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

[Richard Purdie]

On Mon, 2022-08-15 at 17:15 +0000, Joseph Myers wrote:
> On Mon, 15 Aug 2022, Richard Purdie via Gcc wrote:
> 
> > Currently it works well for absolute paths but if a file uses a
> > relative path or a path with a symlink in, or a non-absolute path, it
> > will miss those cases. For relative paths in particular it is
> > problematic as you can't easily construct a compiler commandline that
> > would cover all relative path options.
> 
> I'd expect a relative path to be naturally relocatable without needing to 
> be remapped.  (For example, DW_AT_comp_dir would be relocated in debug 
> info, but there would be no need to relocate the paths to individual files 
> that are relative to DW_AT_comp_dir.)
> 
> Is the issue that you're using relative paths between two directories that 
> don't have a fixed relative path between them, such as between the build 
> and source directories, as opposed to relative paths within the source 
> directory or within the build directory?

Yes, that is the issue. We build with separate build and source
directories with a relative path between them and that relation doesn't
work on target, particularly for installed binaries.

It is a general problem but particularly where we build multiple
components from the one shared source tree as those paths become
multiple levels deep which makes creating a source layout on target to
match even harder.

> > which address a realpath() call into the prefix mapping code. I did
> 
> That would run the risk of breaking relocation for anyone who has 
> deliberately used the paths they pass to the compiler (possibly involving 
> symlinks, for example) in their remapping options - not expecting a 
> further level of processing to be applied to those paths before remapping.

It would be a change in behaviour but I'm not sure how many people
would be relying on that or if they even should be. A relative path
would only be relocated if it did match something in the prefix-map
variables so this would be controllable.

Cheers,

Richard