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:
23 //config: "Data" starts on the next page boundary, but is not padded
24 //config: to a full page at the end. "Bss" starts wherever "data" ends.
25 //config: At runtime, "data" pages are mapped RW and they are file-backed
26 //config: (this includes a small portion of "bss" which may live in the last
27 //config: partial page of "data").
28 //config: Pages which are fully in "bss" are mapped to anonymous memory.
29 //config:
30 //config: "Bss" end is usually not page-aligned. There is an unused space
31 //config: in the last page. Linker marks its start with the "_end" symbol.
32 //config:
33 //config: This option will attempt to use that space for bb_common_bufsiz1[]
34 //config: array. If it fits after _end, it will be used, and COMMON_BUFSIZE
35 //config: will be enlarged from its guaranteed minimum size of 1 kbyte.
36 //config: This may require recompilation a second time, since value of _end
37 //config: is known only after final link.
38 //config:
39 //config: If you are getting a build error like this:
40 //config: appletlib.c:(.text.main+0xd): undefined reference to '_end'
41 //config: disable this option.
42
43 //kbuild:lib-y += common_bufsiz.o
44
45 #include "libbb.h"
46 #include "common_bufsiz.h"
47
48 #if !ENABLE_FEATURE_USE_BSS_TAIL
49
50 /* We use it for "global" data via *(struct global*)bb_common_bufsiz1.
51 * Since gcc insists on aligning struct global's members, it would be a pity
52 * (and an alignment fault on some CPUs) to mess it up. */
53 char bb_common_bufsiz1[COMMON_BUFSIZE] ALIGNED(sizeof(long long));
54
55 #else
56
57 # ifndef setup_common_bufsiz
58 /* For now, this is never used:
59 * scripts/generate_BUFSIZ.sh never generates "malloced" bufsiz1:
60 * enum { COMMON_BUFSIZE = 1024 };
61 * extern char *const bb_common_bufsiz1;
62 * void setup_common_bufsiz(void);
63 * This has proved to be worse than the approach of defining
64 * larger bb_common_bufsiz1[] array.
65 */
66
67 /*
68 * It is not defined as a dummy macro.
69 * It means we have to provide this function.
70 */
71 char *const bb_common_bufsiz1 __attribute__ ((section (".data")));
setup_common_bufsiz(void)72 void setup_common_bufsiz(void)
73 {
74 if (!bb_common_bufsiz1)
75 *(char**)&bb_common_bufsiz1 = xzalloc(COMMON_BUFSIZE);
76 }
77 # else
78 # ifndef bb_common_bufsiz1
79 /* bb_common_bufsiz1[] is not aliased to _end[] */
80 char bb_common_bufsiz1[COMMON_BUFSIZE] ALIGNED(sizeof(long long));
81 # endif
82 # endif
83
84 #endif
85