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    Subject[PATCH 4.9 151/264] hwrng: remember rng chosen by user
    Date
    From: Harald Freudenberger <freude@linux.vnet.ibm.com>

    commit 10a515ddb5f19a1ff0b9882c430b4427843169f3 upstream.

    When a user chooses a rng source via sysfs attribute
    this rng should be sticky, even when other sources
    with better quality to register. This patch introduces
    a simple way to remember the user's choice. This is
    reflected by a new sysfs attribute file 'rng_selected'
    which shows if the current rng has been chosen by
    userspace. The new attribute file shows '1' for user
    selected rng and '0' otherwise.

    Signed-off-by: Harald Freudenberger <freude@linux.vnet.ibm.com>
    Reviewed-by: PrasannaKumar Muralidharan <prasannatsmkumar@gmail.com>
    Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
    Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    ---
    drivers/char/hw_random/core.c | 21 +++++++++++++++++++--
    1 file changed, 19 insertions(+), 2 deletions(-)

    --- a/drivers/char/hw_random/core.c
    +++ b/drivers/char/hw_random/core.c
    @@ -27,6 +27,8 @@
    #define RNG_MODULE_NAME "hw_random"

    static struct hwrng *current_rng;
    +/* the current rng has been explicitly chosen by user via sysfs */
    +static int cur_rng_set_by_user;
    static struct task_struct *hwrng_fill;
    /* list of registered rngs, sorted decending by quality */
    static LIST_HEAD(rng_list);
    @@ -303,6 +305,7 @@ static ssize_t hwrng_attr_current_store(
    list_for_each_entry(rng, &rng_list, list) {
    if (sysfs_streq(rng->name, buf)) {
    err = 0;
    + cur_rng_set_by_user = 1;
    if (rng != current_rng)
    err = set_current_rng(rng);
    break;
    @@ -351,16 +354,27 @@ static ssize_t hwrng_attr_available_show
    return strlen(buf);
    }

    +static ssize_t hwrng_attr_selected_show(struct device *dev,
    + struct device_attribute *attr,
    + char *buf)
    +{
    + return snprintf(buf, PAGE_SIZE, "%d\n", cur_rng_set_by_user);
    +}
    +
    static DEVICE_ATTR(rng_current, S_IRUGO | S_IWUSR,
    hwrng_attr_current_show,
    hwrng_attr_current_store);
    static DEVICE_ATTR(rng_available, S_IRUGO,
    hwrng_attr_available_show,
    NULL);
    +static DEVICE_ATTR(rng_selected, S_IRUGO,
    + hwrng_attr_selected_show,
    + NULL);

    static struct attribute *rng_dev_attrs[] = {
    &dev_attr_rng_current.attr,
    &dev_attr_rng_available.attr,
    + &dev_attr_rng_selected.attr,
    NULL
    };

    @@ -443,10 +457,12 @@ int hwrng_register(struct hwrng *rng)

    old_rng = current_rng;
    err = 0;
    - if (!old_rng || (rng->quality > old_rng->quality)) {
    + if (!old_rng ||
    + (!cur_rng_set_by_user && rng->quality > old_rng->quality)) {
    /*
    * Set new rng as current as the new rng source
    - * provides better entropy quality.
    + * provides better entropy quality and was not
    + * chosen by userspace.
    */
    err = set_current_rng(rng);
    if (err)
    @@ -478,6 +494,7 @@ void hwrng_unregister(struct hwrng *rng)
    list_del(&rng->list);
    if (current_rng == rng) {
    drop_current_rng();
    + cur_rng_set_by_user = 0;
    /* rng_list is sorted by quality, use the best (=first) one */
    if (!list_empty(&rng_list)) {
    struct hwrng *new_rng;

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