = copy_struct_to_user(uarg, usize, &karg, sizeof(karg));
+ *      if (err)
+ *        return err;
+ *
+ *      // ...
+ *   }
+ *
+ * There are three cases to consider:
+ *  * If @usize == @ksize, then it's copied verbatim.
+ *  * If @usize < @ksize, then kernel space is "returning" a newer struct to an
+ *    older user space. In order to avoid user space getting incomplete
+ *    information (new fields might be important), all trailing bytes in @src
+ *    (@ksize - @usize) must be zerored

s/zerored/zero/, right?

                                         , otherwise -EFBIG is returned.

'Funny' that, copy_struct_from_user() below seems to use E2BIG.

+ *  * If @usize > @ksize, then the kernel is "returning" an older struct to a
+ *    newer user space. The trailing bytes in @dst (@usize - @ksize) will be
+ *    zero-filled.
+ *
+ * Returns (in all cases, some data may have been copied):
+ *  * -EFBIG:  (@usize < @ksize) and there are non-zero trailing bytes in @src.
+ *  * -EFAULT: access to user space failed.
+ */
+int copy_struct_to_user(void __user *dst, size_t usize,
+			const void *src, size_t ksize)
+{
+	size_t size = min(ksize, usize);
+	size_t rest = abs(ksize - usize);
+
+	if (unlikely(usize > PAGE_SIZE))
+		return -EFAULT;

Not documented above. Implementation consistent with *from*, but see
below.

+	if (unlikely(!access_ok(dst, usize)))
+		return -EFAULT;
+
+	/* Deal with trailing bytes. */
+	if (usize < ksize) {
+		if (memchr_inv(src + size, 0, rest))
+			return -EFBIG;
+	} else if (usize > ksize) {
+		if (__memzero_user(dst + size, rest))
+			return -EFAULT;
+	}
+	/* Copy the interoperable parts of the struct. */
+	if (__copy_to_user(dst, src, size))
+		return -EFAULT;
+	return 0;
+}
+EXPORT_SYMBOL(copy_struct_to_user);
+
+/**
+ * copy_struct_from_user: copy a struct from user space
+ * @dst:   Destination address, in kernel space. This buffer must be @ksize
+ *         bytes long.
+ * @ksize: Size of @dst struct.
+ * @src:   Source address, in user space.
+ * @usize: (Alleged) size of @src struct.
+ *
+ * Copies a struct from user space to kernel space, in a way that guarantees
+ * backwards-compatibility for struct syscall arguments (as long as future
+ * struct extensions are made such that all new fields are *appended* to the
+ * old struct, and zeroed-out new fields have the same meaning as the old
+ * struct).
+ *
+ * @ksize is just sizeof(*dst), and @usize should've been passed by user space.
+ * The recommended usage is something like the following:
+ *
+ *   SYSCALL_DEFINE2(foobar, const struct foo __user *, uarg, size_t, usize)
+ *   {
+ *      int err;
+ *      struct foo karg = {};
+ *
+ *      err = copy_struct_from_user(&karg, sizeof(karg), uarg, size);
+ *      if (err)
+ *        return err;
+ *
+ *      // ...
+ *   }
+ *
+ * There are three cases to consider:
+ *  * If @usize == @ksize, then it's copied verbatim.
+ *  * If @usize < @ksize, then the user space has passed an old struct to a
+ *    newer kernel. The rest of the trailing bytes in @dst (@ksize - @usize)
+ *    are to be zero-filled.
+ *  * If @usize > @ksize, then the user space has passed a new struct to an
+ *    older kernel. The trailing bytes unknown to the kernel (@usize - @ksize)
+ *    are checked to ensure they are zeroed, otherwise -E2BIG is returned.
+ *
+ * Returns (in all cases, some data may have been copied):
+ *  * -E2BIG:  (@usize > @ksize) and there are non-zero trailing bytes in @src.
+ *  * -E2BIG:  @usize is "too big" (at time of writing, >PAGE_SIZE).
+ *  * -EFAULT: access to user space failed.
+ */
+int copy_struct_from_user(void *dst, size_t ksize,
+			  const void __user *src, size_t usize)
+{
+	size_t size = min(ksize, usize);
+	size_t rest = abs(ksize - usize);
+
+	if (unlikely(usize > PAGE_SIZE))
+		return -EFAULT;

Documented above as returning -E2BIG.

+	if (unlikely(!access_ok(src, usize)))
+		return -EFAULT;
+
+	/* Deal with trailing bytes. */
+	if (usize < ksize)
+		memset(dst + size, 0, rest);
+	else if (usize > ksize) {
+		const void __user *addr = src + size;
+		char buffer[BUFFER_SIZE] = {};

Isn't that too big for on-stack?

+
+		while (rest > 0) {
+			size_t bufsize = min(rest, sizeof(buffer));
+
+			if (__copy_from_user(buffer, addr, bufsize))
+				return -EFAULT;
+			if (memchr_inv(buffer, 0, bufsize))
+				return -E2BIG;
+
+			addr += bufsize;
+			rest -= bufsize;
+		}

The perf implementation uses get_user(); but if that is too slow, surely
we can do something with uaccess_try() here?

+	}
+	/* Copy the interoperable parts of the struct. */
+	if (__copy_from_user(dst, src, size))
+		return -EFAULT;
+	return 0;
+}
+EXPORT_SYMBOL(copy_struct_from_user);

And personally I'm not a big fan of EXPORT_SYMBOL().