export: Fix use-after-free on multiple calls overwriting same FILE

CMake 3.16 and below allow multiple `export()` calls with the same output
file even without using `APPEND`.  The implementation worked by accident
by leaking memory.  Refactoring in commit 5444a8095d (cmGlobalGenerator:
modernize memrory managemenbt, 2019-12-29, v3.17.0-rc1~239^2) cleaned up
that memory leak and converted it to a use-after-free instead.

The problem is caused by using the `cmGlobalGenerator::BuildExportSets`
map to own `cmExportBuildFileGenerator` instances.  It can own only
one instance per output FILE name at a time, so repeating use of the
same file now frees the old `cmExportBuildFileGenerator` instance
and leaves the pointer in the `cmMakefile::ExportBuildFileGenerators`
vector dangling.  Move ownership of the instances into `cmMakefile`'s
vector since its entries are not replaced on a repeat output FILE.

In future work we should introduce a policy to error out on this case.
For now simply fix the use-after-free to restore CMake <= 3.16 behavior.

Fixes: #20469

1 files changed, 4 insertions, 6 deletions

diff --git a/Source/cmGlobalGenerator.h b/Source/cmGlobalGenerator.h
index ba997b2..7dc4822 100644
--- a/Source/cmGlobalGenerator.h
+++ b/Source/cmGlobalGenerator.h
@@ -463,13 +463,12 @@ public:
 
   void ProcessEvaluationFiles();
 
-  std::map<std::string, std::unique_ptr<cmExportBuildFileGenerator>>&
-  GetBuildExportSets()
+  std::map<std::string, cmExportBuildFileGenerator*>& GetBuildExportSets()
   {
     return this->BuildExportSets;
   }
-  void AddBuildExportSet(std::unique_ptr<cmExportBuildFileGenerator>);
-  void AddBuildExportExportSet(std::unique_ptr<cmExportBuildFileGenerator>);
+  void AddBuildExportSet(cmExportBuildFileGenerator* gen);
+  void AddBuildExportExportSet(cmExportBuildFileGenerator* gen);
   bool IsExportedTargetsFile(const std::string& filename) const;
   bool GenerateImportFile(const std::string& file);
   cmExportBuildFileGenerator* GetExportedTargetsFile(
@@ -580,8 +579,7 @@ protected:
   std::set<std::string> InstallComponents;
   // Sets of named target exports
   cmExportSetMap ExportSets;
-  std::map<std::string, std::unique_ptr<cmExportBuildFileGenerator>>
-    BuildExportSets;
+  std::map<std::string, cmExportBuildFileGenerator*> BuildExportSets;
   std::map<std::string, cmExportBuildFileGenerator*> BuildExportExportSets;
 
   std::map<std::string, std::string> AliasTargets;