[rb-general] [PATCH] gcc: read -fdebug-prefix-map OLD from environment (improved reproducibility)

[Daniel Kahn Gillmor]

Work on the reproducible-builds project [0] has identified that build
paths are one cause of output variation between builds.  This
changeset allows users to avoid this variation when building C objects
with debug symbols, while leaving the default behavior unchanged.

Background
----------

gcc includes the build path in any generated DWARF debugging symbols,
specifically in DW_AT_comp_dir, but allows the embedded path to be
changed via -fdebug-prefix-map.

When -fdebug-prefix-map is used with the current build path, it
removes the build path from DW_AT_comp_dir but places it instead in
DW_AT_producer, so the reproducibility problem isn't resolved.

When building software for binary redistribution, the actual build
path on the build machine is irrelevant, and doesn't need to be
exposed in the debug symbols.

Resolution
----------

This patch extends the first argument to -fdebug-prefix-map ("old") to
be able to read from the environment, which allows a packager to avoid
embedded build paths in the debugging symbols with something like:

  export SOURCE_BUILD_DIR="$(pwd)"
  gcc -fdebug-prefix-map=\$SOURCE_BUILD_DIR=/usr/src

Details
-------

Specifically, if the first character of the "old" argument is a
literal $, then gcc will treat it as an environment variable name, and
use the value of the env var for prefix mapping.

As a result, DW_AT_producer contains the literal envvar name,
DW_AT_comp_dir contains the transformed build path, and the actual
build path is not at all present in the generated object file.

This has been tested successfully on amd64 machines, and i see no
reason why it would be platform-specific.

More discussion of alternate approaches considered and discarded in
the development of this change can be found at [1] for those
interested.

Feedback welcome!

[0] https://reproducible-builds.org
[1] https://lists.alioth.debian.org/pipermail/reproducible-builds/Week-of-Mon-20151130/004051.html
---
 gcc/doc/invoke.texi |  4 +++-
 gcc/final.c         | 27 +++++++++++++++++++++++++--
 2 files changed, 28 insertions(+), 3 deletions(-)

diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
index 5256031..234432f 100644
--- a/gcc/doc/invoke.texi
+++ b/gcc/doc/invoke.texi
@@ -6440,7 +6440,9 @@ link processing time.  Merging is enabled by default.
 @item -fdebug-prefix-map=@var{old}=@var{new}
 @opindex fdebug-prefix-map
 When compiling files in directory @file{@var{old}}, record debugging
-information describing them as in @file{@var{new}} instead.
+information describing them as in @file{@var{new}} instead.  If
+ at file{@var{old}} starts with a @samp{$}, the corresponding environment
+variable will be dereferenced, and its value will be used instead.
 
 @item -fno-dwarf2-cfi-asm
 @opindex fdwarf2-cfi-asm
diff --git a/gcc/final.c b/gcc/final.c
index 8cb5533..bc43b61 100644
--- a/gcc/final.c
+++ b/gcc/final.c
@@ -1525,6 +1525,9 @@ add_debug_prefix_map (const char *arg)
 {
   debug_prefix_map *map;
   const char *p;
+  char *env;
+  const char *old;
+  size_t oldlen;
 
   p = strchr (arg, '=');
   if (!p)
@@ -1532,9 +1535,29 @@ add_debug_prefix_map (const char *arg)
       error ("invalid argument %qs to -fdebug-prefix-map", arg);
       return;
     }
+  if (*arg == '$')
+    {
+      env = xstrndup (arg+1, p - (arg+1));
+      old = getenv(env);
+      if (!old)
+	{
+	  warning (0, "environment variable %qs not set in argument to "
+		   "-fdebug-prefix-map", env);
+	  free(env);
+	  return;
+	}
+      oldlen = strlen(old);
+      free(env);
+    }
+  else
+    {
+      old = xstrndup (arg, p - arg);
+      oldlen = p - arg;
+    }
+
   map = XNEW (debug_prefix_map);
-  map->old_prefix = xstrndup (arg, p - arg);
-  map->old_len = p - arg;
+  map->old_prefix = old;
+  map->old_len = oldlen;
   p++;
   map->new_prefix = xstrdup (p);
   map->new_len = strlen (p);
-- 
2.6.2