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

[Richard Purdie <richard.purdie@linuxfoundation.org>]

On Mon, 2022-08-15 at 12:13 +0100, Richard Purdie via Gcc wrote:
> Hi,
> 
> I'm wondering if we'd be able to improve path handling in the -f*-
> prefix-map compiler options to cover relative paths?
> 
> 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.
> 
> At first glance this is relatively straight forward, for example:
> 
> Index: gcc-12.1.0/gcc/file-prefix-map.cc
> ===================================================================
> --- gcc-12.1.0.orig/gcc/file-prefix-map.cc
> +++ gcc-12.1.0/gcc/file-prefix-map.cc
> @@ -70,19 +70,25 @@ remap_filename (file_prefix_map *maps, c
>    file_prefix_map *map;
>    char *s;
>    const char *name;
> +  char *realname;
>    size_t name_len;
>  
> +  realname = lrealpath (filename);
> +
>    for (map = maps; map; map = map->next)
> -    if (filename_ncmp (filename, map->old_prefix, map->old_len) == 0)
> +    if (filename_ncmp (realname, map->old_prefix, map->old_len) == 0)
>        break;
> -  if (!map)
> +  if (!map) {
> +    free (realname);
>      return filename;
> -  name = filename + map->old_len;
> +  }
> +  name = realname + map->old_len;
>    name_len = strlen (name) + 1;
>  
>    s = (char *) ggc_alloc_atomic (name_len + map->new_len);
>    memcpy (s, map->new_prefix, map->new_len);
>    memcpy (s + map->new_len, name, name_len);
> +  free (realname);
>    return s;
>  }
>  
> which address a realpath() call into the prefix mapping code. I did
> experiment with this and found it breaks compiling ruby and xen-tools
> which both have code which does:
> 
> #include __FILE__
> 
> It may be possible to make the remapping conditional of not being
> directly in a #include statement but I didn't find the gcc code
> responsible for that as yet. I also noticed some valgrind tests fails
> after it, I've not looked into why that would be yet.
> 
> I wanted to ask if there would be any interest in adding support for
> something like this? I suspect the include/__FILE__ issue is probably a
> latent bug anyway. If anyone has any pointers to the code I could
> improve my patch with I'm also happy to have them!

To answer my own question, something like:

+Index: gcc-12.1.0/libcpp/macro.cc
+===================================================================
+--- gcc-12.1.0.orig/libcpp/macro.cc
++++ gcc-12.1.0/libcpp/macro.cc
+@@ -563,7 +563,7 @@ _cpp_builtin_macro_text (cpp_reader *pfi
+           if (!name)
+             abort ();
+         }
+-      if (pfile->cb.remap_filename)
++      if (pfile->cb.remap_filename && !pfile->state.in_directive)
+         name = pfile->cb.remap_filename (name);
+       len = strlen (name);
+       buf = _cpp_unaligned_alloc (pfile, len * 2 + 3);

seems to do roughly what I was wondering about.

I'd be interested to understand whether some patch along the lines I've
mentioned here would stand a chance of being accepted or not.

Cheers,

Richard