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    SubjectRe: [PATCH v2] remove no longer use of pdflush interface
    On Tue,  5 Jun 2012 17:26:11 +0800
    Wanpeng Li <liwp.linux@gmail.com> wrote:

    > From: Wanpeng Li <liwp@linux.vnet.ibm.com>
    >
    > ------------------
    >
    > 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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