Re: [RFC PATCH 1/3] modpost: fix TO_NATIVE() with expressions and consts

[Alexander Lobakin]

From: Greg KH <gregkh@xxxxxxxxxxxxxxxxxxx>
Date: Thu, 18 Aug 2022 14:26:14 +0200

> On Thu, Aug 18, 2022 at 01:53:04PM +0200, Alexander Lobakin wrote:
> > Macro TO_NATIVE() directly takes a reference to its argument @x
> > without making an intermediate variable. This makes compilers
> > emit build warnings and errors if @x is an expression or a deref
> > of a const pointer (when target Endianness != host Endianness):
> > 
> > >> scripts/mod/modpost.h:87:18: error: lvalue required as unary '&' operand
> >       87 |         __endian(&(x), &(__x), sizeof(__x));                    \
> >          |                  ^
> >    scripts/mod/sympath.c:19:25: note: in expansion of macro 'TO_NATIVE'
> >       19 | #define t(x)            TO_NATIVE(x)
> >          |                         ^~~~~~~~~
> >    scripts/mod/sympath.c:100:31: note: in expansion of macro 't'
> >      100 |                 eh->e_shoff = t(h(eh->e_shoff) + off);
> > 
> > >> scripts/mod/modpost.h:87:24: warning: passing argument 2 of '__endian'
> > discards 'const' qualifier from pointer target type [-Wdiscarded-qualifiers]
> >       87 |         __endian(&(x), &(__x), sizeof(__x));                    \
> >          |                        ^~~~~~
> >    scripts/mod/sympath.c:18:25: note: in expansion of macro 'TO_NATIVE'
> >       18 | #define h(x)            TO_NATIVE(x)
> >          |                         ^~~~~~~~~
> >    scripts/mod/sympath.c:178:48: note: in expansion of macro 'h'
> >      178 |              iter < end; iter = (void *)iter + h(eh->e_shentsize)) {
> 
> How come this hasn't shown up in cross-builds today?

It doesn't happen with the current code.

> 
> 
> > 
> > Create a temporary variable, assign @x to it and don't use @x after
> > that. This makes it possible to pass expressions as an argument.
> > Also, do a cast-away for the second argument when calling __endian()
> > to avoid 'discarded qualifiers' warning, as typeof() preserves
> > qualifiers and makes compilers think that we're passing pointer
> > to a const.
> > 
> > Reported-by: kernel test robot <lkp@xxxxxxxxx>
> > Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
> > Cc: stable@xxxxxxxxxxxxxxx # 4.9+
> 
> Where are these build warnings showing up at that we don't see them
> today, yet this is needed to go back to all stable trees?

I thought all fixes should go to the applicable stable trees, am I
wrong? If so, I'll drop the tag in the next spin.

I remember we had such discussion already regarding fixing stuff in
modpost, which can happen only with never mainlained GCC LTO or with
the in-dev code. At the end that fix made it into the stables IIRC.

> 
> still confused,
> 
> greg k-h

Thanks,
Olek