On 28/05/2026 5:52 pm, Runyu Xiao wrote:

The etb10 miscdevice uses drvdata->reading as a shared exclusivity gate for userspace buffer access. etb_open() claims that gate with local_cmpxchg(), and etb_release() clears it with local_set().

That gate is shared per-device state rather than CPU-local state. A running system can reach it whenever /dev/<etb> is opened, closed, and reopened by different tasks while the device remains registered, so the same drvdata->reading variable may be claimed on one CPU and later cleared on another.

This code used to use atomic_t for the same gate, but commit 27b10da8fff2 ("coresight: etb10: moving to local atomic operations") changed it to local_t even though the access pattern remained cross-task and cross-CPU. Restore atomic_t together with atomic_cmpxchg() and atomic_set() so the exclusivity gate again uses a primitive intended for shared state.

The issue was found on Linux v6.18.21 by our static analysis tool while scanning surviving local_t-on-shared-state sites, and then manually reviewed against the live etb10 file-op path.

It was runtime-validated with a reproducible QEMU no-device KCSAN PoC that kept the same report-local contract:

1. use one shared struct etb_drvdata carrier and its drvdata->reading gate;
2. call etb_open() and etb_release() sequentially on that gate to confirm the original claim/clear path;
3. bind the open side to CPU0 and the release side to CPU1 for the same gate to show cross-CPU ownership;
4. run bound workers that repeatedly race etb_open() and etb_release() on the same gate until KCSAN reports a target hit.

The harness recorded:

L1 passed open=1 release=1 reading_after_open=1 reading_after_release=0 L2 passed open_cpu=0 release_cpu=1 cross_cpu_release=1 reading_after=0 open_ret=0

Representative KCSAN excerpt from the no-device validation run:

BUG: KCSAN: data-race in etb_open.constprop.0.isra.0 [vuln_msv]

write to 0xffffffffc0003810 of 4 bytes by task 216 on cpu 1: etb_open.constprop.0.isra.0+0x38/0x80 [vuln_msv] l3_worker_thread_fn+0x4f/0xf0 [vuln_msv] kthread+0x17e/0x1c0 ret_from_fork+0x22/0x30

read to 0xffffffffc0003810 of 4 bytes by task 215 on cpu 0: etb_open.constprop.0.isra.0+0x18/0x80 [vuln_msv] l3_worker_thread_fn+0x4f/0xf0 [vuln_msv] kthread+0x17e/0x1c0 ret_from_fork+0x22/0x30

value changed: 0x00000000 -> 0x00000001

Reported by Kernel Concurrency Sanitizer on: CPU: 0 PID: 215 Comm: etb10_l3_a Tainted: G O 6.1.66 #2

This no-device harness is not a real ETB10 hardware end-to-end run, but it preserves the same shared drvdata->reading gate and the same etb_open()/etb_release() claim/clear contract. No real ETB10 hardware was available for runtime testing.

Build-tested with: make olddefconfig make -j"$(nproc)" drivers/hwtracing/coresight/coresight-etb10.o

Fixes: 27b10da8fff2 ("coresight: etb10: moving to local atomic operations") Cc: stable@vger.kernel.org Signed-off-by: Runyu Xiao runyu.xiao@seu.edu.cn

---

drivers/hwtracing/coresight/coresight-etb10.c | 6 +++--- 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/drivers/hwtracing/coresight/coresight-etb10.c b/drivers/hwtracing/coresight/coresight-etb10.c index 35db1b6093d1..98269ea6f7ae 100644 --- a/drivers/hwtracing/coresight/coresight-etb10.c +++ b/drivers/hwtracing/coresight/coresight-etb10.c @@ -85,7 +85,7 @@ struct etb_drvdata { struct coresight_device *csdev; struct miscdevice miscdev; raw_spinlock_t spinlock;

• local_t reading;

• atomic_t reading; pid_t pid; u8 *buf; u32 buffer_depth;

@@ -603,7 +603,7 @@ static int etb_open(struct inode *inode, struct file *file) struct etb_drvdata *drvdata = container_of(file->private_data, struct etb_drvdata, miscdev);

• if (local_cmpxchg(&drvdata->reading, 0, 1))

• if (atomic_cmpxchg(&drvdata->reading, 0, 1)) return -EBUSY;

dev_dbg(&drvdata->csdev->dev, "%s: successfully opened\n", __func__); @@ -641,7 +641,7 @@ static int etb_release(struct inode *inode, struct file *file) { struct etb_drvdata *drvdata = container_of(file->private_data, struct etb_drvdata, miscdev);

• local_set(&drvdata->reading, 0);

• atomic_set(&drvdata->reading, 0);

dev_dbg(&drvdata->csdev->dev, "%s: released\n", __func__); return 0;

Reviewed-by: James Clark james.clark@linaro.org

Semi-related to this change, etb_read() doesn't have any lock when reading drvdata->buffer_dept or drvdata->buf. It locks in etb_dump(), but then unlocks before actually calling copy_to_user().

Seems like concurrent calls to etb_read() might end up with corrupt data, although I'm not sure if that would ever happen in practice because it only allows one open file handle.