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Message-ID: <aBDs9aMQSCLIqD96@google.com>
Date: Tue, 29 Apr 2025 15:15:01 +0000
From: Alice Ryhl <aliceryhl@...gle.com>
To: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc: Danilo Krummrich <kernel@...r.org>, 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 01:48:19PM +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
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