Replace __libc_multiple_libcs with __libc_initial flag

Change sbrk to fail for !__libc_initial (in the generic
implementation).  As a result, sbrk is (relatively) safe to use
for the __libc_initial case (from the main libc).  It is therefore
no longer necessary to avoid using it in that case (or updating the
brk cache), and the __libc_initial flag does not need to be updated
as part of dlmopen or static dlopen.

As before, direct brk system calls on Linux may lead to memory
corruption.

Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>

csu/init-first.c

@@ -28,10 +28,6 @@
 
 #include <ldsodefs.h>
 
-/* Set nonzero if we have to be prepared for more than one libc being
-   used in the process.  Safe assumption if initializer never runs.  */
-int __libc_multiple_libcs attribute_hidden = 1;
-
 /* Remember the command line argument and enviroment contents for
    later calls of initializers for dynamic libraries.  */
 int __libc_argc attribute_hidden;
@@ -50,16 +46,16 @@ _init_first (int argc, char **argv, char **envp)
 {
 #endif
 
-  __libc_multiple_libcs = &_dl_starting_up && !_dl_starting_up;
-
   /* Make sure we don't initialize twice.  */
-  if (!__libc_multiple_libcs)
+#ifdef SHARED
+  if (__libc_initial)
     {
       /* Set the FPU control word to the proper default value if the
 	 kernel would use a different value.  */
       if (__fpu_control != GLRO(dl_fpu_control))
 	__setfpucw (__fpu_control);
     }
+#endif
 
   /* Save the command-line arguments.  */
   __libc_argc = argc;

csu/libc-start.c

@@ -141,8 +141,6 @@ LIBC_START_MAIN (int (*main) (int, char **, char ** MAIN_AUXVEC_DECL),
   /* Result of the 'main' function.  */
   int result;
 
-  __libc_multiple_libcs = &_dl_starting_up && !_dl_starting_up;
-
 #ifndef SHARED
   _dl_relocate_static_pie ();
 
@@ -213,12 +211,11 @@ LIBC_START_MAIN (int (*main) (int, char **, char ** MAIN_AUXVEC_DECL),
 # endif
 
 # ifdef DL_SYSDEP_OSCHECK
-  if (!__libc_multiple_libcs)
-    {
-      /* This needs to run to initiliaze _dl_osversion before TLS
-	 setup might check it.  */
-      DL_SYSDEP_OSCHECK (__libc_fatal);
-    }
+  {
+    /* This needs to run to initiliaze _dl_osversion before TLS
+       setup might check it.  */
+    DL_SYSDEP_OSCHECK (__libc_fatal);
+  }
 # endif
 
   /* Initialize libpthread if linked in.  */

elf/dl-open.c

@@ -787,12 +787,6 @@ dl_open_worker (void *a)
   if (mode & RTLD_GLOBAL)
     add_to_global_update (new);
 
-#ifndef SHARED
-  /* We must be the static _dl_open in libc.a.  A static program that
-     has loaded a dynamic object now has competition.  */
-  __libc_multiple_libcs = 1;
-#endif
-
   /* Let the user know about the opencount.  */
   if (__glibc_unlikely (GLRO(dl_debug_mask) & DL_DEBUG_FILES))
     _dl_debug_printf ("opening file=%s [%lu]; direct_opencount=%u\n\n",

elf/dl-sysdep.c

@@ -58,8 +58,6 @@ ElfW(Addr) _dl_base_addr;
 #endif
 int __libc_enable_secure attribute_relro = 0;
 rtld_hidden_data_def (__libc_enable_secure)
-int __libc_multiple_libcs = 0;	/* Defining this here avoids the inclusion
-				   of init-first.  */
 /* This variable contains the lowest stack address ever used.  */
 void *__libc_stack_end attribute_relro = NULL;
 rtld_hidden_data_def(__libc_stack_end)

elf/libc_early_init.c

@@ -18,8 +18,13 @@
 
 #include <ctype.h>
 #include <libc-early-init.h>
+#include <libc-internal.h>
 #include <sys/single_threaded.h>
 
+#ifdef SHARED
+_Bool __libc_initial;
+#endif
+
 void
 __libc_early_init (_Bool initial)
 {
@@ -28,4 +33,8 @@ __libc_early_init (_Bool initial)
 
   /* Only the outer namespace is marked as single-threaded.  */
   __libc_single_threaded = initial;
+
+#ifdef SHARED
+  __libc_initial = initial;
+#endif
 }

include/libc-internal.h

@@ -47,6 +47,11 @@ extern void __init_misc (int, char **, char **) attribute_hidden;
 extern __typeof (__profile_frequency) __profile_frequency attribute_hidden;
 # endif
 
-extern int __libc_multiple_libcs attribute_hidden;
+#ifdef SHARED
+/* True if this libc belongs to the initially loaded program (i.e., it
+   is not for an audit module, not loaded via dlmopen, and not loaded
+   via static dlopen either).  */
+extern _Bool __libc_initial attribute_hidden;
+#endif
 
 #endif /* _LIBC_INTERNAL  */

misc/sbrk.c

@@ -16,9 +16,10 @@
    <https://www.gnu.org/licenses/>.  */
 
 #include <errno.h>
+#include <libc-internal.h>
+#include <stdbool.h>
 #include <stdint.h>
 #include <unistd.h>
-#include <libc-internal.h>
 
 /* Defined in brk.c.  */
 extern void *__curbrk;
@@ -30,21 +31,34 @@ extern int __brk (void *addr);
 void *
 __sbrk (intptr_t increment)
 {
-  void *oldbrk;
+  /* Controls whether __brk (0) is called to read the brk value from
+     the kernel.  */
+  bool update_brk = __curbrk == NULL;
 
-  /* If this is not part of the dynamic library or the library is used
-     via dynamic loading in a statically linked program update
-     __curbrk from the kernel's brk value.  That way two separate
-     instances of __brk and __sbrk can share the heap, returning
-     interleaved pieces of it.  */
-  if (__curbrk == NULL || __libc_multiple_libcs)
+#if defined (SHARED) && ! IS_IN (rtld)
+  if (!__libc_initial)
+    {
+      if (increment != 0)
+	{
+	  /* Do not allow changing the brk from an inner libc because
+	     it cannot be synchronized with the outer libc's brk.  */
+	  __set_errno (ENOMEM);
+	  return (void *) -1;
+	}
+      /* Querying the kernel's brk value from an inner namespace is
+	 fine.  */
+      update_brk = true;
+    }
+#endif
+
+  if (update_brk)
     if (__brk (0) < 0)		/* Initialize the break.  */
       return (void *) -1;
 
   if (increment == 0)
     return __curbrk;
 
-  oldbrk = __curbrk;
+  void *oldbrk = __curbrk;
   if (increment > 0
       ? ((uintptr_t) oldbrk + (uintptr_t) increment < (uintptr_t) oldbrk)
       : ((uintptr_t) oldbrk < (uintptr_t) -increment))

sysdeps/mach/hurd/dl-sysdep.c

@@ -57,8 +57,6 @@ extern char **_environ;
 
 int __libc_enable_secure = 0;
 rtld_hidden_data_def (__libc_enable_secure)
-int __libc_multiple_libcs = 0;	/* Defining this here avoids the inclusion
-				   of init-first.  */
 /* This variable contains the lowest stack address ever used.  */
 void *__libc_stack_end = NULL;
 rtld_hidden_data_def(__libc_stack_end)

sysdeps/mach/hurd/i386/init-first.c

@@ -30,6 +30,7 @@
 #include <ldsodefs.h>
 #include <fpu_control.h>
 #include <libc-diag.h>
+#include <libc-internal.h>
 
 extern void __mach_init (void);
 extern void __init_misc (int, char **, char **);
@@ -40,7 +41,6 @@ unsigned long int __hurd_threadvar_stack_mask;
 #ifndef SHARED
 int __libc_enable_secure;
 #endif
-int __libc_multiple_libcs attribute_hidden = 1;
 
 extern int __libc_argc attribute_hidden;
 extern char **__libc_argv attribute_hidden;
@@ -56,13 +56,12 @@ DEFINE_HOOK (_hurd_preinit_hook, (void));
 static void
 posixland_init (int argc, char **argv, char **envp)
 {
-  __libc_multiple_libcs = &_dl_starting_up && !_dl_starting_up;
-
   /* Now we have relocations etc. we can start signals etc.  */
   _hurd_libc_proc_init (argv);
 
+#ifdef SHARED
   /* Make sure we don't initialize twice.  */
-  if (!__libc_multiple_libcs)
+  if (__libc_initial)
     {
       /* Set the FPU control word to the proper default value.  */
       __setfpucw (__fpu_control);
@@ -72,6 +71,9 @@ posixland_init (int argc, char **argv, char **envp)
       /* Initialize data structures so the additional libc can do RPCs.  */
       __mach_init ();
     }
+#else /* !SHARED */
+  __setfpucw (__fpu_control);
+#endif
 
   /* Save the command-line arguments.  */
   __libc_argc = argc;