Re: [Patch][RFC] Disabling per-tgid stats on task exit in taskstats

[Shailabh Nagar <nagar@watson.ibm.com>]

Paul Jackson wrote:

> Shailabh wrote:
>  
>
> > I don't know if there are buffer overflow 
> > issues in passing a string
> >    
>
> I don't know if this comment applies to "the standard netlink way of
> passing it up using NLA_STRING", but the way I deal with buffer length
> issues in the cpuset code is to insist that the user code express the
> list in no fewer than 100 + 6 * NR_CPUS bytes:
>
> From kernel/cpuset.c:
>
>        /* Crude upper limit on largest legitimate cpulist user might write. */
>        if (nbytes > 100 + 6 * NR_CPUS)
>                return -E2BIG;
>
> This lets the user specify the buffer size passed in, but prevents
> them from trying a denial of service attack on the kernel by trying
> to pass in a huge buffer.
>
> If the user can't figure out how to write the desired cpulist in
> that size, then tough toenails.
>  
Paul,

Perhaps I should use the the other ascii format for specifying cpumasks 
since its more amenable
to specifying an upper bound for the length of the ascii string and is 
more compact ?

That format (the one used in lib/bitmap.c:bitmap_parse) is comma 
separated chunks of hex digits
with each chunk specifying 32 bits of the desired cpumask.

So
((NR_CPUS + 32) / 32) * 8 + 1
(8 hex characters for each 32 cpus, and 1 extra character for null 
terminator)
would be an upper bound that would accomodate all the cpus for sure.

Thoughts ?

--Shailabh

--Shailabh
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