linux-kernel - Re: [PATCH v6 2/4] drm: Add API for capturing frame CRCs

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Message-ID: <CACvgo53-BsMyejWCHT7uPWbrQjq4BxEYkdBFxVCZWy2mwSL5gw@mail.gmail.com>
Date:   Thu, 8 Sep 2016 16:08:08 +0100
From:   Emil Velikov <emil.l.velikov@...il.com>
To:     Tomeu Vizoso <tomeu.vizoso@...labora.com>
Cc:     Jonathan Corbet <corbet@....net>,
        "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
        "Linux-Kernel@...r. Kernel. Org" <linux-kernel@...r.kernel.org>,
        ML dri-devel <dri-devel@...ts.freedesktop.org>,
        Daniel Vetter <daniel.vetter@...el.com>,
        Emil Velikov <emil.velikov@...labora.com>
Subject: Re: [PATCH v6 2/4] drm: Add API for capturing frame CRCs

On 8 September 2016 at 15:49, Tomeu Vizoso <tomeu.vizoso@...labora.com> wrote:
> On 8 September 2016 at 15:24, Emil Velikov <emil.l.velikov@...il.com> wrote:
>> Hi Tomeu,
>>
>> A couple of small nitpicks and a rather nasty looking bug, related to
>> your earlier question.
>>
>> On 7 September 2016 at 11:27, Tomeu Vizoso <tomeu.vizoso@...labora.com> wrote:
>>
>>> +static ssize_t crc_control_write(struct file *file, const char __user *ubuf,
>>> +                                size_t len, loff_t *offp)
>>> +{
>>
>>> +       if (source[len] == '\n')
>>> +               source[len] = '\0';
>>> +
>> Considering the bug below, I'm considering if there's a case were we
>> don't want to explicitly set the terminating byte ?
>>
>>
>>> +/*
>>> + * 1 frame field of 10 chars plus a number of CRC fields of 10 chars each, space
>>> + * separated, with a newline at the end and null-terminated.
>>> + */
>> NULL-terminated what the things that was missing, explaining the
>> maths. Yet note the code sort of contradicts it.
>>
>> TL;DR: above we conditionally NULL terminate the data, yet (below) we
>> feed garbage (always) instead of \0 byte.
>>
>>> +#define LINE_LEN(values_cnt)   (10 + 11 * values_cnt + 1 + 1)
>>> +#define MAX_LINE_LEN           (LINE_LEN(DRM_MAX_CRC_NR))
>>> +
>>> +static ssize_t crtc_crc_read(struct file *filep, char __user *user_buf,
>>> +                            size_t count, loff_t *pos)
>>> +{
>>> +       struct drm_crtc *crtc = filep->f_inode->i_private;
>>> +       struct drm_crtc_crc *crc = &crtc->crc;
>>> +       struct drm_crtc_crc_entry *entry;
>>> +       char buf[MAX_LINE_LEN];
>> Here buf is filled with garbage...
>>
>>
>>> +       if (entry->has_frame_counter)
>>> +               sprintf(buf, "0x%08x", entry->frame);
>>> +       else
>>> +               sprintf(buf, "XXXXXXXXXX");
>>> +
>>> +       for (i = 0; i < crc->values_cnt; i++)
>>> +               sprintf(buf + 10 + i * 11, " 0x%08x", entry->crcs[i]);
>>> +       sprintf(buf + 10 + crc->values_cnt * 11, "\n");
>>> +
>> ... and now all the data is in, incl. the \n byte.
>>
>>> +       if (copy_to_user(user_buf, buf, LINE_LEN(crc->values_cnt)))
>> And here we copy the whole thing incl. the 'should be \0 but is
>> actually garbage' byte.
>
> As discussed offline, sprintf does terminate the string for us, so I
> think this is fine.
>
Forget I said anything /me hides in shame

>> This doesn't look good :-\
>>
>>> --- a/drivers/gpu/drm/drm_drv.c
>>> +++ b/drivers/gpu/drm/drm_drv.c
>>
>>> @@ -221,6 +222,14 @@ static int drm_minor_register(struct drm_device *dev, unsigned int type)
>>>                 return ret;
>>>         }
>>>
>>> +       if (type == DRM_MINOR_PRIMARY) {
>>> +               drm_for_each_crtc(crtc, dev) {
>>> +                       ret = drm_debugfs_crtc_add(crtc);
>>> +                       if (ret)
>>> +                               DRM_ERROR("DRM: Failed to initialize CRC debugfs.\n");
>>> +               }
>>> +       }
>>> +
>> Minor: We're missing teardown in the error path.
>
> Isn't drm_debugfs_cleanup taking care of it?
>
Maybe ? It goes through the debugfs_list and pulls down individual
files. Yet most/all of drm users of debugfs_create_dir explicitly
cleanup after themselves via debugfs_remove_recursive() so I'd play
along.

Thanks
Emil