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SubjectRe: [PATCH] Allow threads to rename siblings via /proc/pid/tasks/tid/comm
FromAndi Kleen <>
DateSat, 07 Nov 2009 23:52:20 +0100
john stultz <johnstul@us.ibm.com> writes:
> - strlcpy(tsk->comm, buf, sizeof(tsk->comm));
> +
> + /*
> + * Threads may access current->comm without holding
> + * the task lock, so write the string carefully
> + * to avoid non-terminating reads. Readers without a lock
> + * will get the oldname, the newname or an empty string.
> + */
> + tsk->comm[0] = 0;
> + wmb();
> + strlcpy(tsk->comm+1, buf+1, sizeof(tsk->comm)-1);
> + wmb();
> + tsk->comm[0] = buf[0];

Is this really safe?

reader writer


read comm[0]
set comm[0] to 0
overwrites comm[1]
read comm[1]
read comm[2]
writes comm[2] to 0
read comm[3]

...
goes beyond the end

Better way probably is to replace tsk->comm with a pointer
and exchange that using xchg. Drawback: 4-8 bytes more per task.

Or perhaps make comm one byte longer and make sure the last
byte is always 0, but the drawback is that a reader can
read random (but at least safe) junk then.

-Andi

--
ak@linux.intel.com -- Speaking for myself only.


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