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

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Message-ID: <20250716182220.203631-1-marcelomoreira1905@gmail.com>
Date: Wed, 16 Jul 2025 15:20:37 -0300
From: Marcelo Moreira <marcelomoreira1905@...il.com>
To: cem@...nel.org,
	linux-xfs@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	skhan@...uxfoundation.org,
	linux-kernel-mentees@...ts.linuxfoundation.org
Subject: [PATCH] xfs: Replace strncpy with strscpy

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.

Signed-off-by: Marcelo Moreira <marcelomoreira1905@...il.com>
---
 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