On Thu, Aug 18, 2022 at 04:01:53PM +0200, Alexander Lobakin wrote:
> 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.
Great, so there is no bug that you are trying to fix :)
> > > 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.
But this isn't fixing a bug in the code today that anyone can hit, so
why would you mark it as such?
> 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.
I don't remember taking fixes for out-of-tree LTO stuff, but I shouldn't
have :)
thanks,
greg k-h
