Re: [PATCH] [v3] warn on performance-impacting configs aka. TAINT_PERFORMANCE

Ingo Molnar
2014-08-24T14:49:53Z

* Dave Hansen <dave@sr71.net> wrote:

> On 08/22/2014 12:20 AM, Ingo Molnar wrote:
> > Essentially all DEBUG_OBJECTS_* options are expensive, assuming
> > they are enabled, i.e. DEBUG_OBJECTS_ENABLE_DEFAULT=y.
> >
> > Otherwise they should only be warned about if the debugobjects
> > boot option got enabled.
> >
> > I.e. you'll need a bit of a runtime check for this one.
>
> At that point, what do we print, and when do we print it? We're not
> saying that the config option should be disabled because it's really the
> boot option plus the config option that is causing the problem.
>
> I'll just put the DEBUG_OBJECTS_ENABLE_DEFAULT in here which is
> analogous to what we're doing with SLUB_DEBUG_ON.
>
> >> +static ssize_t performance_taint_read(struct file *file, char __user *user_buf,
> >> + size_t count, loff_t *ppos)
> >> +{
> >> + int i;
> >> + int ret;
> >> + char *buf;
> >> + size_t buf_written = 0;
> >> + size_t buf_left;
> >> + size_t buf_len;
> >> +
> >> + if (!ARRAY_SIZE(perfomance_killing_configs))
> >> + return 0;
> >> +
> >> + buf_len = 1;
> >> + for (i = 0; i < ARRAY_SIZE(perfomance_killing_configs); i++)
> >> + buf_len += strlen(config_prefix) +
> >> + strlen(perfomance_killing_configs[i]);
> >> + /* Add a byte for for each entry in the array for a \n */
> >> + buf_len += ARRAY_SIZE(perfomance_killing_configs);
> >> +
> >> + buf = kmalloc(buf_len, GFP_KERNEL);
> >> + if (!buf)
> >> + return -ENOMEM;
> >> +
> >> + buf_left = buf_len;
> >> + for (i = 0; i < ARRAY_SIZE(perfomance_killing_configs); i++) {
> >> + buf_written += snprintf(buf + buf_written, buf_left,
> >> + "%s%s\n", config_prefix,
> >> + perfomance_killing_configs[i]);
> >> + buf_left = buf_len - buf_written;
> >
> > So, ARRAY_SIZE(performance_killing_configs) is written out four
> > times, a temporary variable would be in order I suspect.
>
> If one of them had gone over 80 chars, I probably would have. :) I put
> one in anyway.
>
> > Also, do you want to check buf_left and break out early from
> > the loop if it goes non-positive?
>
> You're slowly inflating my patch for no practical gain. :)

AFAICS it's a potential memory corruption and security bug,
should the array ever grow large enough to overflow the passed
in buffer size.

Thanks,

Ingo