linux-kernel - [PATCH 2/2] set_task_comm: kill the pointless memset() + wmb()

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Message-ID: <20130316202411.GC18613@redhat.com>
Date:	Sat, 16 Mar 2013 21:24:11 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andi Kleen <andi@...stfloor.org>,
	Lucas De Marchi <lucas.de.marchi@...il.com>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Paul Mackerras <paulus@...ba.org>, david@...son.dropbear.id.au,
	Kees Cook <keescook@...omium.org>,
	Serge Hallyn <serge.hallyn@...onical.com>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	Feng Hong <hongfeng@...vell.com>,
	Lucas De Marchi <lucas.demarchi@...fusion.mobi>
Subject: [PATCH 2/2] set_task_comm: kill the pointless memset() + wmb()

set_task_comm() does memset() + wmb() before strlcpy(). This buys
nothing but adds the confusion, the comment is wrong.

- We do not need memset() to be "safe from non-terminating string
  reads", the final char is always zero and we never change it.

- wmb() is paired with nothing, it can't not prevent from printing
  the mixture of the old/new data unless the reader takes the lock.

Signed-off-by: Oleg Nesterov <oleg@...hat.com>
---
 fs/exec.c |   10 ----------
 1 files changed, 0 insertions(+), 10 deletions(-)

diff --git a/fs/exec.c b/fs/exec.c
index bea2f7d..b270844 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1030,17 +1030,7 @@ EXPORT_SYMBOL_GPL(get_task_comm);
 void set_task_comm(struct task_struct *tsk, char *buf)
 {
 	task_lock(tsk);
-
 	trace_task_rename(tsk, buf);
-
-	/*
-	 * Threads may access current->comm without holding
-	 * the task lock, so write the string carefully.
-	 * Readers without a lock may see incomplete new
-	 * names but are safe from non-terminating string reads.
-	 */
-	memset(tsk->comm, 0, TASK_COMM_LEN);
-	wmb();
 	strlcpy(tsk->comm, buf, sizeof(tsk->comm));
 	task_unlock(tsk);
 	perf_event_comm(tsk);
-- 
1.5.5.1


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