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SubjectRe: [PATCH v2 1/1] leds: remove PAGE_SIZE limit of /sys/class/leds/<led>/trigger
On Sat, Sep 28, 2019 at 01:47:21AM +0900, Akinobu Mita wrote:
> 2019年9月27日(金) 15:39 Greg Kroah-Hartman <gregkh@linuxfoundation.org>:
> >
> > On Sat, Sep 14, 2019 at 12:03:24AM +0900, Akinobu Mita wrote:
> > > Reading /sys/class/leds/<led>/trigger returns all available LED triggers.
> > > However, the size of this file is limited to PAGE_SIZE because of the
> > > limitation for sysfs attribute.
> > >
> > > Enabling LED CPU trigger on systems with thousands of CPUs easily hits
> > > PAGE_SIZE limit, and makes it impossible to see all available LED triggers
> > > and which trigger is currently activated.
> > >
> > > We work around it here by converting /sys/class/leds/<led>/trigger to
> > > binary attribute, which is not limited by length. This is _not_ good
> > > design, do not copy it.
> > >
> > > Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> > > Cc: "Rafael J. Wysocki" <rafael@kernel.org>
> > > Cc: Jacek Anaszewski <jacek.anaszewski@gmail.com>
> > > Cc: Pavel Machek <pavel@ucw.cz>
> > > Cc: Dan Murphy <dmurphy@ti.com>
> > > Acked-by: Pavel Machek <pavel@ucw.cz>
> > > Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
> > > ---
> > > drivers/leds/led-class.c | 8 ++--
> > > drivers/leds/led-triggers.c | 90 ++++++++++++++++++++++++++++++++++-----------
> > > drivers/leds/leds.h | 6 +++
> > > include/linux/leds.h | 5 ---
> > > 4 files changed, 79 insertions(+), 30 deletions(-)
> > >
> > > diff --git a/drivers/leds/led-class.c b/drivers/leds/led-class.c
> > > index 4793e77..8b5a1d1 100644
> > > --- a/drivers/leds/led-class.c
> > > +++ b/drivers/leds/led-class.c
> > > @@ -73,13 +73,13 @@ static ssize_t max_brightness_show(struct device *dev,
> > > static DEVICE_ATTR_RO(max_brightness);
> > >
> > > #ifdef CONFIG_LEDS_TRIGGERS
> > > -static DEVICE_ATTR(trigger, 0644, led_trigger_show, led_trigger_store);
> > > -static struct attribute *led_trigger_attrs[] = {
> > > - &dev_attr_trigger.attr,
> > > +static BIN_ATTR(trigger, 0644, led_trigger_read, led_trigger_write, 0);
> > > +static struct bin_attribute *led_trigger_bin_attrs[] = {
> > > + &bin_attr_trigger,
> > > NULL,
> > > };
> > > static const struct attribute_group led_trigger_group = {
> > > - .attrs = led_trigger_attrs,
> > > + .bin_attrs = led_trigger_bin_attrs,
> > > };
> > > #endif
> > >
> > > diff --git a/drivers/leds/led-triggers.c b/drivers/leds/led-triggers.c
> > > index 8d11a5e..ed5a311 100644
> > > --- a/drivers/leds/led-triggers.c
> > > +++ b/drivers/leds/led-triggers.c
> > > @@ -16,6 +16,7 @@
> > > #include <linux/rwsem.h>
> > > #include <linux/leds.h>
> > > #include <linux/slab.h>
> > > +#include <linux/mm.h>
> > > #include "leds.h"
> > >
> > > /*
> > > @@ -26,9 +27,11 @@ LIST_HEAD(trigger_list);
> > >
> > > /* Used by LED Class */
> > >
> > > -ssize_t led_trigger_store(struct device *dev, struct device_attribute *attr,
> > > - const char *buf, size_t count)
> > > +ssize_t led_trigger_write(struct file *filp, struct kobject *kobj,
> > > + struct bin_attribute *bin_attr, char *buf,
> > > + loff_t pos, size_t count)
> > > {
> > > + struct device *dev = kobj_to_dev(kobj);
> > > struct led_classdev *led_cdev = dev_get_drvdata(dev);
> > > struct led_trigger *trig;
> > > int ret = count;
> > > @@ -64,39 +67,84 @@ ssize_t led_trigger_store(struct device *dev, struct device_attribute *attr,
> > > mutex_unlock(&led_cdev->led_access);
> > > return ret;
> > > }
> > > -EXPORT_SYMBOL_GPL(led_trigger_store);
> > > +EXPORT_SYMBOL_GPL(led_trigger_write);
> > >
> > > -ssize_t led_trigger_show(struct device *dev, struct device_attribute *attr,
> > > - char *buf)
> > > +__printf(4, 5)
> > > +static int led_trigger_snprintf(char *buf, size_t size, bool query,
> > > + const char *fmt, ...)
> > > +{
> > > + va_list args;
> > > + int i;
> > > +
> > > + va_start(args, fmt);
> > > + if (query)
> > > + i = vsnprintf(NULL, 0, fmt, args);
> > > + else
> > > + i = vscnprintf(buf, size, fmt, args);
> > > + va_end(args);
> > > +
> > > + return i;
> > > +}
> >
> > You only call this in one place, why is it needed like this? The "old"
> > code open-coded this, what is this helping with here?
> >
> > And what does "query" mean here? I have no idea how that variable
> > matters, or what it does. Why not just test if buf is NULL or not if
> > you don't want to use it?
> >
> > Ah, you are trying to see how "long" the buffer is going to be. That
> > makes more sense, but just trigger off of the NULL buffer or not, making
> > this a bit more "obvious" what you are doing and not tieing two
> > parameters to each other (meaning one always reflects the other one).
>
> We cannot simply replace the "query" by checking the buffer is NULL or not.
> Because led_trigger_snprintf() is repeatedly called by led_trigger_format()
> while increasing 'buf' and decreasing 'size' by the return value of
> led_trigger_snprintf() every time.

Odd, but ok, I'll trust you, this looks like very odd code :(

> > > +static int led_trigger_format(char *buf, size_t size, bool query,
> > > + struct led_classdev *led_cdev)
> > > {
> > > - struct led_classdev *led_cdev = dev_get_drvdata(dev);
> > > struct led_trigger *trig;
> > > - int len = 0;
> > > + int len = led_trigger_snprintf(buf, size, query, "%s",
> > > + led_cdev->trigger ? "none" : "[none]");
> > > +
> > > + list_for_each_entry(trig, &trigger_list, next_trig) {
> > > + bool hit = led_cdev->trigger &&
> > > + !strcmp(led_cdev->trigger->name, trig->name);
> > > +
> > > + len += led_trigger_snprintf(buf + len, size - len, query,
> > > + " %s%s%s", hit ? "[" : "",
> > > + trig->name, hit ? "]" : "");
> > > + }
> > > +
> > > + len += led_trigger_snprintf(buf + len, size - len, query, "\n");
> > > +
> > > + return len;
> > > +}
> > > +
> > > +/*
> > > + * It was stupid to create 10000 cpu triggers, but we are stuck with it now.
> > > + * Don't make that mistake again. We work around it here by creating binary
> > > + * attribute, which is not limited by length. This is _not_ good design, do not
> > > + * copy it.
> > > + */
> > > +ssize_t led_trigger_read(struct file *filp, struct kobject *kobj,
> > > + struct bin_attribute *attr, char *buf,
> > > + loff_t pos, size_t count)
> > > +{
> > > + struct device *dev = kobj_to_dev(kobj);
> > > + struct led_classdev *led_cdev = dev_get_drvdata(dev);
> > > + void *data;
> > > + int len;
> > >
> > > down_read(&triggers_list_lock);
> > > down_read(&led_cdev->trigger_lock);
> > >
> > > - if (!led_cdev->trigger)
> > > - len += scnprintf(buf+len, PAGE_SIZE - len, "[none] ");
> > > + len = led_trigger_format(NULL, 0, true, led_cdev);
> > > + data = kvmalloc(len + 1, GFP_KERNEL);
> >
> > Why kvmalloc() and not just kmalloc()? How big is this buffer you are
> > expecting to have here?
>
> The ledtrig-cpu supports upto 9999 cpus. If all these cpus were available,
> the buffer size would be 78,890 bytes.
> (for i in `seq 0 9999`;do echo -n " cpu$i"; done | wc -c)
>
> The intention of this kvmalloc() allocation is to avoid costly allocation
> if possible.

Ah, forgot it could get that big.

> > > + if (data)
> > > + len = led_trigger_format(data, len + 1, false, led_cdev);
> > > else
> > > - len += scnprintf(buf+len, PAGE_SIZE - len, "none ");
> > > + len = -ENOMEM;
> > >
> > > - list_for_each_entry(trig, &trigger_list, next_trig) {
> > > - if (led_cdev->trigger && !strcmp(led_cdev->trigger->name,
> > > - trig->name))
> > > - len += scnprintf(buf+len, PAGE_SIZE - len, "[%s] ",
> > > - trig->name);
> > > - else
> > > - len += scnprintf(buf+len, PAGE_SIZE - len, "%s ",
> > > - trig->name);
> > > - }
> > > up_read(&led_cdev->trigger_lock);
> > > up_read(&triggers_list_lock);
> >
> > Two locks? Why not 3? 5? How about just 1? :)
>
> I don't touch these locks in this patch :)
>
> Locking both locks seems to be necessary to prevent someone from changing
> trigger_list or led_cdev->trigger.

Ok, it just looked odd to me.

> > > - len += scnprintf(len+buf, PAGE_SIZE - len, "\n");
> > > + if (len < 0)
> > > + return len;
> >
> > You just leaked data if led_trigger_format() returned an error :(
> >
> > Just return -ENOMEM above if !data, which makes this much simpler.
>
> We are holding the two locks, so we need to release them before return.
> Which one do you prefer?
>
> ...
> data = kvmalloc(len + 1, GFP_KERNEL);
> if (data)
> len = led_trigger_format(data, len + 1, false, led_cdev);
> else
> len = -ENOMEM;
>
> up_read(&led_cdev->trigger_lock);
> up_read(&triggers_list_lock);
>
> if (len < 0)
> return len;
>
> vs.
>
> ...
> data = kvmalloc(len + 1, GFP_KERNEL);
> if (!data) {
> up_read(&led_cdev->trigger_lock);
> up_read(&triggers_list_lock);
> return -ENOMEM;
> }
> len = led_trigger_format(data, len + 1, false, led_cdev);
>
> up_read(&led_cdev->trigger_lock);
> up_read(&triggers_list_lock);

I don't care, as long as you do not leak memory I'm happy :)

thanks,

greg k-h

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