Commit 5304877936c0 ("NFSD: Fix strncpy() fortify warning") replaced
strncpy(,, sizeof(..)) with strlcpy(,, sizeof(..) - 1), but strlcpy()
already guaranteed NUL-termination of the destination buffer and
subtracting one byte potentially truncated the source string.

The incorrect size was then carried over in commit 72f78ae00a8e ("NFSD:
move from strlcpy with unused retval to strscpy") when switching from
strlcpy() to strscpy().

Fix this off-by-one error by using the full size of the destination
buffer again.

Cc: stable@vger.kernel.org
Fixes: 5304877936c0 ("NFSD: Fix strncpy() fortify warning")
Signed-off-by: Thorsten Blum <thorsten.blum@linux.dev>
---
Changes in v2:
- Use three parameter variant of strscpy() for easier backporting
- Link to v1: https://lore.kernel.org/lkml/20250805175302.29386-2-thorsten.blum@linux.dev/
---
 fs/nfsd/nfs4proc.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/nfsd/nfs4proc.c b/fs/nfsd/nfs4proc.c
index 71b428efcbb5..954543e92988 100644
--- a/fs/nfsd/nfs4proc.c
+++ b/fs/nfsd/nfs4proc.c
@@ -1469,7 +1469,7 @@ static __be32 nfsd4_ssc_setup_dul(struct nfsd_net *nn, char *ipaddr,
 		return 0;
 	}
 	if (work) {
-		strscpy(work->nsui_ipaddr, ipaddr, sizeof(work->nsui_ipaddr) - 1);
+		strscpy(work->nsui_ipaddr, ipaddr, sizeof(work->nsui_ipaddr));
 		refcount_set(&work->nsui_refcnt, 2);
 		work->nsui_busy = true;
 		list_add_tail(&work->nsui_list, &nn->nfsd_ssc_mount_list);
-- 
2.50.1

From: Chuck Lever <chuck.lever@oracle.com>

On Wed, 06 Aug 2025 03:10:01 +0200, Thorsten Blum wrote:
> Commit 5304877936c0 ("NFSD: Fix strncpy() fortify warning") replaced
> strncpy(,, sizeof(..)) with strlcpy(,, sizeof(..) - 1), but strlcpy()
> already guaranteed NUL-termination of the destination buffer and
> subtracting one byte potentially truncated the source string.
> 
> The incorrect size was then carried over in commit 72f78ae00a8e ("NFSD:
> move from strlcpy with unused retval to strscpy") when switching from
> strlcpy() to strscpy().
> 
> [...]

Applied to nfsd-testing, thanks!

[1/1] NFSD: Fix destination buffer size in nfsd4_ssc_setup_dul()
      commit: 387d7905fb60d28ae3f2ff8956de626c797e4508

--
Chuck Lever