1 /* vi: set sw=4 ts=4: */
2 /*
3 * Utility routines.
4 *
5 * Copyright (C) 2016 Denys Vlasenko
6 *
7 * Licensed under GPLv2, see file LICENSE in this source tree.
8 */
9 //config:config FEATURE_USE_BSS_TAIL
10 //config: bool "Use the end of BSS page"
11 //config: default n
12 //config: help
13 //config: Attempt to reclaim a small unused part of BSS.
14 //config:
15 //config: Executables have the following parts:
16 //config: = read-only executable code and constants, also known as "text"
17 //config: = read-write data
18 //config: = non-initialized (zeroed on demand) data, also known as "bss"
19 //config:
20 //config: At link time, "text" is padded to a full page. At runtime, all "text"
21 //config: pages are mapped RO and executable.
22 //config: "Data" starts on the next page boundary, but is not padded
23 //config: to a full page at the end. "Bss" starts wherever "data" ends.
24 //config: At runtime, "data" pages are mapped RW and they are file-backed
25 //config: (this includes a small portion of "bss" which may live in the last
26 //config: partial page of "data").
27 //config: Pages which are fully in "bss" are mapped to anonymous memory.
28 //config:
29 //config: "Bss" end is usually not page-aligned. There is an unused space
30 //config: in the last page. Linker marks its start with the "_end" symbol.
31 //config:
32 //config: This option will attempt to use that space for bb_common_bufsiz1[]
33 //config: array. If it fits after _end, it will be used, and COMMON_BUFSIZE
34 //config: will be enlarged from its guaranteed minimum size of 1 kbyte.
35 //config: This may require recompilation a second time, since value of _end
36 //config: is known only after final link.
37 //config:
38 //config: If you are getting a build error like this:
39 //config: appletlib.c:(.text.main+0xd): undefined reference to '_end'
40 //config: disable this option.
41
42 //kbuild:lib-y += common_bufsiz.o
43
44 #include "libbb.h"
45 #include "common_bufsiz.h"
46
47 #if !ENABLE_FEATURE_USE_BSS_TAIL
48
49 /* We use it for "global" data via *(struct global*)bb_common_bufsiz1.
50 * Since gcc insists on aligning struct global's members, it would be a pity
51 * (and an alignment fault on some CPUs) to mess it up. */
52 char bb_common_bufsiz1[COMMON_BUFSIZE] ALIGNED(sizeof(long long));
53
54 #else
55
56 # ifndef setup_common_bufsiz
57 /* For now, this is never used:
58 * scripts/generate_BUFSIZ.sh never generates "malloced" bufsiz1:
59 * enum { COMMON_BUFSIZE = 1024 };
60 * extern char *const bb_common_bufsiz1;
61 * void setup_common_bufsiz(void);
62 * This has proved to be worse than the approach of defining
63 * larger bb_common_bufsiz1[] array.
64 */
65
66 /*
67 * It is not defined as a dummy macro.
68 * It means we have to provide this function.
69 */
70 char *const bb_common_bufsiz1 __attribute__ ((section (".data")));
setup_common_bufsiz(void)71 void setup_common_bufsiz(void)
72 {
73 if (!bb_common_bufsiz1)
74 *(char**)&bb_common_bufsiz1 = xzalloc(COMMON_BUFSIZE);
75 }
76 # else
77 # ifndef bb_common_bufsiz1
78 /* bb_common_bufsiz1[] is not aliased to _end[] */
79 char bb_common_bufsiz1[COMMON_BUFSIZE] ALIGNED(sizeof(long long));
80 # endif
81 # endif
82
83 #endif
84