Re: Reproducible builds - supporting relative paths in *-prefix-map

[Mark Wielaard <mark@klomp.org>]

Hi Richard,

On Mon, Aug 15, 2022 at 09:29:03PM +0100, Richard Purdie wrote:
> On Mon, 2022-08-15 at 21:55 +0200, Mark Wielaard wrote:
> > I might be misinterpreting the issue you are seeing.
> > 
> > But one problem with debuginfo/DWARF is that relative source paths
> > aren't clearly defined. If you move or install the executable or
> > (split) debug file out of the build directory a DWARF reader has no
> > way to know what the paths are relative to.
> > 
> > So for DWARF the paths always have to be absolute (they can still be
> > relative to the compilation dir (DW_AT_comp_dir), but at least that
> > has to be absolute (and the compiler should turn any relative path
> > into an absolute one or make sure they are relative to an absolute
> > compilation directory path).
> 
> It gets slightly more complicated as we build in a directory separate
> to the source where we can. Some source files are generated source
> files and placed in the build directory whilst many are in the source
> directory. DW_AT_comp_dir can be set to one or the other but it is the
> relative path between build and source which is problematic.

Could you give an example directory structure when that creates a
problem? Any debug paths generated should be absolute or relative to
an absolute path.  In particular the DW_AT_comp_dir is normally an
absolute path (getpwd). So normally just remapping some prefix of the
pwd (either srcdir and/or builddir) should work to keep the relative
paths correct when moving the (generated) sources under that new
absolute path. I am probably missing something about the directory
setup that makes that reasoning invalid.

> We split the debuginfo into a separate package. We also look at the
> sources it references and those go into a different separate package
> too. We support populating a remote debuginfod server with these or
> installing them onto the target.

Nice. How do you lookup the referenced sources and where are they
then installed?

> > Using known absolute paths generated with debugedit or
> > -fdebug-prefix-map makes sure the paths used in the debuginfo/DWARF
> > are always the same independent from the current srcdir or builddir to
> > make them reproducible. And the user/tools don't have to guess what
> > the relative paths are relative to.
> 
> We have that working and set debug-prefix-map today. What is
> problematic is trying to recreate the relative paths on target between
> our source and build directories. Currently, most generated files in
> the build directory just don't get handled correctly on target. We'd
> like to fix that. There is currently no way to remap a relative path
> though, at least as far as I could determine.

So is the problem that when collecting the generated source files you
cannot map the debug-prefix-map back anymore? The relative paths are
relative to the original (absolute) prefix, but you only have the
remapped prefix?

Cheers,

Mark