libbb: make bb_common_bufsiz1 1 kbyte, add capability to use bss tail for it

The config item is FEATURE_USE_BSS_TAIL. When it is off (default):

function                                             old     new   delta
read_config                                          210     228     +18
doCommands                                          2279    2294     +15
ipneigh_list_or_flush                                763     772      +9
ipaddr_list_or_flush                                1256    1261      +5
display_process_list                                1301    1306      +5
conspy_main                                         1378    1383      +5
do_lzo_compress                                      352     355      +3
do_lzo_decompress                                    565     567      +2
push                                                  46      44      -2
inetd_main                                          2136    2134      -2
uevent_main                                          421     418      -3
addLines                                              97      92      -5
bb_common_bufsiz1                                   8193    1024   -7169
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 8/5 up/down: 62/-7181)        Total: -7119 bytes
   text	   data	    bss	    dec	    hex	filename
 829850	   4086	   9080	 843016	  cdd08	busybox_old
 829901	   4086	   1904	 835891	  cc133	busybox_unstripped

FEATURE_USE_BSS_TAIL=y:

read_config                                          210     228     +18
doCommands                                          2279    2294     +15
ipneigh_list_or_flush                                763     772      +9
ipaddr_list_or_flush                                1256    1261      +5
display_process_list                                1301    1306      +5
conspy_main                                         1378    1383      +5
do_lzo_compress                                      352     355      +3
do_lzo_decompress                                    565     567      +2
inetd_main                                          2136    2134      -2
bb_common_bufsiz1                                   8193       -   -8193
------------------------------------------------------------------------------
(add/remove: 0/1 grow/shrink: 8/1 up/down: 62/-8195)        Total: -8133 bytes
   text	   data	    bss	    dec	    hex	filename
 829850	   4086	   9080	 843016	  cdd08	busybox_old
 829911	   4086	    880	 834877	  cbd3d	busybox_unstripped

FIXME: setup_common_bufsiz() calls are missing.

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

2 files changed, 74 insertions, 5 deletions

diff --git a/libbb/common_bufsiz.c b/libbb/common_bufsiz.c
new file mode 100644
index 0000000..c16c361
--- /dev/null
+++ b/libbb/common_bufsiz.c
@@ -0,0 +1,74 @@
+/* vi: set sw=4 ts=4: */
+/*
+ * Utility routines.
+ *
+ * Copyright (C) 2016 Denys Vlasenko
+ *
+ * Licensed under GPLv2, see file LICENSE in this source tree.
+ */
+//config:config FEATURE_USE_BSS_TAIL
+//config:	bool "Use the end of BSS page"
+//config:	default n
+//config:	help
+//config:	  Attempt to reclaim a small unused part of BSS.
+//config:
+//config:	  Executables have the following parts:
+//config:	  = read-only executable code and constants, also known as "text"
+//config:	  = read-write data
+//config:	  = non-initialized (zeroed on demand) data, also known as "bss"
+//config:
+//config:	  At link time, "text" is padded to a full page. At runtime, all "text"
+//config:	  pages are mapped RO and executable.
+//config:	  "Data" starts on the next page boundary, but is not padded
+//config:	  to a full page at the end. "Bss" starts wherever "data" ends.
+//config:	  At runtime, "data" pages are mapped RW and they are file-backed
+//config:	  (this includes a small portion of "bss" which may live in the last
+//config:	  partial page of "data").
+//config:	  Pages which are fully in "bss" are mapped to anonymous memory.
+//config:
+//config:	  "Bss" end is usually not page-aligned. There is an unused space
+//config:	  in the last page. Linker marks its start with the "_end" symbol.
+//config:
+//config:	  This option will attempt to use that space for bb_common_bufsiz1[]
+//config:	  array. If it fits after _end, it will be used, and COMMON_BUFSIZE
+//config:	  will be enlarged from its guaranteed minimum size of 1 kbyte.
+//config:	  This may require recompilation a second time, since value of _end
+//config:	  is known only after final link.
+//config:
+//config:	  If you are getting a build error like this:
+//config:	        appletlib.c:(.text.main+0xd): undefined reference to '_end'
+//config:	  disable this option.
+
+//kbuild:lib-y += common_bufsiz.o
+
+#include "libbb.h"
+#include "common_bufsiz.h"
+
+#if !ENABLE_FEATURE_USE_BSS_TAIL
+
+/* We use it for "global" data via *(struct global*)bb_common_bufsiz1.
+ * Since gcc insists on aligning struct global's members, it would be a pity
+ * (and an alignment fault on some CPUs) to mess it up. */
+char bb_common_bufsiz1[COMMON_BUFSIZE] ALIGNED(sizeof(long long));
+
+#else
+
+# ifndef setup_common_bufsiz
+/*
+ * It is not a "((void)0)" macro. It means we have to provide this function.
+ */
+char* bb_common_bufsiz1;
+char* setup_common_bufsiz(void)
+{
+	if (!bb_common_bufsiz1)
+		bb_common_bufsiz1 = xzalloc(COMMON_BUFSIZE);
+	return bb_common_bufsiz1;
+}
+# else
+#  ifndef bb_common_bufsiz1
+   /* bb_common_bufsiz1[] is not aliased to _end[] */
+char bb_common_bufsiz1[COMMON_BUFSIZE] ALIGNED(sizeof(long long));
+#  endif
+# endif
+
+#endif
diff --git a/libbb/messages.c b/libbb/messages.c
index 23e440b..cb0836d 100644
--- a/libbb/messages.c
+++ b/libbb/messages.c
@@ -59,8 +59,3 @@ const char bb_path_wtmp_file[] ALIGN1 =
 #  error unknown path to wtmp file
 # endif
 #endif
-
-/* We use it for "global" data via *(struct global*)&bb_common_bufsiz1.
- * Since gcc insists on aligning struct global's members, it would be a pity
- * (and an alignment fault on some CPUs) to mess it up. */
-char bb_common_bufsiz1[COMMON_BUFSIZE] ALIGNED(sizeof(long long));