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

[Aleksa Sarai]

On 2019-09-05, Al Viro <viro@xxxxxxxxxxxxxxxxxx> wrote:
> 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().

Already switched, I didn't know about clear_user() -- I assumed it
would've been called bzero_user() or memzero_user() and didn't find it
when looking.

> > +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()?

I'm not sure I understand what you mean -- are you asking why we need to
do memchr_inv(src + size, 0, rest) earlier?

> 
> > +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.

I would argue it's harder to mess up using "rest" compared to getting
"ksize - usize" and "usize - ksize" mixed up (and it's a bit more
readable).

> > +
> > +	if (unlikely(usize > PAGE_SIZE))
> > +		return -EFAULT;
> 
> Again, why?

As discussed in a sister thread, I will leave this in the callers
(though I would argue callers should always do some kind of sanity check
like this).

> 
> > +	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...

Why is memchr_inv() overkill?

But yes, breaking this out to an asm-generic is_all_zeroes_user()
wouldn't hurt -- and I'll put a cleaned-up version of the alignment
handling there too. Should I drop it in asm-generic/uaccess.h, or
somewhere else?

-- 
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
<https://www.cyphar.com/>

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