Re: [PATCH v2 2/2] uaccess: rust: add UserSliceReader::strcpy_into_buf

[Alice Ryhl <aliceryhl@google.com>]

On Tue, Apr 29, 2025 at 01:48:19PM +0200, Greg Kroah-Hartman wrote:
> On Tue, Apr 29, 2025 at 01:38:26PM +0200, Danilo Krummrich wrote:
> > On 2025-04-29 13:09, Greg Kroah-Hartman wrote:
> > > On Tue, Apr 29, 2025 at 09:02:23AM +0000, Alice Ryhl wrote:
> > > > +        let mut len = raw_strncpy_from_user(self.ptr, dst)?;
> > > > +        if len < dst.len() {
> > > > +            // Add one to include the NUL-terminator.
> > > > +            len += 1;
> > > > +        } else if len < buf.len() {
> > > > +            // We hit the `self.length` limit before `buf.len()`.
> > > > +            return Err(EFAULT);
> > > 
> > > How can this happen?
> > 
> > See my reply here (if I did not get it wrong):
> > 
> > https://lore.kernel.org/rust-for-linux/aBCrqJe4two4I45G@pollux/
> 
> Ah, I should have read ahead :)
> 
> I agree, some comments here would be good.  We want everyone to be able
> to easily read and understand this code, off-by-one errors are rough.

I will add this comment:

if len < dst.len() {
    // Add one to include the NUL-terminator.
    len += 1;
} else if len < buf.len() {
    // This implies that len == dst.len() < buf.len().
    //
    // This means that we could not fill the entire buffer, but we had
    // to stop reading because we hit the `self.length` limit of this
    // `UserSliceReader`. Since we did not fill the buffer, we treat
    // this case as if we tried to read past the `self.length` limit and
    // received a page fault, which is consistent with other
    // `UserSliceReader` methods that also return page faults when you
    // exceed `self.length`.
    return Err(EFAULT);

Alice