[glibc/fw/bug25097] slotinfo in struct dtv_slotinfo_list should be flexible array [BZ #25097]

[Florian Weimer <fw at sourceware dot org>]

https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=51ac7d6bf2e05053044b6fabe77bb80753deb80a

commit 51ac7d6bf2e05053044b6fabe77bb80753deb80a
Author: Florian Weimer <fweimer@redhat.com>
Date:   Sat Nov 2 17:03:06 2019 +0100

    slotinfo in struct dtv_slotinfo_list should be flexible array [BZ #25097]
    
    GCC 10 will warn about subscribing inner length zero arrays.  Use a GCC
    extension in csu/libc-tls.c to allocate space for the static_slotinfo
    variable.  Adjust nptl_db so that the type description machinery does
    not attempt to determine the size of the flexible array member slotinfo.

Diff:
---
 csu/libc-tls.c             | 32 +++++++++++++-------------------
 nptl_db/db-symbols.h       |  3 +++
 nptl_db/db_info.c          |  8 +++++++-
 nptl_db/structs.def        |  2 +-
 nptl_db/thread_dbP.h       |  4 ++++
 sysdeps/generic/ldsodefs.h |  2 +-
 6 files changed, 29 insertions(+), 22 deletions(-)

diff --git a/csu/libc-tls.c b/csu/libc-tls.c
index 33d8b61..23d6cde 100644
--- a/csu/libc-tls.c
+++ b/csu/libc-tls.c
@@ -23,7 +23,7 @@
 #include <unistd.h>
 #include <stdio.h>
 #include <sys/param.h>
-
+#include <array_length.h>
 
 #ifdef SHARED
  #error makefile bug, this file is for static only
@@ -32,17 +32,11 @@
 dtv_t _dl_static_dtv[2 + TLS_SLOTINFO_SURPLUS];
 
 
-static struct
-{
-  struct dtv_slotinfo_list si;
-  /* The dtv_slotinfo_list data structure does not include the actual
-     information since it is defined as an array of size zero.  We define
-     here the necessary entries.  Note that it is not important whether
-     there is padding or not since we will always access the information
-     through the 'si' element.  */
-  struct dtv_slotinfo info[2 + TLS_SLOTINFO_SURPLUS];
-} static_slotinfo;
-
+static struct dtv_slotinfo_list static_slotinfo =
+  {
+   /* Allocate an array of 2 + TLS_SLOTINFO_SURPLUS elements.  */
+   .slotinfo =  { [array_length (_dl_static_dtv) - 1] = { } },
+  };
 
 /* Highest dtv index currently needed.  */
 size_t _dl_tls_max_dtv_idx;
@@ -72,16 +66,16 @@ TLS_INIT_HELPER
 static void
 init_slotinfo (void)
 {
-  /* Create the slotinfo list.  */
-  static_slotinfo.si.len = (((char *) (&static_slotinfo + 1)
-			     - (char *) &static_slotinfo.si.slotinfo[0])
-			    / sizeof static_slotinfo.si.slotinfo[0]);
-  // static_slotinfo.si.next = NULL;	already zero
+  /* Create the slotinfo list.  Note that the type of static_slotinfo
+     has effectively a zero-length array, so we cannot use the size of
+     static_slotinfo to determine the array length.  */
+  static_slotinfo.len = array_length (_dl_static_dtv);
+  /* static_slotinfo.si.next = NULL; -- Already zero.  */
 
   /* The slotinfo list.  Will be extended by the code doing dynamic
      linking.  */
   GL(dl_tls_max_dtv_idx) = 1;
-  GL(dl_tls_dtv_slotinfo_list) = &static_slotinfo.si;
+  GL(dl_tls_dtv_slotinfo_list) = &static_slotinfo;
 }
 
 static void
@@ -206,7 +200,7 @@ __libc_setup_tls (void)
 
   init_slotinfo ();
   // static_slotinfo.si.slotinfo[1].gen = 0; already zero
-  static_slotinfo.si.slotinfo[1].map = main_map;
+  static_slotinfo.slotinfo[1].map = main_map;
 
   memsz = roundup (memsz, align ?: 1);
 
diff --git a/nptl_db/db-symbols.h b/nptl_db/db-symbols.h
index 8b078b0..7c53d80 100644
--- a/nptl_db/db-symbols.h
+++ b/nptl_db/db-symbols.h
@@ -25,6 +25,7 @@
   DB_LOOKUP_NAME (SYM_SIZEOF_##type, _thread_db_sizeof_##type)
 #define DB_STRUCT_FIELD(type, field) \
   DB_LOOKUP_NAME (SYM_##type##_FIELD_##field, _thread_db_##type##_##field)
+#define DB_STRUCT_FLEXIBLE_ARRAY(type, field) DB_STRUCT_FIELD (type, field)
 #define DB_SYMBOL(name) \
   DB_LOOKUP_NAME (SYM_##name, name)
 #define DB_FUNCTION(name) \
@@ -36,6 +37,8 @@
 # include "structs.def"
 
 # undef DB_STRUCT
+# undef DB_STRUCT_FIELD
+# undef DB_STRUCT_FLEXIBLE_ARRAY
 # undef DB_FUNCTION
 # undef DB_SYMBOL
 # undef DB_VARIABLE
diff --git a/nptl_db/db_info.c b/nptl_db/db_info.c
index af7f754..5a4baa8 100644
--- a/nptl_db/db_info.c
+++ b/nptl_db/db_info.c
@@ -54,8 +54,11 @@ extern bool __nptl_initial_report_events;
   DB_DEFINE_DESC (name, 8 * sizeof (obj), 1, offset);
 #define ARRAY_DESC(name, offset, obj) \
   DB_DEFINE_DESC (name, \
-		  8 * sizeof (obj)[0], sizeof (obj) / sizeof (obj)[0], \
+		  8 * sizeof (obj)[0], sizeof (obj) / sizeof (obj)[0],	\
 		  offset);
+/* Flexible arrays do not have a length that can be determined.  */
+#define FLEXIBLE_ARRAY_DESC(name, offset, obj) \
+  DB_DEFINE_DESC (name, 8 * sizeof (obj)[0], 0, offset);
 
 #if TLS_TCB_AT_TP
 # define dtvp header.dtv
@@ -77,6 +80,9 @@ DESC (_thread_db_pthread_dtvp,
 #define DB_STRUCT_ARRAY_FIELD(type, field) \
   ARRAY_DESC (_thread_db_##type##_##field, \
 	      offsetof (type, field), ((type *) 0)->field)
+#define DB_STRUCT_FLEXIBLE_ARRAY(type, field) \
+  FLEXIBLE_ARRAY_DESC (_thread_db_##type##_##field, \
+		       offsetof (type, field), ((type *) 0)->field)
 #define DB_VARIABLE(name) DESC (_thread_db_##name, 0, name)
 #define DB_ARRAY_VARIABLE(name) ARRAY_DESC (_thread_db_##name, 0, name)
 #define DB_SYMBOL(name)	/* Nothing.  */
diff --git a/nptl_db/structs.def b/nptl_db/structs.def
index f834d1d..a3aa71a 100644
--- a/nptl_db/structs.def
+++ b/nptl_db/structs.def
@@ -110,7 +110,7 @@ DB_RTLD_GLOBAL_FIELD (dl_tls_dtv_slotinfo_list)
 DB_STRUCT (dtv_slotinfo_list)
 DB_STRUCT_FIELD (dtv_slotinfo_list, len)
 DB_STRUCT_FIELD (dtv_slotinfo_list, next)
-DB_STRUCT_ARRAY_FIELD (dtv_slotinfo_list, slotinfo)
+DB_STRUCT_FLEXIBLE_ARRAY (dtv_slotinfo_list, slotinfo)
 
 DB_STRUCT (dtv_slotinfo)
 DB_STRUCT_FIELD (dtv_slotinfo, gen)
diff --git a/nptl_db/thread_dbP.h b/nptl_db/thread_dbP.h
index 1dbb543..cf6a2b0 100644
--- a/nptl_db/thread_dbP.h
+++ b/nptl_db/thread_dbP.h
@@ -37,12 +37,14 @@ enum
   {
 # define DB_STRUCT(type)		SYM_SIZEOF_##type,
 # define DB_STRUCT_FIELD(type, field)	SYM_##type##_FIELD_##field,
+# define DB_STRUCT_FLEXIBLE_ARRAY(type, field) DB_STRUCT_FIELD (type, field)
 # define DB_SYMBOL(name)		SYM_##name,
 # define DB_FUNCTION(name)		SYM_##name,
 # define DB_VARIABLE(name)		SYM_##name, SYM_DESC_##name,
 # include "structs.def"
 # undef DB_STRUCT
 # undef DB_STRUCT_FIELD
+# undef DB_STRUCT_FLEXIBLE_ARRAY
 # undef DB_SYMBOL
 # undef DB_FUNCTION
 # undef DB_VARIABLE
@@ -90,6 +92,7 @@ struct td_thragent
   uint32_t ta_sizeof_##type;
 # define DB_STRUCT_FIELD(type, field) \
   db_desc_t ta_field_##type##_##field;
+# define DB_STRUCT_FLEXIBLE_ARRAY(type, field) DB_STRUCT_FIELD (type, field)
 # define DB_SYMBOL(name) \
   psaddr_t ta_addr_##name;
 # define DB_FUNCTION(name) \
@@ -100,6 +103,7 @@ struct td_thragent
 # include "structs.def"
 # undef DB_STRUCT
 # undef DB_STRUCT_FIELD
+# undef DB_STRUCT_FLEXIBLE_ARRAY
 # undef DB_FUNCTION
 # undef DB_SYMBOL
 # undef DB_VARIABLE
diff --git a/sysdeps/generic/ldsodefs.h b/sysdeps/generic/ldsodefs.h
index eb6cbea..4d67c05 100644
--- a/sysdeps/generic/ldsodefs.h
+++ b/sysdeps/generic/ldsodefs.h
@@ -421,7 +421,7 @@ struct rtld_global
     {
       size_t gen;
       struct link_map *map;
-    } slotinfo[0];
+    } slotinfo[];
   } *_dl_tls_dtv_slotinfo_list;
   /* Number of modules in the static TLS block.  */
   EXTERN size_t _dl_tls_static_nelem;