linux-kernel - Re: [PATCH] coresight: Fix erroneous memset in tmc_read_unprepare

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Date:	Sun, 12 Jun 2016 15:06:32 -0600
From:	Mathieu Poirier <mathieu.poirier@...aro.org>
To:	Suzuki K Poulose <suzuki.poulose@....com>
Cc:	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] coresight: Fix erroneous memset in tmc_read_unprepare_etr

On 10 June 2016 at 04:31, Suzuki K Poulose <suzuki.poulose@....com> wrote:
> At the end of a trace collection, we try to clear the entire buffer
> and enable the ETR back if it was already enabled. But, we would have
> adjusted the drvdata->buf to point to the beginning of the trace data
> in the trace buffer @drvdata->vaddr. So, the following code which
> clears the buffer is dangerous and can cause crashes, like below :
>
>         memset(drvdata->buf, 0, drvdata->size);
>
>  Unable to handle kernel paging request at virtual address ffffff800a145000
>  pgd = ffffffc974726000
>  *pgd=00000009f3e91003, *pud=00000009f3e91003, *pmd=0000000000000000
>  PREEMPT SMP
>  Modules linked in:
>  CPU: 4 PID: 1692 Comm: dd Not tainted 4.7.0-rc2+ #1721
>  Hardware name: ARM Juno development board (r0) (DT)
>  task: ffffffc9734a0080 ti: ffffffc974460000 task.ti: ffffffc974460000
>  PC is at __memset+0x1ac/0x200
>  LR is at tmc_read_unprepare_etr+0x144/0x1bc
>  pc : [<ffffff80083a05ac>] lr : [<ffffff800859c984>] pstate: 200001c5
>  ...
>  [<ffffff80083a05ac>] __memset+0x1ac/0x200
>  [<ffffff800859b2e4>] tmc_release+0x90/0x94
>  [<ffffff8008202f58>] __fput+0xa8/0x1ec
>  [<ffffff80082030f4>] ____fput+0xc/0x14
>  [<ffffff80080c3ef8>] task_work_run+0xb0/0xe4
>  [<ffffff8008088bf4>] do_notify_resume+0x64/0x6c
>  [<ffffff8008084d5c>] work_pending+0x10/0x14
>  Code: 91010108 54ffff4a 8b040108 cb050042 (d50b7428)
>
> Since we clear the buffer anyway in the following call to
> tmc_etr_enable_hw(), remove the erroneous memset().
>
> Fixes: commit de5461970b3e9e1 ("coresight: tmc: allocating memory when needed")
> Cc: Mathieu Poirier <mathieu.poirier@...aro.org>
> Signed-off-by: Suzuki K Poulose <suzuki.poulose@....com>
> ---
>
> Mathieu,
>
> I think this should go to 4.7. Please push it.

If this [1] didn't make it, this one won't make it either.  I've
applied it to my tree for merging in the 4.8 cycle.

Thanks,
Mathieu

[1]. https://patchwork.kernel.org/patch/9144589/

>
> ---
>  drivers/hwtracing/coresight/coresight-tmc-etr.c | 9 +++------
>  1 file changed, 3 insertions(+), 6 deletions(-)
>
> diff --git a/drivers/hwtracing/coresight/coresight-tmc-etr.c b/drivers/hwtracing/coresight/coresight-tmc-etr.c
> index 69e104b..24d99ed 100644
> --- a/drivers/hwtracing/coresight/coresight-tmc-etr.c
> +++ b/drivers/hwtracing/coresight/coresight-tmc-etr.c
> @@ -306,13 +306,10 @@ int tmc_read_unprepare_etr(struct tmc_drvdata *drvdata)
>         if (local_read(&drvdata->mode) == CS_MODE_SYSFS) {
>                 /*
>                  * The trace run will continue with the same allocated trace
> -                * buffer. As such zero-out the buffer so that we don't end
> -                * up with stale data.
> -                *
> -                * Since the tracer is still enabled drvdata::buf
> -                * can't be NULL.
> +                * buffer. The trace buffer is cleared in tmc_etr_enable_hw(),
> +                * so we don't have to explicitly clear it. Also, since the

Yes, very good.

> +                * tracer is still enabled drvdata::buf can't be NULL.
>                  */
> -               memset(drvdata->buf, 0, drvdata->size);
>                 tmc_etr_enable_hw(drvdata);
>         } else {
>                 /*
> --
> 1.9.1
>