Darwin stack creation: hack to keep apple pointer conrrect on macOS 12+

From Louis Brunner's port.

diff --git a/coregrind/vg_preloaded.c b/coregrind/vg_preloaded.c
index 0b77e0537f7ee61746d9df25b689b7410435f94d..baf4cc11c2ae8e7fa9c3e07151c6c9d0012fa86b 100644 (file)
--- a/coregrind/vg_preloaded.c
+++ b/coregrind/vg_preloaded.c
@@ -188,9 +188,60 @@ static void env_unsetenv ( HChar **env, const HChar *varname )
       }
    }
    *(to++) = *(from++);
+
    /* fix the 4th "char* apple" pointer (aka. executable path pointer) */
    *(to++) = *(from++);
+
+#if DARWIN_VERS < DARWIN_12_00
+   /* We only do this on older versions of darwin because dyld changed
+      and by the point we do this changes, the apple env ptr is already set,
+      so if we move values around, we'll end up with a pointer pointing inside
+      (and even potentially after) applep.
+
+      Instead we copy the first value of the applelp over and over again
+      so that envp and applep are still separated by NULL,
+      applep is continuous and points to a correct value.
+
+      See the following example, using the following envp and applep:
+
+      ```
+        0xXXXX00: PATH=/bin (envp)
+        0xXXXX08: DYLD_INSERT_LIBRARIES=/lib
+        0xXXXX10: USER=foo
+        0xXXXX18: NULL
+        0xXXXX20: executable_path=/bin/ls (applep)
+        0xXXXX28: NULL
+      ```
+
+      # With this line
+
+      ```
+        0xXXXX00: PATH=/bin (envp)
+        0xXXXX08: USER=foo
+        0xXXXX10: NULL
+        0xXXXX18: executable_path=/bin/ls
+        0xXXXX20: NULL (applep)
+        0xXXXX28: NULL
+      ```
+
+      Notice that the applep is now invalid.
+
+      # Without this line
+
+      ```
+        0xXXXX00: PATH=/bin (envp)
+        0xXXXX08: USER=foo
+        0xXXXX10: NULL
+        0xXXXX18: executable_path=/bin/ls
+        0xXXXX20: executable_path=/bin/ls (applep)
+        0xXXXX28: NULL
+      ```
+
+      Notice that while values in applep are duplicated, this is only the case if browsing frop envp[len+1]
+      but not from applep which is always valid. Duplicated values are also harmless in this case.
+   */
    *to = NULL;
+#endif
 }
 
 static void vg_cleanup_env(void)  __attribute__((constructor));