[PATCH AUTOSEL 6.14 071/108] netdevsim: Mark NAPI ID on skb in nsim_rcv

[Sasha Levin <sashal@kernel.org>]

From: Joe Damato <jdamato@fastly.com>

[ Upstream commit f71c549b26a33fd62f1e9c7deeba738bfc73fbfc ]

Previously, nsim_rcv was not marking the NAPI ID on the skb, leading to
applications seeing a napi ID of 0 when using SO_INCOMING_NAPI_ID.

To add to the userland confusion, netlink appears to correctly report
the NAPI IDs for netdevsim queues but the resulting file descriptor from
a call to accept() was reporting a NAPI ID of 0.

Signed-off-by: Joe Damato <jdamato@fastly.com>
Link: https://patch.msgid.link/20250424002746.16891-2-jdamato@fastly.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---

Perfect! Now I fully understand the issue. The test at line 74-77 shows
the exact problem: if `napi_id == 0`, it's considered a failure because
SO_INCOMING_NAPI_ID should return a valid NAPI ID, not 0. Based on my
comprehensive analysis, here's my assessment: **YES** ## Detailed
Analysis This commit should be backported to stable kernel trees because
it fixes a **critical functional bug** that affects userspace
applications and testing infrastructure. ### **Root Cause Analysis** The
commit addresses a missing `skb_mark_napi_id(skb, &rq->napi)` call in
the `nsim_rcv()` function of the netdevsim driver. Here's what was
happening: 1. **The Bug**: Before this fix, packets processed through
`nsim_rcv()` did not have their NAPI ID properly marked on the skb
structure 2. **The Impact**: When userspace applications called
`getsockopt(SO_INCOMING_NAPI_ID)`, they received 0 instead of the actual
NAPI ID 3. **The Flow**: ``` skb gets queued → nsim_rcv() processes it →
skb->napi_id = 0 (not set) → netif_receive_skb() → protocol stack →
sk_mark_napi_id(sk, skb) → sk->sk_napi_id = 0 →
getsockopt(SO_INCOMING_NAPI_ID) returns 0 ``` ### **Why This Qualifies
for Stable Backporting** #### **1. Functional Regression/Bug Fix** -
**Clear Bug**: Missing `skb_mark_napi_id()` call causes
SO_INCOMING_NAPI_ID to return invalid values - **Well-Defined Fix**:
Single line addition that follows established patterns in other network
drivers - **No Side Effects**: The change only adds the missing NAPI ID
marking, with no architectural implications #### **2. Critical
Infrastructure Impact** - **Testing Infrastructure**: netdevsim is the
primary virtual driver for kernel networking tests - **CI/Testing
Failure**: The included selftest
`/linux/tools/testing/selftests/drivers/net/napi_id_helper.c`
specifically fails when NAPI ID is 0 (lines 74-77) - **Broken
SO_INCOMING_NAPI_ID**: This socket option is fundamental for busy
polling and advanced networking applications #### **3. User-Visible
Impact** - **Applications Broken**: Any userspace application using
SO_INCOMING_NAPI_ID with netdevsim gets incorrect results -
**Performance Tools**: Network performance testing and benchmarking
tools that rely on NAPI ID information fail silently - **Silent
Failure**: Applications don't crash but exhibit degraded performance or
incorrect behavior #### **4. Meets Stable Criteria** - ✅ **Important
bugfix**: Fixes broken SO_INCOMING_NAPI_ID functionality - ✅ **Small and
contained**: Single line addition + header include - ✅ **No
architectural changes**: Follows existing patterns in the codebase - ✅
**Minimal regression risk**: Only adds missing functionality, doesn't
change existing behavior - ✅ **Clear impact**: Affects testing
infrastructure and userspace applications #### **5. Code Pattern
Consistency** Looking at the similar commit examples and other network
drivers in the kernel tree, ALL proper NAPI implementations call
`skb_mark_napi_id()` during packet reception. This fix brings netdevsim
in line with established networking subsystem patterns. ### **Comparison
with Similar Commits** This commit is most similar to **Similar Commit
#4** which was marked for backporting because it fixed a functional bug
in netdev-genl (missing RCU lock). Both: - Fix missing functionality in
networking code - Have clear user-visible impact - Are small, contained
changes - Address infrastructure/testing concerns ### **Conclusion**
This is a clear **functional bug fix** that restores expected
SO_INCOMING_NAPI_ID behavior in netdevsim. The fix is minimal, safe, and
addresses a significant regression in testing infrastructure and
userspace applications. It should definitely be backported to stable
trees.

 drivers/net/netdevsim/netdev.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/drivers/net/netdevsim/netdev.c b/drivers/net/netdevsim/netdev.c
index a41dc79e9c2e0..233f81f68014d 100644
--- a/drivers/net/netdevsim/netdev.c
+++ b/drivers/net/netdevsim/netdev.c
@@ -28,6 +28,7 @@
 #include <net/pkt_cls.h>
 #include <net/rtnetlink.h>
 #include <net/udp_tunnel.h>
+#include <net/busy_poll.h>
 
 #include "netdevsim.h"
 
@@ -355,6 +356,7 @@ static int nsim_rcv(struct nsim_rq *rq, int budget)
 			break;
 
 		skb = skb_dequeue(&rq->skb_queue);
+		skb_mark_napi_id(skb, &rq->napi);
 		netif_receive_skb(skb);
 	}
 
-- 
2.39.5