Ubuntu
thinkfan package

Thinkfan will occasionally send garbage to fan control (setfan_ibm: Error writing to /proc/acpi/ibm/fan)

Bug #1494546 reported by Shuhao on 2015-09-11

This bug affects 3 people

Affects		Status	Importance	Assigned to	Milestone
	thinkfan (Ubuntu)	Confirmed	Undecided	Unassigned

Bug Description

Thinkfan, for some reason, will occasionally send garbage output to /proc/acpi/ibm/fan, which causes the error to error with invalid argument, causing thinkfan to crash.

Thinkfan sometimes will work, but other times will not. I do not know the exact condition to reproduce.

This is due to a really strange bug in the software. I'm not sure I quite figured it out, but here's an analysis anyway (patch for this at the end). Note that I only used this in thinkpad fan mode, so idk about the behaviour with just pwm.

In the file system.c, the setfan_ibm function is defined as

void setfan_ibm() {
int ibm_fan, l = strlen(cur_lvl);

if (unlikely((ibm_fan = open(IBM_FAN, O_RDWR, O_TRUNC)) < 0)) {
  prefix = "\n";
  report(LOG_ERR, LOG_ERR, IBM_FAN ": %s\n", strerror(errno));
  errcnt |= ERR_FAN_SET;
}
else {
  if (unlikely(write(ibm_fan, cur_lvl, l) < l)) {
   prefix = "\n";
   report(LOG_ERR, LOG_ERR, MSG_ERR_FANCTRL);
   errcnt |= ERR_FAN_SET;
  }
  close(ibm_fan);
}
}

Note that it is trying to write `cur_lvl` to `ibm_fan`, which is /proc/acpi/ibm/fan.

If you look around, you'll realize a couple of things:

1. setfan_ibm() and what not is actually never called directly, it is called via config->setfan(), which is also almost never called directly, once in thinkpad.c:159, thinkpad.c:42, and system.c:216. There is a macro defined at around thinkpad.c:40 known as `set_fan` that also sets the cur_lvl.
2. There are only 2 locations where `cur_lvl` is set: in the `set_fan` macro as discussed, once on main before everything boots, and once when we read the configuration.

With some printf debugging, you can see once the program boots, it will first readconfig, which will give cur_lvl a value of "127", which is not in the format of "level <i>". Sending this to /proc/acpi/ibm/fan will fail. The code does set the correct limits (config.c:222), but `cur_lvl` is never changed.

Next, `setfan()` is called directly at thinkfan.c:159, which I believe usually only happens during first boot. This means `cur_lvl` is never set to a correct value. For some reason, adding a printf with cur_lvl's value here results in the string "V". This is what I'm not quite sure about.

In any case, switching line 159 to the `set_fan` macro works.

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