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    Subject[PATCH v7 01/12] driver: platform: Add helper for safer setting of driver_override
    Date
    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).

    Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
    ---
    drivers/base/driver.c | 69 +++++++++++++++++++++++++++++++++
    drivers/base/platform.c | 28 ++-----------
    include/linux/device/driver.h | 2 +
    include/linux/platform_device.h | 6 ++-
    4 files changed, 80 insertions(+), 25 deletions(-)

    diff --git a/drivers/base/driver.c b/drivers/base/driver.c
    index 8c0d33e182fd..1b9d47b10bd0 100644
    --- 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 8cc272fd5c99..b684157b7f2f 100644
    --- 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 15e7c5e15d62..700453017e1c 100644
    --- 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 7c96f169d274..582d83ed9a91 100644
    --- 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;
    --
    2.32.0
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