There is currently no easy way to read NUL-terminated strings from
userspace. Trying to use the ordinary read function on an array of the
maximum length doesn't work because it could fail with EFAULT when the C
string is shorter than the maximum length. In this case,
strncpy_from_user is better because it doesn't return EFAULT even if it
encounters a page fault on bytes that are after the NUL-terminator but
before the maximum length.

Signed-off-by: Alice Ryhl <aliceryhl@google.com>
---
Changes in v3:
- Remove pub from raw_strncpy_from_user.
- Mention that some data may have been copied on EFAULT.
- Add more comments to strcpy_into_buf about tricky cases.
- Rewrite documentation of strcpy_into_buf.
- Pick up Reviewed-by tags.
- Link to v2: https://lore.kernel.org/r/20250429-strncpy-from-user-v2-0-7e6facac0bf0@google.com

Changes in v2:
- Rename the raw wrapper around strncpy_from_user to raw_strncpy_from_user.
- Add a more convenient helper on top that adds the missing
  NUL-terminator when necessary.
- Link to v1: https://lore.kernel.org/r/20250424-strncpy-from-user-v1-1-f983fe21685a@google.com

---
Alice Ryhl (2):
      uaccess: rust: add strncpy_from_user
      uaccess: rust: add UserSliceReader::strcpy_into_buf

 rust/kernel/uaccess.rs | 86 +++++++++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 85 insertions(+), 1 deletion(-)
---
base-commit: 9c32cda43eb78f78c73aee4aa344b777714e259b
change-id: 20250424-strncpy-from-user-1f2d06b0cdde

Best regards,
-- 
Alice Ryhl <aliceryhl@google.com>

On Mon, May 5, 2025 at 2:17 PM Alice Ryhl <aliceryhl@google.com> wrote:
>
> There is currently no easy way to read NUL-terminated strings from
> userspace. Trying to use the ordinary read function on an array of the
> maximum length doesn't work because it could fail with EFAULT when the C
> string is shorter than the maximum length. In this case,
> strncpy_from_user is better because it doesn't return EFAULT even if it
> encounters a page fault on bytes that are after the NUL-terminator but
> before the maximum length.
>
> Signed-off-by: Alice Ryhl <aliceryhl@google.com>
> ---
> Changes in v3:
> - Remove pub from raw_strncpy_from_user.
> - Mention that some data may have been copied on EFAULT.
> - Add more comments to strcpy_into_buf about tricky cases.
> - Rewrite documentation of strcpy_into_buf.
> - Pick up Reviewed-by tags.
> - Link to v2: https://lore.kernel.org/r/20250429-strncpy-from-user-v2-0-7e6facac0bf0@google.com

Oh I forgot to mention that I added a documentation alias so that
searching for strncpy_from_user using the search box on
https://rust.docs.kernel.org/ will show this function in the search
results.

Alice