Re: [PATCH] coverage: filter out lib{fdt,elf}-temp.o

[Anthony PERARD <anthony.perard@xxxxxxxxxx>]

On Thu, Jan 18, 2024 at 02:12:21PM +0100, Jan Beulich wrote:
> On 18.01.2024 13:06, Michal Orzel wrote:
> > At the moment, trying to run xencov read/reset (calling SYSCTL_coverage_op
> > under the hood) results in a crash. This is due to an attempt to
> > access code in the .init.* sections (libfdt for Arm and libelf for x86)
> > that are stripped after boot. Normally, the build system compiles any
> > *.init.o file without COV_FLAGS. However, these two libraries are
> > handled differently as sections will be renamed to init after linking.
> > 
> > This worked until e321576f4047 ("xen/build: start using if_changed")
> > that added lib{fdt,elf}-temp.o to extra-y. Any file listed there without
> > *.init.o suffix will be part of non-init-objects for which COV_FLAGS
> > will be appended.
> 
> While this is true, aiui COV_FLAGS would be empty for anything listed
> in nocov-y and all of the prerequisites of those objects (iirc target-
> specific variable settings propagate to prerequisites). Therefore ...
> 
> > In such case, the solution is to add a file to nocov-y.
> 
> ... libelf.o / libfdt.o already being listed there ought to suffice.
> Alternatively listing only libelf-temp.o / libfdt-temp.o ought to
> suffice as well.
> 
> Since you apparently observed things not working, I must be missing
> something.

Yes, $(extra-y) is like $(obj-y), but objects there will not be added
"built_in.o". So, make is likely building "libelf-temp.o" and deps
because it's in $(extra-y) rather than because it's a prerequisite of
"libelf.o". We could ask make to process prerequisite in a reverse
order, and suddenly, the command line to make all "libelf-*.o" is
different: `make --shuffle=reverse V=2`.

So, adding extra object to $(nocov-y) is a workaround, but I think a
better fix would be to add those objects to $(targets) instead of
$(extra-y). I think I've made a mistake by using $(extra-y) instead of
$(targets) in that original commit.

Cheers,

-- 
Anthony PERARD