[PATCH AUTOSEL 6.17-5.4] 9p: fix /sys/fs/9p/caches overwriting itself

[Sasha Levin <sashal@kernel.org>]

From: "Randall P. Embry" <rpembry@gmail.com>

[ Upstream commit 86db0c32f16c5538ddb740f54669ace8f3a1f3d7 ]

caches_show() overwrote its buffer on each iteration,
so only the last cache tag was visible in sysfs output.

Properly append with snprintf(buf + count, …).

Signed-off-by: Randall P. Embry <rpembry@gmail.com>
Message-ID: <20250926-v9fs_misc-v1-2-a8b3907fc04d@codewreck.org>
Signed-off-by: Dominique Martinet <asmadeus@codewreck.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---

LLM Generated explanations, may be completely bogus:

YES. The change in `fs/9p/v9fs.c:563` switches the sysfs formatter from
`snprintf(buf, …)` to `snprintf(buf + count, …)`, so each cache tag is
appended rather than overwriting the start of the buffer. Without this
adjustment, `/sys/fs/9p/caches` only ever reported the final cache tag,
which is a real user-visible bug for multi-session configurations and
makes the sysfs knob effectively unusable. The fix is a one-line
adjustment behind `CONFIG_9P_FSCACHE`, touches no other logic, and keeps
the existing `count`/`limit` bookkeeping, so the regression risk is
negligible. There are no prerequisites or follow-up changes in this
area, making it an ideal low-risk candidate for the stable trees.

 fs/9p/v9fs.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/9p/v9fs.c b/fs/9p/v9fs.c
index 77e9c4387c1df..714cfe76ee651 100644
--- a/fs/9p/v9fs.c
+++ b/fs/9p/v9fs.c
@@ -561,7 +561,7 @@ static ssize_t caches_show(struct kobject *kobj,
 	spin_lock(&v9fs_sessionlist_lock);
 	list_for_each_entry(v9ses, &v9fs_sessionlist, slist) {
 		if (v9ses->cachetag) {
-			n = snprintf(buf, limit, "%s\n", v9ses->cachetag);
+			n = snprintf(buf + count, limit, "%s\n", v9ses->cachetag);
 			if (n < 0) {
 				count = n;
 				break;
-- 
2.51.0