On Tue, 5 Jun 2012 17:26:11 +0800
Wanpeng Li <liwp.linux@xxxxxxxxx> wrote:
> From: Wanpeng Li <liwp@xxxxxxxxxxxxxxxxxx>
>
> ------------------
>
> Change in v2:
>
> * add printk warning
> * add description in Documentation
>
This is not a changelog. The patch should be accompanied by text
explaining the reasons for the removal, the back-compatibility issues,
how they were addressed, etc.
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.
> --- 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().
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