Actual default linker script and settings gcc uses
Where can I find the actual linker script and settings gcc uses?
Things I've tried:
For concreteness, let's consider a small program: empty.c
int main(void)
{
return 0;
}
build it statically, and look at the result:
$ gcc -static -o empty empty.c
$ readelf -W -l empty
Elf file type is EXEC (Executable file)
Entry point 0x400f4e
There are 6 program headers, starting at offset 64
Program Headers:
Type Offset VirtAddr PhysAddr FileSiz MemSiz Flg Align
LOAD 0x000000 0x0000000000400000 0x0000000000400000 0x0bf581 0x0bf581 R E 0x200000
LOAD 0x0bfeb0 0x00000000006bfeb0 0x00000000006bfeb0 0x001d80 0x0042d8 RW 0x200000
NOTE 0x000190 0x0000000000400190 0x0000000000400190 0x000044 0x000044 R 0x4
TLS 0x0bfeb0 0x00000000006bfeb0 0x00000000006bfeb0 0x000020 0x000058 R 0x10
GNU_STACK 0x000000 0x0000000000000000 0x0000000000000000 0x000000 0x000000 RW 0x10
GNU_RELRO 0x0bfeb0 0x00000000006bfeb0 0x00000000006bfeb0 0x000150 0x000150 R 0x1
Section to Segment mapping:
Segment Sections...
00 .note.ABI-tag .note.gnu.build-id .rela.plt .init .plt .text __libc_freeres_fn __libc_thread_freeres_fn .fini .rodata __libc_subfreeres __libc_atexit __libc_thread_subfreeres .eh_frame .gcc_except_table
01 .tdata .init_array .fini_array .jcr .data.rel.ro .got .got.plt .data .bss __libc_freeres_ptrs
02 .note.ABI-tag .note.gnu.build-id
03 .tdata .tbss
04
05 .tdata .init_array .fini_array .jcr .data.rel.ro .got
Note the various sections, grouped into segments, and placed into memory regions of various permissions.
Now let's attempt to get as much information as possible on how it did this linking.
$ gcc -static -o empty empty.c -Wl,--verbose
GNU ld (GNU Binutils for Ubuntu) 2.24
Supported emulations:
elf_x86_64
elf32_x86_64
elf_i386
i386linux
elf_l1om
elf_k1om
i386pep
i386pe
using internal linker script:
==================================================
/* Script for -z combreloc: combine and sort reloc sections */
OUTPUT_FORMAT("elf64-x86-64", "elf64-x86-64",
"elf64-x86-64")
OUTPUT_ARCH(i386:x86-64)
ENTRY(_start)
SEARCH_DIR("/usr/x86_64-linux-gnu/lib64"); SEARCH_DIR("=/usr/local/lib/x86_64-linux-gnu"); SEARCH_DIR("=/usr/local/lib64"); SEARCH_DIR("=/lib/x86_64-linux-gnu"); SEARCH_DIR("=/lib64"); SEARCH_DIR("=/usr/lib/x86_64-linux-gnu"); SEARCH_DIR("=/usr/lib64"); SEARCH_DIR("=/usr/local/lib"); SEARCH_DIR("=/lib"); SEARCH_DIR("=/usr/lib");
SECTIONS
{
/* Read-only sections, merged into text segment: */
PROVIDE (__executable_start = SEGMENT_START("text-segment", 0x400000)); . = SEGMENT_START("text-segment", 0x400000) + SIZEOF_HEADERS;
.interp : { *(.interp) }
.note.gnu.build-id : { *(.note.gnu.build-id) }
.hash : { *(.hash) }
.gnu.hash : { *(.gnu.hash) }
.dynsym : { *(.dynsym) }
.dynstr : { *(.dynstr) }
.gnu.version : { *(.gnu.version) }
.gnu.version_d : { *(.gnu.version_d) }
.gnu.version_r : { *(.gnu.version_r) }
.rela.dyn :
{
*(.rela.init)
*(.rela.text .rela.text.* .rela.gnu.linkonce.t.*)
*(.rela.fini)
*(.rela.rodata .rela.rodata.* .rela.gnu.linkonce.r.*)
*(.rela.data .rela.data.* .rela.gnu.linkonce.d.*)
*(.rela.tdata .rela.tdata.* .rela.gnu.linkonce.td.*)
*(.rela.tbss .rela.tbss.* .rela.gnu.linkonce.tb.*)
*(.rela.ctors)
*(.rela.dtors)
*(.rela.got)
*(.rela.bss .rela.bss.* .rela.gnu.linkonce.b.*)
*(.rela.ldata .rela.ldata.* .rela.gnu.linkonce.l.*)
*(.rela.lbss .rela.lbss.* .rela.gnu.linkonce.lb.*)
*(.rela.lrodata .rela.lrodata.* .rela.gnu.linkonce.lr.*)
*(.rela.ifunc)
}
.rela.plt :
{
*(.rela.plt)
PROVIDE_HIDDEN (__rela_iplt_start = .);
*(.rela.iplt)
PROVIDE_HIDDEN (__rela_iplt_end = .);
}
.init :
{
KEEP (*(SORT_NONE(.init)))
}
.plt : { *(.plt) *(.iplt) }
.text :
{
*(.text.unlikely .text.*_unlikely .text.unlikely.*)
*(.text.exit .text.exit.*)
*(.text.startup .text.startup.*)
*(.text.hot .text.hot.*)
*(.text .stub .text.* .gnu.linkonce.t.*)
/* .gnu.warning sections are handled specially by elf32.em. */
*(.gnu.warning)
}
.fini :
{
KEEP (*(SORT_NONE(.fini)))
}
PROVIDE (__etext = .);
PROVIDE (_etext = .);
PROVIDE (etext = .);
.rodata : { *(.rodata .rodata.* .gnu.linkonce.r.*) }
.rodata1 : { *(.rodata1) }
.eh_frame_hdr : { *(.eh_frame_hdr) }
.eh_frame : ONLY_IF_RO { KEEP (*(.eh_frame)) }
.gcc_except_table : ONLY_IF_RO { *(.gcc_except_table
.gcc_except_table.*) }
/* These sections are generated by the Sun/Oracle C++ compiler. */
.exception_ranges : ONLY_IF_RO { *(.exception_ranges
.exception_ranges*) }
/* Adjust the address for the data segment. We want to adjust up to
the same address within the page on the next page up. */
. = ALIGN (CONSTANT (MAXPAGESIZE)) - ((CONSTANT (MAXPAGESIZE) - .) & (CONSTANT (MAXPAGESIZE) - 1)); . = DATA_SEGMENT_ALIGN (CONSTANT (MAXPAGESIZE), CONSTANT (COMMONPAGESIZE));
/* Exception handling */
.eh_frame : ONLY_IF_RW { KEEP (*(.eh_frame)) }
.gcc_except_table : ONLY_IF_RW { *(.gcc_except_table .gcc_except_table.*) }
.exception_ranges : ONLY_IF_RW { *(.exception_ranges .exception_ranges*) }
/* Thread Local Storage sections */
.tdata : { *(.tdata .tdata.* .gnu.linkonce.td.*) }
.tbss : { *(.tbss .tbss.* .gnu.linkonce.tb.*) *(.tcommon) }
.preinit_array :
{
PROVIDE_HIDDEN (__preinit_array_start = .);
KEEP (*(.preinit_array))
PROVIDE_HIDDEN (__preinit_array_end = .);
}
.init_array :
{
PROVIDE_HIDDEN (__init_array_start = .);
KEEP (*(SORT_BY_INIT_PRIORITY(.init_array.*) SORT_BY_INIT_PRIORITY(.ctors.*)))
KEEP (*(.init_array EXCLUDE_FILE (*crtbegin.o *crtbegin?.o *crtend.o *crtend?.o ) .ctors))
PROVIDE_HIDDEN (__init_array_end = .);
}
.fini_array :
{
PROVIDE_HIDDEN (__fini_array_start = .);
KEEP (*(SORT_BY_INIT_PRIORITY(.fini_array.*) SORT_BY_INIT_PRIORITY(.dtors.*)))
KEEP (*(.fini_array EXCLUDE_FILE (*crtbegin.o *crtbegin?.o *crtend.o *crtend?.o ) .dtors))
PROVIDE_HIDDEN (__fini_array_end = .);
}
.ctors :
{
/* gcc uses crtbegin.o to find the start of
the constructors, so we make sure it is
first. Because this is a wildcard, it
doesn't matter if the user does not
actually link against crtbegin.o; the
linker won't look for a file to match a
wildcard. The wildcard also means that it
doesn't matter which directory crtbegin.o
is in. */
KEEP (*crtbegin.o(.ctors))
KEEP (*crtbegin?.o(.ctors))
/* We don't want to include the .ctor section from
the crtend.o file until after the sorted ctors.
The .ctor section from the crtend file contains the
end of ctors marker and it must be last */
KEEP (*(EXCLUDE_FILE (*crtend.o *crtend?.o ) .ctors))
KEEP (*(SORT(.ctors.*)))
KEEP (*(.ctors))
}
.dtors :
{
KEEP (*crtbegin.o(.dtors))
KEEP (*crtbegin?.o(.dtors))
KEEP (*(EXCLUDE_FILE (*crtend.o *crtend?.o ) .dtors))
KEEP (*(SORT(.dtors.*)))
KEEP (*(.dtors))
}
.jcr : { KEEP (*(.jcr)) }
.data.rel.ro : { *(.data.rel.ro.local* .gnu.linkonce.d.rel.ro.local.*) *(.data.rel.ro .data.rel.ro.* .gnu.linkonce.d.rel.ro.*) }
.dynamic : { *(.dynamic) }
.got : { *(.got) *(.igot) }
. = DATA_SEGMENT_RELRO_END (SIZEOF (.got.plt) >= 24 ? 24 : 0, .);
.got.plt : { *(.got.plt) *(.igot.plt) }
.data :
{
*(.data .data.* .gnu.linkonce.d.*)
SORT(CONSTRUCTORS)
}
.data1 : { *(.data1) }
_edata = .; PROVIDE (edata = .);
. = .;
__bss_start = .;
.bss :
{
*(.dynbss)
*(.bss .bss.* .gnu.linkonce.b.*)
*(COMMON)
/* Align here to ensure that the .bss section occupies space up to
_end. Align after .bss to ensure correct alignment even if the
.bss section disappears because there are no input sections.
FIXME: Why do we need it? When there is no .bss section, we don't
pad the .data section. */
. = ALIGN(. != 0 ? 64 / 8 : 1);
}
.lbss :
{
*(.dynlbss)
*(.lbss .lbss.* .gnu.linkonce.lb.*)
*(LARGE_COMMON)
}
. = ALIGN(64 / 8);
. = SEGMENT_START("ldata-segment", .);
.lrodata ALIGN(CONSTANT (MAXPAGESIZE)) + (. & (CONSTANT (MAXPAGESIZE) - 1)) :
{
*(.lrodata .lrodata.* .gnu.linkonce.lr.*)
}
.ldata ALIGN(CONSTANT (MAXPAGESIZE)) + (. & (CONSTANT (MAXPAGESIZE) - 1)) :
{
*(.ldata .ldata.* .gnu.linkonce.l.*)
. = ALIGN(. != 0 ? 64 / 8 : 1);
}
. = ALIGN(64 / 8);
_end = .; PROVIDE (end = .);
. = DATA_SEGMENT_END (.);
/* Stabs debugging sections. */
.stab 0 : { *(.stab) }
.stabstr 0 : { *(.stabstr) }
.stab.excl 0 : { *(.stab.excl) }
.stab.exclstr 0 : { *(.stab.exclstr) }
.stab.index 0 : { *(.stab.index) }
.stab.indexstr 0 : { *(.stab.indexstr) }
.comment 0 : { *(.comment) }
/* DWARF debug sections.
Symbols in the DWARF debugging sections are relative to the beginning
of the section so we begin them at 0. */
/* DWARF 1 */
.debug 0 : { *(.debug) }
.line 0 : { *(.line) }
/* GNU DWARF 1 extensions */
.debug_srcinfo 0 : { *(.debug_srcinfo) }
.debug_sfnames 0 : { *(.debug_sfnames) }
/* DWARF 1.1 and DWARF 2 */
.debug_aranges 0 : { *(.debug_aranges) }
.debug_pubnames 0 : { *(.debug_pubnames) }
/* DWARF 2 */
.debug_info 0 : { *(.debug_info .gnu.linkonce.wi.*) }
.debug_abbrev 0 : { *(.debug_abbrev) }
.debug_line 0 : { *(.debug_line .debug_line.* .debug_line_end ) }
.debug_frame 0 : { *(.debug_frame) }
.debug_str 0 : { *(.debug_str) }
.debug_loc 0 : { *(.debug_loc) }
.debug_macinfo 0 : { *(.debug_macinfo) }
/* SGI/MIPS DWARF 2 extensions */
.debug_weaknames 0 : { *(.debug_weaknames) }
.debug_funcnames 0 : { *(.debug_funcnames) }
.debug_typenames 0 : { *(.debug_typenames) }
.debug_varnames 0 : { *(.debug_varnames) }
/* DWARF 3 */
.debug_pubtypes 0 : { *(.debug_pubtypes) }
.debug_ranges 0 : { *(.debug_ranges) }
/* DWARF Extension. */
.debug_macro 0 : { *(.debug_macro) }
.gnu.attributes 0 : { KEEP (*(.gnu.attributes)) }
/DISCARD/ : { *(.note.GNU-stack) *(.gnu_debuglink) *(.gnu.lto_*) }
}
==================================================
... <snip searching and linking actual object files>
But the script, while long, is missing most of the import information previously listed.
How does it know which sections to gather into different load segments?
There is no PHDRS command, and while the use of SEGMENT_START suggests there are some standard segments for this system defined somewhere else, none of the sections are listed with an associated segment.
Furthermore, how does it know where to load these segments, or what permissions these memory regions have?
There is no MEMORY command. And again, if there are some standard memory regions for this system defined somewhere else, none of the sections list which memory region to use.
When I've seen default linker scripts before for microcontrollers, they were incredibly detailed. This output however suggests there are more scripts and settings somewhere.
Where are these other linker script definitions and settings stored?
Answer
Well, I know that this is an old question, but I also found it frustrating that there is no precise info about options that are used during the linking process. This answer shows my journey to find them.
First of all, I was looking into official docs https://gcc.gnu.org/onlinedocs/ - I searched the GCC Manual and GCC Internals Manual. The only meaningful information that I found is that gcc uses an internal tool called collect2 to invoke the linker. According to https://gcc.gnu.org/onlinedocs/gccint/Collect2.html "The program collect2 works by linking the program once and looking through the linker output file for symbols with particular names indicating they are constructor functions". So it's used to make linking possible.
Next thing that I tried is getting through source code. You can browse code here https://code.woboq.org/gcc/gcc/collect2.c.html . The problem is that it wasn't really helpful. But I noticed that collect2 fork_execute function to invoke ld. You can deep dive into fork_execute to find out that it will fork (execute a new program in the forked program) and wait for it to finish. Because both forks and execs are system calls (to put it quickly - system calls are the way/functions the application uses to communicate with a system). I decided to give it a try.
So I made the simple program that doesn't require any compilation (it's already compiled to object file - so everything that gcc have to do is linking).
[Alex@Normandy tmp]$ gcc hello.c.s -o hello_gcc
[Alex@Normandy tmp]$ ./hello_gcc
Hello, World!
Then I use strace with following options:
-o forked.logsave the output to forked.log-s 1024variables shorter than 1024 chars are not truncated (default 32 was not enough)-f- enables strace on forked processes-e trace=/exec- filter system calls so only ones starting withexecare shown
The final output was following.
[Alex@Normandy tmp]$ strace -o forked.log -s 1024 -f -e trace=/exec gcc hello.c.s -o hello_gcc
[Alex@Normandy tmp]$ grep 'ld' forked.log
2153 execve("/usr/libexec/gcc/x86_64-redhat-linux/4.8.5/collect2", ["/usr/libexec/gcc/x86_64-redhat-linux/4.8.5/collect2", "--build-id", "--no-add-needed", "--eh-frame-hdr", "--hash-style=gnu", "-m", "elf_x86_64", "-dynamic-linker", "/lib64/ld-linux-x86-64.so.2", "-o", "hello_gcc", "/usr/lib/gcc/x86_64-redhat-linux/4.8.5/../../../../lib64/crt1.o", "/usr/lib/gcc/x86_64-redhat-linux/4.8.5/../../../../lib64/crti.o", "/usr/lib/gcc/x86_64-redhat-linux/4.8.5/crtbegin.o", "-L/usr/lib/gcc/x86_64-redhat-linux/4.8.5", "-L/usr/lib/gcc/x86_64-redhat-linux/4.8.5/../../../../lib64", "-L/lib/../lib64", "-L/usr/lib/../lib64", "-L/usr/lib/gcc/x86_64-redhat-linux/4.8.5/../../..", "/tmp/ccyl36jf.o", "-lgcc", "--as-needed", "-lgcc_s", "--no-as-needed", "-lc", "-lgcc", "--as-needed", "-lgcc_s", "--no-as-needed", "/usr/lib/gcc/x86_64-redhat-linux/4.8.5/crtend.o", "/usr/lib/gcc/x86_64-redhat-linux/4.8.5/../../../../lib64/crtn.o"], 0x17b9da0 /* 61 vars */) = 0
2154 execve("/usr/bin/ld", ["/usr/bin/ld", "--build-id", "--no-add-needed", "--eh-frame-hdr", "--hash-style=gnu", "-m", "elf_x86_64", "-dynamic-linker", "/lib64/ld-linux-x86-64.so.2", "-o", "hello_gcc", "/usr/lib/gcc/x86_64-redhat-linux/4.8.5/../../../../lib64/crt1.o", "/usr/lib/gcc/x86_64-redhat-linux/4.8.5/../../../../lib64/crti.o", "/usr/lib/gcc/x86_64-redhat-linux/4.8.5/crtbegin.o", "-L/usr/lib/gcc/x86_64-redhat-linux/4.8.5", "-L/usr/lib/gcc/x86_64-redhat-linux/4.8.5/../../../../lib64", "-L/lib/../lib64", "-L/usr/lib/../lib64", "-L/usr/lib/gcc/x86_64-redhat-linux/4.8.5/../../..", "/tmp/ccyl36jf.o", "-lgcc", "--as-needed", "-lgcc_s", "--no-as-needed", "-lc", "-lgcc", "--as-needed", "-lgcc_s", "--no-as-needed", "/usr/lib/gcc/x86_64-redhat-linux/4.8.5/crtend.o", "/usr/lib/gcc/x86_64-redhat-linux/4.8.5/../../../../lib64/crtn.o"], 0x7fff14226a98 /* 61 vars */) = 0
So used ld command was
/usr/bin/ld --build-id --no-add-needed --eh-frame-hdr --hash-style=gnu -m elf_x86_64 -dynamic-linker /lib64/ld-linux-x86-64.so.2 -o hello_gcc /usr/lib/gcc/x86_64-redhat-linux/4.8.5/../../../../lib64/crt1.o /usr/lib/gcc/x86_64-redhat-linux/4.8.5/../../../../lib64/crti.o /usr/lib/gcc/x86_64-redhat-linux/4.8.5/crtbegin.o -L/usr/lib/gcc/x86_64-redhat-linux/4.8.5 -L/usr/lib/gcc/x86_64-redhat-linux/4.8.5/../../../../lib64 -L/lib/../lib64 -L/usr/lib/../lib64 -L/usr/lib/gcc/x86_64-redhat-linux/4.8.5/../../.. /tmp/ccyl36jf.o -lgcc --as-needed -lgcc_s --no-as-needed -lc -lgcc --as-needed -lgcc_s --no-as-needed /usr/lib/gcc/x86_64-redhat-linux/4.8.5/crtend.o /usr/lib/gcc/x86_64-redhat-linux/4.8.5/../../../../lib64/crtn.o
So what the f*** was made? Well, all options can be found in the manual. Here is decomposed output.
/usr/bin/ld- linker program--build-id- add build-id to binary. In my system default it is sha1.--no-add-needed- it is depracaceted name for--no-copy-dt-needed-entries- it is connected with DT_NEEDED tags inside ELF, if I get that correctly it means that DT_NEEDED tag won't be copied from input libraries.--eh-frame-hdr- "Request creation of ".eh_frame_hdr" section and ELF "PT_GNU_EH_FRAME" segment header." Whatever that means.--hash-style=gnu- "Set the type of linker's hash table(s)." Default is sysv, but there is a newer formatgnu. Binary can also have a hash table(s) in both formats.-m elf_x86_64- linkers emulates (makes elf type binary for x86_64)-dynamic-linker /lib64/ld-linux-x86-64.so.2- set name of expected dynamic linker-o hello_gcc- set output binary/usr/lib/gcc/x86_64-redhat-linux/4.8.5/../../../../lib64/crt1.o- code that is run before main of actual program/usr/lib/gcc/x86_64-redhat-linux/4.8.5/../../../../lib64/crti.o- code that is run before main of actual program/usr/lib/gcc/x86_64-redhat-linux/4.8.5/crtbegin.o- code that is run before main of actual program-L/usr/lib/gcc/x86_64-redhat-linux/4.8.5- additional library search path-L/usr/lib/gcc/x86_64-redhat-linux/4.8.5/../../../../lib64- additional library search path-L/lib/../lib64- additional library search path-L/usr/lib/../lib64- additional library search path-L/usr/lib/gcc/x86_64-redhat-linux/4.8.5/../../..- additional library search path/tmp/ccyl36jf.o- this is actual program (binary object) with it's main function-lgcc--loption - "Add the archive or object file specified by namespec to the list of files to link." In that case it is gcc.--as-needed- enable "as-needed" mode that checks if on particular point following library (namespace?) is needed-lgcc_s- add gcc_s note that only if it's really needed at this moment.--no-as-needed- disable "as-needed" mode that checks if on particular point following library (namespace?) is needed-lc- standard C namespace/library-lgcc- this lib should be already set. There might be something between this and previous usage of this option.--as-needed- set "as-needed mode. There might be something between this and previous usage of this option.-lgcc_s- already described. There might be something between this and previous usage of this option.--no-as-needed-- disable "as-needed mode"./usr/lib/gcc/x86_64-redhat-linux/4.8.5/crtend.o- additional code that run when program finish/usr/lib/gcc/x86_64-redhat-linux/4.8.5/../../../../lib64/crtn.o- additional code that run when program finish.
More about: crt1.o, crti.o, crtbegin.o, crtend.o, crtn.o - they are startup, initialization, constructor, destructor and finalization files (according to Building Embedded Linux Systems By Karim Yaghmour).
Probably simpler way
During writing this answer I also "discovered" that you can invoke gcc with -v option and it will return COLLECT_GCC_OPTIONS, that is identical to invoked ld
COLLECT_GCC_OPTIONS='-v' '-o' 'hello_gcc' '-mtune=generic' '-march=x86-64'
/usr/libexec/gcc/x86_64-redhat-linux/4.8.5/collect2 --build-id --no-add-needed --eh-frame-hdr --hash-style=gnu -m elf_x86_64 -dynamic-linker /lib64/ld-linux-x86-64.so.2 -o hello_gcc /usr/lib/gcc/x86_64-redhat-linux/4.8.5/../../../../lib64/crt1.o /usr/lib/gcc/x86_64-redhat-linux/4.8.5/../../../../lib64/crti.o /usr/lib/gcc/x86_64-redhat-linux/4.8.5/crtbegin.o -L/usr/lib/gcc/x86_64-redhat-linux/4.8.5 -L/usr/lib/gcc/x86_64-redhat-linux/4.8.5/../../../../lib64 -L/lib/../lib64 -L/usr/lib/../lib64 -L/usr/lib/gcc/x86_64-redhat-linux/4.8.5/../../.. hello_gcc.o -lgcc --as-needed -lgcc_s --no-as-needed -lc -lgcc --as-needed -lgcc_s --no-as-needed /usr/lib/gcc/x86_64-redhat-linux/4.8.5/crtend.o /usr/lib/gcc/x86_64-redhat-linux/4.8.5/../../../../lib64/crtn.o
Still, if you want to be sure for 100% how ld was invoked - the strace is your best bet.
Lastly, note that I used Enterprise Linux v7 and v8 system to check if I'm right. Both of them uses the x86_64 arch, and the results might be different on different architectures.