Fix dynamic library linking.

nacl/dyn-link/ldscripts/elf64_nacl.xs

 /* Script for ld --shared: link shared library */
 OUTPUT_FORMAT("elf64-nacl", "elf64-nacl",
               "elf64-nacl")
 OUTPUT_ARCH(i386:x86-64)
 ENTRY(_start)
 SEARCH_DIR("=/usr/local/lib"); SEARCH_DIR("=/lib"); SEARCH_DIR("=/usr/lib");
 PHDRS
 {
   seg_code     PT_LOAD FLAGS(5) ;       /* read + execute */
   seg_rodata   PT_LOAD FLAGS(4) ;       /* read */
   seg_rwdata   PT_LOAD FLAGS(6) ;       /* read + write */
+  seg_bss      PT_LOAD FLAGS(6) ;       /* read + write */
   seg_dynamic  PT_DYNAMIC FLAGS(6) ;
   seg_stack    PT_GNU_STACK FLAGS(6) ;
   seg_tls      PT_TLS FLAGS(4) ;
 }
 SECTIONS
 {
   /* ELF headers are not included in any PT_LOAD segment */
   . = SEGMENT_START("text", 0);
   _begin = .;
   /* The ALIGN(32) instructions below are workarounds.
@@ skipping 134 matching lines @@
   .got.plt        : { *(.got.plt) }
   . = ALIGN(CONSTANT (MAXPAGESIZE)); /* nacl wants page alignment */
   .data           :
   {
     *(.data .data.* .gnu.linkonce.d.*)
     KEEP (*(.gnu.linkonce.d.*personality*))
     SORT(CONSTRUCTORS)
   }
   .data1          : { *(.data1) }
   _edata = .; PROVIDE (edata = .);
+
+  /* We place the BSS in a separate segment as a workaround for the
+     behaviour of NaCl's mmap() on the last page in a file.  When using
+     copy-on-write MAP_PRIVATE, we cannot necessarily write beyond the
+     file's extent within the 64k page.
+     See http://code.google.com/p/nativeclient/issues/detail?id=824.
+     TODO(mseaborn): A better fix would be to change the dynamic linker
+     to copy the last page.  This would save a page of memory because
+     the BSS could share a page with the data segment.  But for now, it
+     is simpler to change the linker script. */
+  . = DATA_SEGMENT_END (.);
+  . = ALIGN (CONSTANT (COMMONPAGESIZE));

[pasko-google - do not use, 2011/08/24 16:38:14]

please, take a look it it did not significantly increase the overall size of
library files on disk, when files are not sparse

[khim, 2011/08/24 17:03:07]

It adds few kilobytes to file with debug info (but these are large anyway),
after strip size is the same.

+
   __bss_start = .;
   .bss            :
   {
    *(.dynbss)
    *(.bss .bss.* .gnu.linkonce.b.*)
    *(COMMON)
    /* Align here to ensure that the .bss section occupies space up to
       _end.  Align after .bss to ensure correct alignment even if the
       .bss section disappears because there are no input sections.
       FIXME: Why do we need it? When there is no .bss section, we don't
       pad the .data section.  */
    . = ALIGN(. != 0 ? 32 / 8 : 1);
-  }
+  } :seg_bss
   . = ALIGN(32 / 8);
   . = ALIGN(32 / 8);
   _end = .; PROVIDE (end = .);
-  . = DATA_SEGMENT_END (.);
   /* Stabs debugging sections.  */
   .stab          0 : { *(.stab) }
   .stabstr       0 : { *(.stabstr) }
   .stab.excl     0 : { *(.stab.excl) }
   .stab.exclstr  0 : { *(.stab.exclstr) }
   .stab.index    0 : { *(.stab.index) }
   .stab.indexstr 0 : { *(.stab.indexstr) }
   .comment       0 : { *(.comment) }
   /* DWARF debug sections.
      Symbols in the DWARF debugging sections are relative to the beginning
@@ skipping 20 matching lines @@
   .debug_funcnames 0 : { *(.debug_funcnames) }
   .debug_typenames 0 : { *(.debug_typenames) }
   .debug_varnames  0 : { *(.debug_varnames) }
   /* DWARF 3 */
   .debug_pubtypes 0 : { *(.debug_pubtypes) }
   .debug_ranges   0 : { *(.debug_ranges) }
   .gnu.attributes 0 : { KEEP (*(.gnu.attributes)) }
   /DISCARD/ : { *(.note.GNU-stack) *(.gnu_debuglink) }
   /DISCARD/ : { *(.note.ABI-tag) }
 }