linux-kernel - Re: [PATCH 2/3] printk: Add %ptc to safely print a task's comm

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Date:	Tue, 10 May 2011 17:51:30 -0700
From:	Joe Perches <joe@...ches.com>
To:	John Stultz <john.stultz@...aro.org>
Cc:	LKML <linux-kernel@...r.kernel.org>, Ted Ts'o <tytso@....edu>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	David Rientjes <rientjes@...gle.com>,
	Dave Hansen <dave@...ux.vnet.ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org
Subject: Re: [PATCH 2/3] printk: Add %ptc to safely print a task's comm

On Tue, 2011-05-10 at 17:23 -0700, John Stultz wrote:
> Acessing task->comm requires proper locking. However in the past
> access to current->comm could be done without locking. This
> is no longer the case, so all comm access needs to be done
> while holding the comm_lock.
> 
> In my attempt to clean up unprotected comm access, I've noticed
> most comm access is done for printk output. To simpify correct
> locking in these cases, I've introduced a new %ptc format,
> which will safely print the corresponding task's comm.

Hi John.

Couple of tyops for Accessing and simplify in your commit message
and a few comments on the patch.

Could misuse of %ptc (not using current) cause system lockup?

> Example use:
> printk("%ptc: unaligned epc - sending SIGBUS.\n", current);
 

> diff --git a/lib/vsprintf.c b/lib/vsprintf.c
> index bc0ac6b..b9c97b8 100644
> --- a/lib/vsprintf.c
> +++ b/lib/vsprintf.c
> @@ -797,6 +797,26 @@ char *uuid_string(char *buf, char *end, const u8 *addr,
>  	return string(buf, end, uuid, spec);
>  }
>  
> +static noinline_for_stack
> +char *task_comm_string(char *buf, char *end, u8 *addr,
> +			 struct printf_spec spec, const char *fmt)

addr should be void * not u8 *

> +{
> +	struct task_struct *tsk = (struct task_struct *) addr;

no cast.

Maybe it'd be better to use current inside this routine and not
pass the pointer at all.

static noinline_for_stack
char *task_comm_string(char *buf, char *end,
		       struct printf_spec spec, const char *fmt)

> +	char *ret;
> +	unsigned long seq;
> +
> +	do {
> +		seq = read_seqbegin(&tsk->comm_lock);
> +
> +		ret = string(buf, end, tsk->comm, spec);
> +
> +	} while (read_seqretry(&tsk->comm_lock, seq));


> @@ -864,6 +884,12 @@ char *pointer(const char *fmt, char *buf, char *end, void *ptr,
>  	}
>  
>  	switch (*fmt) {
> +	case 't':
> +		switch (fmt[1]) {
> +		case 'c':
> +			return task_comm_string(buf, end, ptr, spec, fmt);

maybe
			return task_comm_string(buf, end, spec, fmt);


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