[PATCH] nvmem: Don't let a NULL cell_id for nvmem_cell_get() crash us

From: Douglas Anderson
Date: Tue May 15 2018 - 17:17:09 EST

In commit ca04d9d3e1b1 ("phy: qcom-qusb2: New driver for QUSB2 PHY on
Qcom chips") you can see a call like:

devm_nvmem_cell_get(dev, NULL);

Note that the cell ID passed to the function is NULL. This is because
the qcom-qusb2 driver is expected to work only on systems where the
PHY node is hooked up via device-tree and is nameless.

This works OK for the most part. The first thing nvmem_cell_get()
does is to call of_nvmem_cell_get() and there it's documented that a
NULL name is fine. The problem happens when the call to
of_nvmem_cell_get() returns -EINVAL. In such a case we'll fall back
to nvmem_cell_get_from_list() and eventually might (if nvmem_cells
isn't an empty list) crash with something that looks like:


There are several different ways we could fix this problem:

One could argue that perhaps the qcom-qusb2 driver should be changed
to use of_nvmem_cell_get() which is allowed to have a NULL name. In
that case, we'd need to add a patche to introduce
devm_of_nvmem_cell_get() since the qcom-qusb2 driver is using devm
managed resources.

One could also argue that perhaps we could just add a name to
qcom-qusb2. That would be OK but I believe it effectively changes the
device tree bindings, so maybe it's a no-go.

In this patch I have chosen to fix the problem by simply not crashing
when a NULL cell_id is passed to nvmem_cell_get().

NOTE: that for the qcom-qusb2 driver the "nvmem-cells" property is
defined to be optional and thus it's expected to be a common case that
we would hit this crash and this is more than just a theoretical fix.

Fixes: ca04d9d3e1b1 ("phy: qcom-qusb2: New driver for QUSB2 PHY on Qcom chips")
Signed-off-by: Douglas Anderson <dianders@xxxxxxxxxxxx>

drivers/nvmem/core.c | 4 ++++
1 file changed, 4 insertions(+)

diff --git a/drivers/nvmem/core.c b/drivers/nvmem/core.c
index b5b0cdc21d01..514d1dfc5630 100644
--- a/drivers/nvmem/core.c
+++ b/drivers/nvmem/core.c
@@ -936,6 +936,10 @@ struct nvmem_cell *nvmem_cell_get(struct device *dev, const char *cell_id)
return cell;

+ /* NULL cell_id only allowed for device tree; invalid otherwise */
+ if (!cell_id)
+ return ERR_PTR(-EINVAL);
return nvmem_cell_get_from_list(cell_id);