84 lines
3.0 KiB
C
84 lines
3.0 KiB
C
/* vi: set sw=4 ts=4: */
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/*
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* Utility routines.
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*
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* Copyright (C) 2016 Denys Vlasenko
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*
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* Licensed under GPLv2, see file LICENSE in this source tree.
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*/
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//config:config FEATURE_USE_BSS_TAIL
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//config: bool "Use the end of BSS page"
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//config: default n
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//config: help
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//config: Attempt to reclaim a small unused part of BSS.
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//config:
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//config: Executables have the following parts:
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//config: = read-only executable code and constants, also known as "text"
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//config: = read-write data
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//config: = non-initialized (zeroed on demand) data, also known as "bss"
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//config:
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//config: At link time, "text" is padded to a full page. At runtime, all "text"
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//config: pages are mapped RO and executable.
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//config: "Data" starts on the next page boundary, but is not padded
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//config: to a full page at the end. "Bss" starts wherever "data" ends.
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//config: At runtime, "data" pages are mapped RW and they are file-backed
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//config: (this includes a small portion of "bss" which may live in the last
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//config: partial page of "data").
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//config: Pages which are fully in "bss" are mapped to anonymous memory.
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//config:
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//config: "Bss" end is usually not page-aligned. There is an unused space
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//config: in the last page. Linker marks its start with the "_end" symbol.
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//config:
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//config: This option will attempt to use that space for bb_common_bufsiz1[]
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//config: array. If it fits after _end, it will be used, and COMMON_BUFSIZE
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//config: will be enlarged from its guaranteed minimum size of 1 kbyte.
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//config: This may require recompilation a second time, since value of _end
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//config: is known only after final link.
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//config:
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//config: If you are getting a build error like this:
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//config: appletlib.c:(.text.main+0xd): undefined reference to '_end'
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//config: disable this option.
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//kbuild:lib-y += common_bufsiz.o
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#include "libbb.h"
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#include "common_bufsiz.h"
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#if !ENABLE_FEATURE_USE_BSS_TAIL
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/* We use it for "global" data via *(struct global*)bb_common_bufsiz1.
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* Since gcc insists on aligning struct global's members, it would be a pity
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* (and an alignment fault on some CPUs) to mess it up. */
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char bb_common_bufsiz1[COMMON_BUFSIZE] ALIGNED(sizeof(long long));
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#else
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# ifndef setup_common_bufsiz
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/* For now, this is never used:
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* scripts/generate_BUFSIZ.sh never generates "malloced" bufsiz1:
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* enum { COMMON_BUFSIZE = 1024 };
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* extern char *const bb_common_bufsiz1;
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* void setup_common_bufsiz(void);
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* This has proved to be worse than the approach of defining
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* larger bb_common_bufsiz1[] array.
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*/
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/*
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* It is not defined as a dummy macro.
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* It means we have to provide this function.
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*/
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char *const bb_common_bufsiz1 __attribute__ ((section (".data")));
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void setup_common_bufsiz(void)
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{
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if (!bb_common_bufsiz1)
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*(char**)&bb_common_bufsiz1 = xzalloc(COMMON_BUFSIZE);
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}
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# else
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# ifndef bb_common_bufsiz1
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/* bb_common_bufsiz1[] is not aliased to _end[] */
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char bb_common_bufsiz1[COMMON_BUFSIZE] ALIGNED(sizeof(long long));
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# endif
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# endif
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#endif
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