driver: platform: Add helper for safer setting of driver_override

Several core drivers and buses expect that driver_override is a
dynamically allocated memory thus later they can kfree() it.

However such assumption is not documented, there were in the past and
there are already users setting it to a string literal. This leads to
kfree() of static memory during device release (e.g. in error paths or
during unbind):

    kernel BUG at ../mm/slub.c:3960!
    Internal error: Oops - BUG: 0 [#1] PREEMPT SMP ARM
    ...
    (kfree) from [<c058da50>] (platform_device_release+0x88/0xb4)
    (platform_device_release) from [<c0585be0>] (device_release+0x2c/0x90)
    (device_release) from [<c0a69050>] (kobject_put+0xec/0x20c)
    (kobject_put) from [<c0f2f120>] (exynos5_clk_probe+0x154/0x18c)
    (exynos5_clk_probe) from [<c058de70>] (platform_drv_probe+0x6c/0xa4)
    (platform_drv_probe) from [<c058b7ac>] (really_probe+0x280/0x414)
    (really_probe) from [<c058baf4>] (driver_probe_device+0x78/0x1c4)
    (driver_probe_device) from [<c0589854>] (bus_for_each_drv+0x74/0xb8)
    (bus_for_each_drv) from [<c058b48c>] (__device_attach+0xd4/0x16c)
    (__device_attach) from [<c058a638>] (bus_probe_device+0x88/0x90)
    (bus_probe_device) from [<c05871fc>] (device_add+0x3dc/0x62c)
    (device_add) from [<c075ff10>] (of_platform_device_create_pdata+0x94/0xbc)
    (of_platform_device_create_pdata) from [<c07600ec>] (of_platform_bus_create+0x1a8/0x4fc)
    (of_platform_bus_create) from [<c0760150>] (of_platform_bus_create+0x20c/0x4fc)
    (of_platform_bus_create) from [<c07605f0>] (of_platform_populate+0x84/0x118)
    (of_platform_populate) from [<c0f3c964>] (of_platform_default_populate_init+0xa0/0xb8)
    (of_platform_default_populate_init) from [<c01031f8>] (do_one_initcall+0x8c/0x404)

Provide a helper which clearly documents the usage of driver_override.
This will allow later to reuse the helper and reduce the amount of
duplicated code.

Convert the platform driver to use a new helper and make the
driver_override field const char (it is not modified by the core).

Reviewed-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Acked-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20220419113435.246203-2-krzysztof.kozlowski@linaro.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/base/driver.c b/drivers/base/driver.c
index 8c0d33e182fd5b0781aa7ba5e811da418fee168a..1b9d47b10bd0a88411ed3b7bfc5d5583e632f0b3 100644 (file)
--- a/drivers/base/driver.c
+++ b/drivers/base/driver.c
@@ -30,6 +30,75 @@ static struct device *next_device(struct klist_iter *i)
        return dev;
 }
 
+/**
+ * driver_set_override() - Helper to set or clear driver override.
+ * @dev: Device to change
+ * @override: Address of string to change (e.g. &device->driver_override);
+ *            The contents will be freed and hold newly allocated override.
+ * @s: NUL-terminated string, new driver name to force a match, pass empty
+ *     string to clear it ("" or "\n", where the latter is only for sysfs
+ *     interface).
+ * @len: length of @s
+ *
+ * Helper to set or clear driver override in a device, intended for the cases
+ * when the driver_override field is allocated by driver/bus code.
+ *
+ * Returns: 0 on success or a negative error code on failure.
+ */
+int driver_set_override(struct device *dev, const char **override,
+                       const char *s, size_t len)
+{
+       const char *new, *old;
+       char *cp;
+
+       if (!override || !s)
+               return -EINVAL;
+
+       /*
+        * The stored value will be used in sysfs show callback (sysfs_emit()),
+        * which has a length limit of PAGE_SIZE and adds a trailing newline.
+        * Thus we can store one character less to avoid truncation during sysfs
+        * show.
+        */
+       if (len >= (PAGE_SIZE - 1))
+               return -EINVAL;
+
+       if (!len) {
+               /* Empty string passed - clear override */
+               device_lock(dev);
+               old = *override;
+               *override = NULL;
+               device_unlock(dev);
+               kfree(old);
+
+               return 0;
+       }
+
+       cp = strnchr(s, len, '\n');
+       if (cp)
+               len = cp - s;
+
+       new = kstrndup(s, len, GFP_KERNEL);
+       if (!new)
+               return -ENOMEM;
+
+       device_lock(dev);
+       old = *override;
+       if (cp != s) {
+               *override = new;
+       } else {
+               /* "\n" passed - clear override */
+               kfree(new);
+               *override = NULL;
+       }
+       device_unlock(dev);
+
+       kfree(old);
+
+       return 0;
+}
+EXPORT_SYMBOL_GPL(driver_set_override);
+
 /**
  * driver_for_each_device - Iterator for devices bound to a driver.
  * @drv: Driver we're iterating.

diff --git a/drivers/base/platform.c b/drivers/base/platform.c
index 8cc272fd5c99aefb601c312e87632288e1ad7382..b684157b7f2f9c01a147f4a51b1f4a33effcbace 100644 (file)
--- a/drivers/base/platform.c
+++ b/drivers/base/platform.c
@@ -1275,31 +1275,11 @@ static ssize_t driver_override_store(struct device *dev,
                                     const char *buf, size_t count)
 {
        struct platform_device *pdev = to_platform_device(dev);
-       char *driver_override, *old, *cp;
-
-       /* We need to keep extra room for a newline */
-       if (count >= (PAGE_SIZE - 1))
-               return -EINVAL;
-
-       driver_override = kstrndup(buf, count, GFP_KERNEL);
-       if (!driver_override)
-               return -ENOMEM;
-
-       cp = strchr(driver_override, '\n');
-       if (cp)
-               *cp = '\0';
-
-       device_lock(dev);
-       old = pdev->driver_override;
-       if (strlen(driver_override)) {
-               pdev->driver_override = driver_override;
-       } else {
-               kfree(driver_override);
-               pdev->driver_override = NULL;
-       }
-       device_unlock(dev);
+       int ret;
 
-       kfree(old);
+       ret = driver_set_override(dev, &pdev->driver_override, buf, count);
+       if (ret)
+               return ret;
 
        return count;
 }

diff --git a/include/linux/device/driver.h b/include/linux/device/driver.h
index 15e7c5e15d629545a757752bf61ffd26c8f24f4b..700453017e1c721b012059993079638d625e4284 100644 (file)
--- a/include/linux/device/driver.h
+++ b/include/linux/device/driver.h
@@ -151,6 +151,8 @@ extern int __must_check driver_create_file(struct device_driver *driver,
 extern void driver_remove_file(struct device_driver *driver,
                               const struct driver_attribute *attr);
 
+int driver_set_override(struct device *dev, const char **override,
+                       const char *s, size_t len);
 extern int __must_check driver_for_each_device(struct device_driver *drv,
                                               struct device *start,
                                               void *data,

diff --git a/include/linux/platform_device.h b/include/linux/platform_device.h
index 7c96f169d2740ca7c843e24ba76d3d925dedb936..582d83ed9a91fb451e0e4dd5b7ddfc83eb999ec8 100644 (file)
--- a/include/linux/platform_device.h
+++ b/include/linux/platform_device.h
@@ -31,7 +31,11 @@ struct platform_device {
        struct resource *resource;
 
        const struct platform_device_id *id_entry;
-       char *driver_override; /* Driver name to force a match */
+       /*
+        * Driver name to force a match.  Do not set directly, because core
+        * frees it.  Use driver_set_override() to set or clear it.
+        */
+       const char *driver_override;
 
        /* MFD cell pointer */
        struct mfd_cell *mfd_cell;