From 30908556fece379ffd7c0da96c774d8bd297e459 Mon Sep 17 00:00:00 2001
From: Sergei Trofimovich <siarheit@google.com>
Date: Fri, 22 Sep 2023 22:41:49 +0100
Subject: [PATCH] gcc/file-prefix-map.cc: always mangle __FILE__ into invalid
 store path

Without the change `__FILE__` used in static inline functions in headers
embed paths to header files into executable images. For local headers
it's not a problem, but for headers in `/nix/store` this causes `-dev`
inputs to be retained in runtime closure.

Typical examples are `nix` -> `nlohmann_json` and `pipewire` ->
`lttng-ust.dev`.

For this reason we want to remove the occurrences of hashes in the
expansion of `__FILE__`. `nuke-references` does it by replacing hashes
by `eeeeee...`. It is handy to be able to invert the transformation to
go back to the original store path. The chosen solution is to make the
hash uppercase:
- it does not trigger runtime references (except for all digit hashes,
  which are unlikely enough)
- it visually looks like a bogus store path
- it is easy to find the original store path if required

Ideally we would like to use `-fmacro-prefix-map=` feature of `gcc` as:

  -fmacro-prefix-map=/nix/store/$hash1-nlohmann-json-ver=/nix/store/$HASH1-nlohmann-json-ver
  -fmacro-prefix-map=/nix/...

In practice it quickly exhausts argument length limit due to `gcc`
deficiency: https://gcc.gnu.org/PR111527

Until it's fixed let's hardcode header mangling if $NIX_STORE variable
is present in the environment.

Tested as:

    $ printf "# 0 \"/nix/store/aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa-pppppp-vvvvvvv\" \nconst char * f(void) { return __FILE__; }" | NIX_STORE=/nix/store ./gcc/xgcc -Bgcc -x c - -S -o -
    ...
    .string "/nix/store/AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA-pppppp-vvvvvvv"
    ...

Mangled successfully.

To reverse the effect of the mangle use new `NIX_GCC_DONT_MANGLE_PREFIX_MAP`
environment variable. It should not normally be needed.
--- a/gcc/file-prefix-map.cc
+++ b/gcc/file-prefix-map.cc
@@ -65,7 +65,7 @@ add_prefix_map (file_prefix_map *&maps, const char *arg, const char *opt)
    remapping was performed.  */
 
 static const char *
-remap_filename (file_prefix_map *maps, const char *filename)
+remap_filename (file_prefix_map *maps, const char *filename, bool mangle_nix_store = false)
 {
   file_prefix_map *map;
   char *s;
@@ -76,7 +76,31 @@ remap_filename (file_prefix_map *maps, const char *filename)
     if (filename_ncmp (filename, map->old_prefix, map->old_len) == 0)
       break;
   if (!map)
-    return filename;
+    {
+      if (mangle_nix_store && getenv("NIX_GCC_DONT_MANGLE_PREFIX_MAP") == NULL)
+	{
+	  /* Remap the 32 characters after $NIX_STORE/ to uppercase
+	  *
+	  * That way we avoid argument parameters explosion
+	  * and still avoid embedding headers into runtime closure:
+	  *   https://gcc.gnu.org/PR111527
+	  */
+	  char * nix_store = getenv("NIX_STORE");
+	  size_t nix_store_len = nix_store ? strlen(nix_store) : 0;
+	  const char * name = filename;
+	  size_t name_len = strlen(name);
+	  if (nix_store && name_len >= nix_store_len + 1 + 32 && memcmp(name, nix_store, nix_store_len) == 0)
+	    {
+	       s = (char *) ggc_alloc_atomic (name_len + 1);
+	       memcpy(s, name, name_len + 1);
+	       for (size_t i = nix_store_len + 1; i < nix_store_len + 1 + 32; i++) {
+		 s[i] = TOUPPER(s[i]);
+	       }
+	       return s;
+	    }
+	}
+      return filename;
+    }
   name = filename + map->old_len;
   name_len = strlen (name) + 1;
 
@@ -129,7 +153,7 @@ add_profile_prefix_map (const char *arg)
 const char *
 remap_macro_filename (const char *filename)
 {
-  return remap_filename (macro_prefix_maps, filename);
+  return remap_filename (macro_prefix_maps, filename, true);
 }
 
 /* Remap using -fdebug-prefix-map.  Return the GC-allocated new name