linux-kernel - Re: [PATCH] xfs: Replace strncpy with strscpy

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Message-ID: <CAPZ3m_jXwp1FfsvtR2s3nwATT3fER=Mc6qj+GzKuUhY5tjQFNQ@mail.gmail.com>
Date: Thu, 17 Jul 2025 14:36:28 -0300
From: Marcelo Moreira <marcelomoreira1905@...il.com>
To: "Darrick J. Wong" <djwong@...nel.org>
Cc: cem@...nel.org, linux-xfs@...r.kernel.org, linux-kernel@...r.kernel.org, 
	skhan@...uxfoundation.org, linux-kernel-mentees@...ts.linuxfoundation.org
Subject: Re: [PATCH] xfs: Replace strncpy with strscpy

Em qui., 17 de jul. de 2025 às 13:39, Darrick J. Wong
<djwong@...nel.org> escreveu:
>
> 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.

This was s/strncpy/strscpy/ , no real bug.

-- 
Cheers,
Marcelo Moreira