linux-kernel - Re: [PATCH v2 2/2] uaccess: rust: add UserSliceReader::strcpy_into

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Message-ID: <aBICQt-CzQkMw4uk@google.com>
Date: Wed, 30 Apr 2025 10:58:10 +0000
From: Alice Ryhl <aliceryhl@...gle.com>
To: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc: Miguel Ojeda <ojeda@...nel.org>, Andrew Morton <akpm@...ux-foundation.org>, 
	Alexander Viro <viro@...iv.linux.org.uk>, Boqun Feng <boqun.feng@...il.com>, 
	Gary Guo <gary@...yguo.net>, 
	"Björn Roy Baron" <bjorn3_gh@...tonmail.com>, Benno Lossin <benno.lossin@...ton.me>, 
	Andreas Hindborg <a.hindborg@...nel.org>, Trevor Gross <tmgross@...ch.edu>, 
	Danilo Krummrich <dakr@...nel.org>, rust-for-linux@...r.kernel.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/2] uaccess: rust: add UserSliceReader::strcpy_into_buf

On Tue, Apr 29, 2025 at 01:09:18PM +0200, Greg Kroah-Hartman wrote:
> On Tue, Apr 29, 2025 at 09:02:23AM +0000, Alice Ryhl wrote:
> > This patch adds a more convenient method for reading C strings from
> > userspace. Logic is added to NUL-terminate the buffer when necessary so
> > that a &CStr can be returned.
> > 
> > Note that we treat attempts to read past `self.length` as a fault, so
> > this returns EFAULT if that limit is exceeded before `buf.len()` is
> > reached.
> > 
> > Signed-off-by: Alice Ryhl <aliceryhl@...gle.com>

> > +        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?  And if it does, why is that a memory fault?
> Doesn't this just mean that we read smaller than our overall size of our
> buffer?  Or am I misreading this completely?
> 
> Maybe a self-test would be good to exercise all of this :)

How can I test userspace access? Is there a way to create a kernel
buffer that strncpy_from_user will let you read from for use in a kunit
test?

Alice