linux-kernel - [PATCH] nvmem: core: Fix race in nvmem

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Date:   Wed, 14 Dec 2022 23:29:53 +0900
From:   Hector Martin <marcan@...can.st>
To:     Srinivas Kandagatla <srinivas.kandagatla@...aro.org>
Cc:     Sven Peter <sven@...npeter.dev>,
        Alyssa Rosenzweig <alyssa@...enzweig.io>,
        asahi@...ts.linux.dev, linux-kernel@...r.kernel.org,
        Hector Martin <marcan@...can.st>
Subject: [PATCH] nvmem: core: Fix race in nvmem_register()

nvmem_register() currently registers the device before adding the nvmem
cells, which creates a race window where consumers may find the nvmem
device (and not get PROBE_DEFERred), but then fail to find the cells and
error out.

Move device registration to the end of nvmem_register(), to close the
race.

Observed when the stars line up on Apple Silicon machines with the (not
yet upstream, but trivial) spmi nvmem driver and the macsmc-rtc client:

[    0.487375] macsmc-rtc macsmc-rtc: error -ENOENT: Failed to get rtc_offset NVMEM cell

Fixes: eace75cfdcf7 ("nvmem: Add a simple NVMEM framework for nvmem providers")
Signed-off-by: Hector Martin <marcan@...can.st>
---
 drivers/nvmem/core.c | 18 ++++++++----------
 1 file changed, 8 insertions(+), 10 deletions(-)

diff --git a/drivers/nvmem/core.c b/drivers/nvmem/core.c
index 321d7d63e068..d255feca5e17 100644
--- a/drivers/nvmem/core.c
+++ b/drivers/nvmem/core.c
@@ -837,20 +837,16 @@ struct nvmem_device *nvmem_register(const struct nvmem_config *config)

 	dev_dbg(&nvmem->dev, "Registering nvmem device %s\n", config->name);

-	rval = device_register(&nvmem->dev);
-	if (rval)
-		goto err_put_device;
-
 	if (nvmem->nkeepout) {
 		rval = nvmem_validate_keepouts(nvmem);
 		if (rval)
-			goto err_device_del;
+			return ERR_PTR(rval);
 	}

 	if (config->compat) {
 		rval = nvmem_sysfs_setup_compat(nvmem, config);
 		if (rval)
-			goto err_device_del;
+			return ERR_PTR(rval);
 	}

 	if (config->cells) {
@@ -867,19 +863,21 @@ struct nvmem_device *nvmem_register(const struct nvmem_config *config)
 	if (rval)
 		goto err_remove_cells;

+	rval = device_register(&nvmem->dev);
+	if (rval)
+		goto err_put_device;
+
 	blocking_notifier_call_chain(&nvmem_notifier, NVMEM_ADD, nvmem);

 	return nvmem;

+err_put_device:
+	put_device(&nvmem->dev);
 err_remove_cells:
 	nvmem_device_remove_all_cells(nvmem);
 err_teardown_compat:
 	if (config->compat)
 		nvmem_sysfs_remove_compat(nvmem, config);
-err_device_del:
-	device_del(&nvmem->dev);
-err_put_device:
-	put_device(&nvmem->dev);

 	return ERR_PTR(rval);
 }
--
2.35.1