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

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().