[RFC patch] vsprintf: Allow %pe to print non PTR_ERR %pe uses as decimal

[Joe Perches]

On Wed, 2021-03-24 at 18:33 +0100, Rasmus Villemoes wrote:
> On 24/03/2021 18.20, Joe Perches wrote:
> > On Wed, 2021-03-24 at 09:52 -0700, Joe Perches wrote:
> > > On Wed, 2021-03-24 at 17:42 +0100, Arnd Bergmann wrote:
> > > > On Wed, Mar 24, 2021 at 3:20 PM Joe Perches <joe at perches.com> wrote:
> > > []
> > > > > > diff --git a/drivers/gpu/drm/imx/imx-ldb.c b/drivers/gpu/drm/imx/imx-ldb.c
> > > > > []
> > > > > > @@ -197,6 +197,12 @@ static void imx_ldb_encoder_enable(struct drm_encoder *encoder)
> > > > > >       int dual = ldb->ldb_ctrl & LDB_SPLIT_MODE_EN;
> > > > > >       int mux = drm_of_encoder_active_port_id(imx_ldb_ch->child, encoder);
> > > > > > 
> > > > > > +     if (mux < 0 || mux >= ARRAY_SIZE(ldb->clk_sel)) {
> > > > > > +             dev_warn(ldb->dev, "%s: invalid mux %d\n",
> > > > > > +                      __func__, ERR_PTR(mux));
> > > > > 
> > > > > This does not compile without warnings.
> > > > > 
> > > > > drivers/gpu/drm/imx/imx-ldb.c: In function ‘imx_ldb_encoder_enable’:
> > > > > drivers/gpu/drm/imx/imx-ldb.c:201:22: warning: format ‘%d’ expects argument of type ‘int’, but argument 4 has type ‘void *’ [-Wformat=]
> > > > >   201 |   dev_warn(ldb->dev, "%s: invalid mux %d\n",
> > > > >       |                      ^~~~~~~~~~~~~~~~~~~~~~
> > > > > 
> > > > > If you want to use ERR_PTR, the %d should be %pe as ERR_PTR
> > > > > is converting an int a void * to decode the error type and
> > > > > emit it as a string.
> > > > 
> > > > Sorry about that.
> > > > 
> > > > I decided against using ERR_PTR() in order to also check for
> > > > positive array overflow, but the version I tested was different from
> > > > the version I sent.
> > > > 
> > > > v3 coming.
> > > 
> > > Thanks.  No worries.
> > > 
> > > Up to you, vsprintf would emit the positive mux as a funky hashed
> > > hex value by default if you use ERR_PTR with mux > ARRAY_SIZE so
> > > perhaps %d without the ERR_PTR use makes the most sense.
> > > 
> 
> > 
> > Maybe it's better to output non PTR_ERR %pe uses as decimal so this
> > sort of code would work.
> 
> No, because that would leak the pointer value when somebody has
> accidentally passed a real kernel pointer to %pe.

I think it's not really an issue.

_All_ code that uses %p<foo> extensions need inspection anyway.

It's already possible to intentionally 'leak' the ptr value
by using %pe, -ptr so I think that's not really an issue.

> 
> If the code wants a cute -EFOO string explaining what's wrong, what
> about "%pe", ERR_PTR(mux < 0 : mux : -ERANGE)? Or two separate error
> messages
> 
> if (mux < 0)
>   ...
> else if (mux >= ARRAY_SIZE())
>   ...

Multiple tests, more unnecessary code, multiple format strings, etc...