Re: [Xen-devel] [RFC v3 10/13] jump_label: port __jump_table to linker tables

["Luis R. Rodriguez" <mcgrof@xxxxxxxxxx>]

On Fri, Jul 22, 2016 at 04:49:45PM -0500, Josh Poimboeuf wrote:
> On Fri, Jul 22, 2016 at 02:24:44PM -0700, Luis R. Rodriguez wrote:
> > diff --git a/tools/objtool/special.c b/tools/objtool/special.c
> > index bff8abb3a4aa..f0ad369f994b 100644
> > --- a/tools/objtool/special.c
> > +++ b/tools/objtool/special.c
> > @@ -26,6 +26,10 @@
> >  #include "special.h"
> >  #include "warn.h"
> >  
> > +#include "../../include/asm-generic/sections.h"
> > +#include "../../include/asm-generic/tables.h"
> > +#include "../../include/linux/stringify.h"
> > +
> >  #define EX_ENTRY_SIZE              12
> >  #define EX_ORIG_OFFSET             0
> >  #define EX_NEW_OFFSET              4
> > @@ -63,7 +67,9 @@ struct special_entry entries[] = {
> >             .feature = ALT_FEATURE_OFFSET,
> >     },
> >     {
> > -           .sec = "__jump_table",
> > +           .sec = __stringify(SECTION_TBL(SECTION_DATA,
> > +                                          __jump_table,
> > +                                          SECTION_ORDER_ANY)),
> >             .jump_or_nop = true,
> >             .size = JUMP_ENTRY_SIZE,
> >             .orig = JUMP_ORIG_OFFSET,
> 
> (continuing our discussion from another thread...)
> 
> I think tools code isn't allowed to include kernel files because the
> tools subdirectory is supposed to be completely independent.

That was also the case for userpsace tools in scripts/ -- for instance
scripts/mod/modpost.c made an exception. What I've proposed here to
keep things simple was to ensure -UKERNEL is passed and then only
include files we know will work.

Refer to the patch "kprobes: port .kprobes.text to section range"
in this series for an extension of the scripts/mod/modpost.c kernel
headers use.

> As far as I can tell, the section name will always be
> ".data.tbl.__jump_table.any".  Is that true?  If so, any reason why we
> can't just hard-code the string here?

Its a fair strategy, however if upstream changes the order name we'd
have to hand code this as well, by using a macro we keep it all in one
place.

> As you saw, if the string
> changes, objtool will complain and 0-day will report it.

Indeed, which is why I was hoping to instead stick to a standard
sections set of header files that lets us get these in on place.
The only place I hand coded in this series was in the perl
file scripts/recordmcount.pl but I suppose if we wanted to get
creative we could even generate a header for it too.

If we want to avoid all this hacker include stuff another option
is to *generate* each respective sections.h for the kernel, and
also, one for tools, and one for perl. What do you think?

  Luis

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