[glibc/fw/bug25097] Avoid zero-length array at the end of struct link_map [BZ #25097]

[Florian Weimer <fw at sourceware dot org>]

https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=6d241445b417089ac7d26976f520f35d3442abc0

commit 6d241445b417089ac7d26976f520f35d3442abc0
Author: Florian Weimer <fweimer@redhat.com>
Date:   Sun Nov 3 11:20:23 2019 +0100

    Avoid zero-length array at the end of struct link_map [BZ #25097]
    
    l_audit ends up as an internal array with _rtld_global, and GCC 10
    warns about this.
    
    This commit does not change the layout of _rtld_global, so it is
    suitable for backporting.  Future changes could allocate more of the
    audit state dynamically and remove it from always-allocated data
    structures, to optimize the common case of inactive auditing.

Diff:
---
 include/link.h             | 18 ++++++++++--------
 sysdeps/generic/ldsodefs.h | 17 +++++++++++++----
 2 files changed, 23 insertions(+), 12 deletions(-)

diff --git a/include/link.h b/include/link.h
index 1184201..be52b97 100644
--- a/include/link.h
+++ b/include/link.h
@@ -325,16 +325,18 @@ struct link_map
     size_t l_relro_size;
 
     unsigned long long int l_serial;
-
-    /* Audit information.  This array apparent must be the last in the
-       structure.  Never add something after it.  */
-    struct auditstate
-    {
-      uintptr_t cookie;
-      unsigned int bindflags;
-    } l_audit[0];
   };
 
+/* Information used by audit modules.  For most link maps, this data
+   immediate follows the link map in memory.  For the dynamic linker,
+   it is allocated separately.  See link_map_audit_state in
+   <ldsodefs.h>.  */
+struct auditstate
+{
+  uintptr_t cookie;
+  unsigned int bindflags;
+};
+
 
 #if __ELF_NATIVE_CLASS == 32
 # define symbind symbind32
diff --git a/sysdeps/generic/ldsodefs.h b/sysdeps/generic/ldsodefs.h
index 04b6d17..eb6cbea 100644
--- a/sysdeps/generic/ldsodefs.h
+++ b/sysdeps/generic/ldsodefs.h
@@ -379,11 +379,12 @@ struct rtld_global
   /* List of search directories.  */
   EXTERN struct r_search_path_elem *_dl_all_dirs;
 
-  /* Structure describing the dynamic linker itself.  We need to
-     reserve memory for the data the audit libraries need.  */
+  /* Structure describing the dynamic linker itself.  */
   EXTERN struct link_map _dl_rtld_map;
 #ifdef SHARED
-  struct auditstate audit_data[DL_NNS];
+  /* Used to store the audit information for the link map of the
+     dynamic loader.  */
+  struct auditstate _dl_rtld_auditstate[DL_NNS];
 #endif
 
 #if defined SHARED && defined _LIBC_REENTRANT \
@@ -1178,7 +1179,15 @@ rtld_active (void)
 static inline struct auditstate *
 link_map_audit_state (struct link_map *l, size_t index)
 {
-  return &l->l_audit[index];
+  if (l == &GL (dl_rtld_map))
+    /* The auditstate array is stored separately.  */
+    return &GL (dl_rtld_auditstate) [index];
+  else
+    {
+      /* The auditstate array follows the link map in memory.  */
+      struct auditstate *base = (struct auditstate *) (l + 1);
+      return &base[index];
+    }
 }
 #endif /* SHARED */