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

[Danilo Krummrich <kernel@dakr.org>]

On 2025-04-29 13:09, 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@google.com>
>> ---
>>  rust/kernel/uaccess.rs | 35 +++++++++++++++++++++++++++++++++++
>>  1 file changed, 35 insertions(+)
>> 
>> diff --git a/rust/kernel/uaccess.rs b/rust/kernel/uaccess.rs
>> index 
>> acb703f074a30e60d42a222dd26aed80d8bdb76a..7cec1b62bd8b816f523c8be12cb29905740789fc 
>> 100644
>> --- a/rust/kernel/uaccess.rs
>> +++ b/rust/kernel/uaccess.rs
>> @@ -293,6 +293,41 @@ pub fn read_all<A: Allocator>(mut self, buf: &mut 
>> Vec<u8, A>, flags: Flags) -> R
>>          unsafe { buf.set_len(buf.len() + len) };
>>          Ok(())
>>      }
>> +
>> +    /// Read a NUL-terminated string from userspace and append it to 
>> `dst`.
>> +    ///
>> +    /// Fails with [`EFAULT`] if the read happens on a bad address.
> 
> Also returns this error:
> 
>> +    pub fn strcpy_into_buf<'buf>(&mut self, buf: &'buf mut [u8]) -> 
>> Result<&'buf CStr> {
>> +        if buf.is_empty() {
>> +            return Err(EINVAL);
> 
> if the buffer is of 0 length.  Don't know if you want to document that
> or not.
> 
>> +        }
>> +
>> +        // SAFETY: The types are compatible and `strncpy_from_user` 
>> doesn't write uninitialized
>> +        // bytes to `buf`.
>> +        let mut dst = unsafe { &mut *(buf as *mut [u8] as *mut 
>> [MaybeUninit<u8>]) };
>> +
>> +        // We never read more than `self.length` bytes.
>> +        if dst.len() > self.length {
>> +            dst = &mut dst[..self.length];
>> +        }
>> +
>> +        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/