Fix relocation to avoid overwriting merged constant strings (#32253)

Compilers and linker optimize string constants for space by aliasing
them when one is a suffix of another. For gcc / binutils this happens
already at -O1, due to -fmerge-constants. This means that we have
to take care during relocation to always preserve a certain length
of the suffix of those prefixes that are C-strings. 

In this commit we pick length 7 as a safe suffix length, assuming the
suffix is typically the 7 characters from the hash (i.e. random), so
it's unlikely to alias with any string constant used in the sources.

In general we now pad shortened strings from the left with leading
dir seperators, but in the case of C-strings that are much shorter
and don't share a common suffix (due to projections), we do allow
shrinking the C-string, appending a null, and retaining the old part
of the prefix.

Also when rewiring, we ensure that the new hash preserves the last
7 bytes of the old hash.

Co-authored-by: Harmen Stoppels <harmenstoppels@gmail.com>

2 files changed, 12 insertions, 2 deletions

diff --git a/var/spack/repos/builtin.mock/packages/garply/package.py b/var/spack/repos/builtin.mock/packages/garply/package.py
index 819da45286..7fc3fd484a 100644
--- a/var/spack/repos/builtin.mock/packages/garply/package.py
+++ b/var/spack/repos/builtin.mock/packages/garply/package.py
@@ -83,7 +83,12 @@ const int garply_version_minor = %s;
             f.write(garply_cc % prefix.config)
         with open("%s/garply/garplinator.cc" % self.stage.source_path, "w") as f:
             f.write(garplinator_cc)
-        gpp = which("/usr/bin/g++")
+        gpp = which(
+            "g++",
+            path=":".join(
+                [s for s in os.environ["PATH"].split(os.pathsep) if "lib/spack/env" not in s]
+            ),
+        )
         if sys.platform == "darwin":
             gpp = which("/usr/bin/clang++")
         gpp(
diff --git a/var/spack/repos/builtin.mock/packages/quux/package.py b/var/spack/repos/builtin.mock/packages/quux/package.py
index 693ef07ba4..de4f8b9327 100644
--- a/var/spack/repos/builtin.mock/packages/quux/package.py
+++ b/var/spack/repos/builtin.mock/packages/quux/package.py
@@ -97,7 +97,12 @@ const int quux_version_minor = %s;
             f.write(quux_h)
         with open("%s/quux/quuxifier.cc" % self.stage.source_path, "w") as f:
             f.write(quuxifier_cc)
-        gpp = which("/usr/bin/g++")
+        gpp = which(
+            "g++",
+            path=":".join(
+                [s for s in os.environ["PATH"].split(os.pathsep) if "lib/spack/env" not in s]
+            ),
+        )
         if sys.platform == "darwin":
             gpp = which("/usr/bin/clang++")
         gpp(