Re: [PATCH] xfs: Replace strncpy with strscpy

[Darrick J. Wong]

On Wed, Jul 16, 2025 at 03:20:37PM -0300, Marcelo Moreira wrote:
> The `strncpy` function is deprecated for NUL-terminated strings as
> explained in the "strncpy() on NUL-terminated strings" section of
> Documentation/process/deprecated.rst.
> 
> In `xrep_symlink_salvage_inline()`, the `target_buf` (which is `sc->buf`)
> is intended to hold a NUL-terminated symlink path. The original code
> used `strncpy(target_buf, ifp->if_data, nr)`, where `nr` is the maximum
> number of bytes to copy. This approach is problematic because `strncpy()`
> does not guarantee NUL-termination if the source string is truncated
> exactly at `nr` bytes, which can lead to out-of-bounds read issues
> if the buffer is later treated as a NUL-terminated string.
> Evidence from `fs/xfs/scrub/symlink.c` (e.g., `strnlen(sc->buf,
> XFS_SYMLINK_MAXLEN)`) confirms that `sc->buf` is indeed expected to be
> NUL-terminated. Furthermore, `sc->buf` is allocated with
> `kvzalloc(XFS_SYMLINK_MAXLEN + 1, ...)`, explicitly reserving space for
> the NUL terminator.
> 
> `strscpy()` is the proper replacement because it guarantees NUL-termination
> of the destination buffer, correctly handles the copy limit, and aligns
> with current kernel string-copying best practices.
> Other recommended functions like `strscpy_pad()`, `memcpy()`, or
> `memcpy_and_pad()` were not used because:
> - `strscpy_pad()` would unnecessarily zero-pad the entire buffer beyond the
>   NUL terminator, which is not required as the function returns `nr` bytes.
> - `memcpy()` and `memcpy_and_pad()` do not guarantee NUL-termination, which
>   is critical given `target_buf` is used as a NUL-terminated string.
> 
> This change improves code safety and clarity by using a safer function for
> string copying.

Did you find an actual bug in online fsck, or is this just
s/strncpy/strscpy/ ?  If the code already works correctly, please leave
it alone.  Unless you want to take on all the online fsck and fuzz
testing to make sure the changes don't break anything.

--D

> Signed-off-by: Marcelo Moreira <marcelomoreira1905@xxxxxxxxx>
> ---
>  fs/xfs/scrub/symlink_repair.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/fs/xfs/scrub/symlink_repair.c b/fs/xfs/scrub/symlink_repair.c
> index 953ce7be78dc..ce21c7f0ef54 100644
> --- a/fs/xfs/scrub/symlink_repair.c
> +++ b/fs/xfs/scrub/symlink_repair.c
> @@ -185,7 +185,7 @@ xrep_symlink_salvage_inline(
>  		return 0;
>  
>  	nr = min(XFS_SYMLINK_MAXLEN, xfs_inode_data_fork_size(ip));
> -	strncpy(target_buf, ifp->if_data, nr);
> +	strscpy(target_buf, ifp->if_data, XFS_SYMLINK_MAXLEN + 1);
>  	return nr;
>  }
>  
> -- 
> 2.50.0
> 
>