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

Al Viro
2019-09-05T18:08:34Z

On Thu, Sep 05, 2019 at 06:19:22AM +1000, Aleksa Sarai wrote:
> +/*
> + * "memset(p, 0, size)" but for user space buffers. Caller must have already
> + * checked access_ok(p, size).
> + */
> +static int __memzero_user(void __user *p, size_t s)
> +{
> + const char zeros[BUFFER_SIZE] = {};
> + while (s > 0) {
> + size_t n = min(s, sizeof(zeros));
> +
> + if (__copy_to_user(p, zeros, n))
> + return -EFAULT;
> +
> + p += n;
> + s -= n;
> + }
> + return 0;
> +}

That's called clear_user().

> +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;

Why?

> + } 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;

Why not simply clear_user() and copy_to_user()?

> +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);

Cute, but... you would be just as well without that 'rest' thing.

> +
> + if (unlikely(usize > PAGE_SIZE))
> + return -EFAULT;

Again, why?

> + if (unlikely(!access_ok(src, usize)))
> + return -EFAULT;

Why not simply copy_from_user() here?

> + /* 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] = {};
> +
> + 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;

Frankly, that looks like a candidate for is_all_zeroes_user().
With the loop like above serving as a dumb default. And on
badly alighed address it _will_ be dumb. Probably too much
so - something like
if ((unsigned long)addr & 1) {
u8 v;
if (get_user(v, (__u8 __user *)addr))
return -EFAULT;
if (v)
return -E2BIG;
addr++;
}
if ((unsigned long)addr & 2) {
u16 v;
if (get_user(v, (__u16 __user *)addr))
return -EFAULT;
if (v)
return -E2BIG;
addr +=2;
}
if ((unsigned long)addr & 4) {
u32 v;
if (get_user(v, (__u32 __user *)addr))
return -EFAULT;
if (v)
return -E2BIG;
}
<read the rest like you currently do>
would be saner, and things like x86 could trivially add an
asm variant - it's not hard. Incidentally, memchr_inv() is
an overkill in this case...