Re: [PATCH 1/3] comm: Introduce comm_lock spinlock to protecttask->comm access

From: David Rientjes
Date: Tue May 17 2011 - 18:56:53 EST

On Tue, 17 May 2011, Peter Zijlstra wrote:

> The changelog also fails to mention _WHY_ this is no longer true. Nor
> does it treat why making it true again isn't an option.

It's been true since:

Author: john stultz <johnstul@xxxxxxxxxx>
Date: Mon Dec 14 18:00:05 2009 -0800

procfs: allow threads to rename siblings via /proc/pid/tasks/tid/comm

Although at the time it appears that nobody was concerned about races so
proper syncronization was never implemented. We always had the
prctl(PR_SET_NAME) so the majority of comm reads, those to current,
required no locking, but this commit changed that. The remainder of comm
dereferences always required task_lock() and the helper get_task_comm() to
read the string into a (usually stack-allocated) buffer.

> Who is changing another task's comm? That's just silly.

I agree, and I suggested taking write privileges away from /proc/pid/comm,
but others find that it is useful to be able to differentiate between
threads in the same thread group without using the prctl() for debugging?
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