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SubjectRe: [PATCH v2] remove no longer use of pdflush interface
On Tue, Jun 05, 2012 at 03:28:45PM -0700, Andrew Morton wrote:


Hi Andrew,
>I do agree with the intent of the patch and the creation of
>proc_deprecated() seems a good idea - something we can use in the
>future.

Yes, actually I think many interfaces in /proc which will be removed in
the near future can take advantage of this function.

>
>> --- a/kernel/sysctl.c
>> +++ b/kernel/sysctl.c
>> @@ -1095,11 +1095,9 @@ static struct ctl_table vm_table[] = {
>> .extra1 = &zero,
>> },
>> {
>> - .procname = "nr_pdflush_threads",
>> - .data = &nr_pdflush_threads,
>> - .maxlen = sizeof nr_pdflush_threads,
>> - .mode = 0444 /* read-only*/,
>> - .proc_handler = proc_dointvec,
>> + .procname = "nr_pdflush_threads",
>> + .mode = 0444 /* read-only */,
>> + .proc_handler = proc_deprecated,
>> },
>> {
>> .procname = "swappiness",
>> @@ -2505,6 +2503,15 @@ int proc_doulongvec_ms_jiffies_minmax(struct ctl_table *table, int write,
>>
>> #endif /* CONFIG_PROC_SYSCTL */
>>
>> +/* notice associated proc deprecated */
>> +int proc_deprecated(struct ctl_table *table, int write,
>> + void __user *buffer, size_t *lenp, loff_t *ppos)
>> +{
>> + printk(KERN_WARNING "%s exported in /proc is deprecated\n",
>> + table->procname);
>> + return -ENOSYS;
>> +}
>
>I see a couple of things here.
>
>Firstly, I'd change the text from "deprecated" to "is scheduled for
>removal". Which implies that the function should be called
>proc_obsolete().
>
>
>Secondly, this code will permit unprivileged users to flood the logs,
>by repeatedly reading /proc/sys/vm/nr_pdflush_threads. We try to avoid
>this, as it is a form of denial-of-service attack.
>
>This is a bit hard to fix. The typical way of addressing this is to
>use printk_once(), so the message only appears once per boot. But that
>doesn't work for a generic function - we'd need to add one bit of state
>to the ctl_table to do this. We can of course do that, but it's not
>obvious that it's _worth_ doing that just for handling obsolete
>entries.
>
>So perhaps the solution is to give up on the generic proc_obsolete()
>idea, and just add a handler specifically for nr_pdflush_threads, whcih
>uses printk_once().

What about modify the generic proc_obsolete just to put the warning message into
the buffer, then transfer to userspace, in order to users can see this warning.Do
you think this is a better idea?

Regards,
Wanpeng



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