blob: 2847eb57d6d7fdbd53173567108b07993e120b1d
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"))); |
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 |