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

From: Shailabh Nagar
Date: Mon Jul 03 2006 - 20:07:58 EST


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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