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. */ 53char 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 */ 71char *const bb_common_bufsiz1 __attribute__ ((section (".data"))); 72void 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[] */ 80char bb_common_bufsiz1[COMMON_BUFSIZE] ALIGNED(sizeof(long long)); 81# endif 82# endif 83 84#endif 85