Re: [PATCH v12 01/12] lib: introduce copy_struct_{to,from}_user helpers

On Thu, Sep 05, 2019 at 07:26:22PM +1000, Aleksa Sarai wrote:

On 2019-09-05, Peter Zijlstra [off-list ref] wrote:

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

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+/**
+ * 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?

It should've been "zeroed".

That reads wrong to me; that way it reads like this function must take
that action and zero out the 'rest'; which is just wrong.

This function must verify those bytes are zero, not make them zero.

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                                         , otherwise -EFBIG is returned.

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

This is a copy of the semantics that sched_[sg]etattr(2) uses -- E2BIG for
a "too big" struct passed to the kernel, and EFBIG for a "too big"
struct passed to user-space. I would personally have preferred EMSGSIZE
instead of EFBIG, but felt using the existing error codes would be less
confusing.

Sadly a recent commit:

  1251201c0d34 ("sched/core: Fix uclamp ABI bug, clean up and robustify sched_read_attr() ABI logic and code")

Made the situation even 'worse'.

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+	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?

Is a 64-byte buffer too big? I picked the number "at random" to be the
size of a cache line, but I could shrink it down to 32 bytes if the size
is an issue (I wanted to avoid needless allocations -- hence it being
on-stack).

Ah, my ctags gave me a definition of BUFFER_SIZE that was 512. I suppose
64 should be OK.

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+
+		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?

Is there a non-x86-specific way to do that (unless I'm mistaken only x86
has uaccess_try() or the other *_try() wrappers)? The main "performance
improvement" (if you can even call it that) is that we use memchr_inv()
which finds non-matching characters more efficiently than just doing a
loop.

Oh, you're right, that's x86 only :/