linux-kernel - Re: [PATCH 05/10] of: Fix available buffer size calculating error in API of_device_uevent

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Message-ID: <CAL_Jsq+Aap9Kjzcd5H5m9ArcXMWRogoT0CdPnz4-d8OeRgadUA@mail.gmail.com>
Date: Tue, 10 Dec 2024 08:10:18 -0600
From: Rob Herring <robh@...nel.org>
To: Zijun Hu <zijun_hu@...oud.com>
Cc: Saravana Kannan <saravanak@...gle.com>, Leif Lindholm <leif.lindholm@...aro.org>, 
	Stephen Boyd <stephen.boyd@...aro.org>, Maxime Ripard <mripard@...nel.org>, 
	Robin Murphy <robin.murphy@....com>, Grant Likely <grant.likely@...retlab.ca>, 
	devicetree@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Zijun Hu <quic_zijuhu@...cinc.com>
Subject: Re: [PATCH 05/10] of: Fix available buffer size calculating error in
 API of_device_uevent_modalias()

On Tue, Dec 10, 2024 at 6:39 AM Zijun Hu <zijun_hu@...oud.com> wrote:
>
> On 2024/12/10 04:34, Rob Herring wrote:
> > On Thu, Dec 5, 2024 at 6:53 PM Zijun Hu <zijun_hu@...oud.com> wrote:
> >>
> >> From: Zijun Hu <quic_zijuhu@...cinc.com>
> >>
> >> of_device_uevent_modalias() saves MODALIAS value from offset
> >> (@env->buflen - 1), so the available buffer size should be
> >> (sizeof(@env->buf) - @env->buflen + 1), but it uses the wrong
> >> size (sizeof(@env->buf) - @env->buflen).
> >>
> >> Fix by using right size (sizeof(@env->buf) - @env->buflen + 1).
> >
> > Just writing what the diff says already is not that useful. The key
> > part you need to know is why we back up by 1 character to begin with.
> >
>
> will correct commit message in v2.
>
> >>
> >> Fixes: dd27dcda37f0 ("of/device: merge of_device_uevent")
> >> Signed-off-by: Zijun Hu <quic_zijuhu@...cinc.com>
> >> ---
> >>  drivers/of/device.c | 4 ++--
> >>  1 file changed, 2 insertions(+), 2 deletions(-)
> >>
> >> diff --git a/drivers/of/device.c b/drivers/of/device.c
> >> index edf3be1972658f6dc165f577da53b10c7eebc116..ee29c07c83b9e6abd9b1c7747dd341026bc79eb0 100644
> >> --- a/drivers/of/device.c
> >> +++ b/drivers/of/device.c
> >> @@ -266,10 +266,10 @@ int of_device_uevent_modalias(const struct device *dev, struct kobj_uevent_env *
> >>                 return -ENOMEM;
> >>
> >>         sl = of_modalias(dev->of_node, &env->buf[env->buflen-1],
> >
> > This could use a comment why we back up by 1. Better to put it in a
> > variable than add/subtract 1 everywhere:
> >
> > /* After add_uevent_event(), buflen is at character after the nul char
> > which needs to be overwritten */
> > buflen = env->buflen - 1;
> >
> > And then use 'buflen' throughout.
> >
>
> good proposal. may use it for v2 after discussion done.
>
> >> -                        sizeof(env->buf) - env->buflen);
> >> +                        sizeof(env->buf) - env->buflen + 1);
> >>         if (sl < 0)
> >>                 return sl;
> >> -       if (sl >= (sizeof(env->buf) - env->buflen))
> >> +       if (sl >= (sizeof(env->buf) - env->buflen + 1))
> >>                 return -ENOMEM;
> >
> > There's another potential problem. If we return before here, we end up
> > with "OF_MODALIAS=\0" or "OF_MODALIAS=some-non-nul-terminated-str".
> > Maybe that doesn't matter? I haven't looked at the caller.
> >
>
> that does not matter since current logic follows below 2 rules
>
> 1) all strings in @env->buf always terminated with '\0'.

Ah, right. However, we still end up with a truncated value though it
is nul terminated.

> 2) both env->buflen and env->envp_idx are not updated once @env->buf
> does not enough spaces then failed.
>
> current logic has no difference with normal add_uevent_var() usage.

There is one major difference. add_uevent_var() will not output
anything if the whole string doesn't fit. Whereas we might output a
truncated value because the add_uevent_var() call updated env->buflen
and env->envp_idx. We could unwind that I suppose, but that involves
even more mucking with the internals of the env struct.

> > I think a better solution to all this would be making add_uevent_var
> > work to construct the full value. We could add "%pOFm" format that
> > calls of_mod_alias(). Then this function becomes just:
> >
> of_modalias() ?

Yes.

>
> agree with you.
> for good practice, users should only use uevent APIs and should not
> depend on  uevent's internal implementation.
>
> > return add_uevent_var(env, "MODALIAS=%pOFm");
> >
> > And of_device_modalias() can be:
> >
> > sl = snprintf(str, len, "%pOFm\n", dev->of_node);
> > if (sl >= len)
> >   return -ENOMEM;
> > return sl;
> >
>
> looks good.
>
> of_request_module() provides another solution.

Yes, that can be simplified to just a kasnprintf() call.

Rob