1 | /* vi: set sw=4 ts=4: */
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2 | /*
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3 | * Utility routines.
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4 | *
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5 | * Copyright (C) 2016 Denys Vlasenko
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6 | *
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7 | * Licensed under GPLv2, see file LICENSE in this source tree.
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8 | */
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9 | //config:config FEATURE_USE_BSS_TAIL
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10 | //config: bool "Use the end of BSS page"
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11 | //config: default n
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12 | //config: help
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13 | //config: Attempt to reclaim a small unused part of BSS.
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14 | //config:
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15 | //config: Executables have the following parts:
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16 | //config: = read-only executable code and constants, also known as "text"
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17 | //config: = read-write data
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18 | //config: = non-initialized (zeroed on demand) data, also known as "bss"
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19 | //config:
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20 | //config: At link time, "text" is padded to a full page. At runtime, all "text"
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21 | //config: pages are mapped RO and executable.
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22 | //config: "Data" starts on the next page boundary, but is not padded
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23 | //config: to a full page at the end. "Bss" starts wherever "data" ends.
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24 | //config: At runtime, "data" pages are mapped RW and they are file-backed
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25 | //config: (this includes a small portion of "bss" which may live in the last
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26 | //config: partial page of "data").
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27 | //config: Pages which are fully in "bss" are mapped to anonymous memory.
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28 | //config:
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29 | //config: "Bss" end is usually not page-aligned. There is an unused space
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30 | //config: in the last page. Linker marks its start with the "_end" symbol.
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31 | //config:
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32 | //config: This option will attempt to use that space for bb_common_bufsiz1[]
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33 | //config: array. If it fits after _end, it will be used, and COMMON_BUFSIZE
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34 | //config: will be enlarged from its guaranteed minimum size of 1 kbyte.
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35 | //config: This may require recompilation a second time, since value of _end
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36 | //config: is known only after final link.
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37 | //config:
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38 | //config: If you are getting a build error like this:
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39 | //config: appletlib.c:(.text.main+0xd): undefined reference to '_end'
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40 | //config: disable this option.
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41 |
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42 | //kbuild:lib-y += common_bufsiz.o
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43 |
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44 | #include "libbb.h"
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45 | #include "common_bufsiz.h"
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46 |
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47 | #if !ENABLE_FEATURE_USE_BSS_TAIL
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48 |
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49 | /* We use it for "global" data via *(struct global*)bb_common_bufsiz1.
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50 | * Since gcc insists on aligning struct global's members, it would be a pity
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51 | * (and an alignment fault on some CPUs) to mess it up. */
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52 | char bb_common_bufsiz1[COMMON_BUFSIZE] ALIGNED(sizeof(long long));
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53 |
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54 | #else
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55 |
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56 | # ifndef setup_common_bufsiz
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57 | /*
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58 | * It is not defined as a dummy macro.
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59 | * It means we have to provide this function.
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60 | */
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61 | char *const bb_common_bufsiz1 __attribute__ ((section (".data")));
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62 | void setup_common_bufsiz(void)
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63 | {
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64 | if (!bb_common_bufsiz1)
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65 | *(char**)&bb_common_bufsiz1 = xzalloc(COMMON_BUFSIZE);
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66 | }
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67 | # else
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68 | # ifndef bb_common_bufsiz1
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69 | /* bb_common_bufsiz1[] is not aliased to _end[] */
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70 | char bb_common_bufsiz1[COMMON_BUFSIZE] ALIGNED(sizeof(long long));
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71 | # endif
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72 | # endif
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73 |
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74 | #endif
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