I've done a bunch of reading on dynamic linker relocations and position independent code including procedure linkage tables and global offset tables. I don't understand why a statically linked executable needs a PLT and GOT. I compiled a hello world program on my ubuntu x86_64 machine and when I dump the section headers with readelf -S it shows PLT and GOT sections.
I also created a shared library with a simple increment function that I compiled with gcc -shared without -fpic and I also see PLT and GOT sections. I didn't expect this either.
I don't understand why a statically linked executable needs a PLT and GOT.
It doesn't.
I compiled a hello world program on my ubuntu x86_64 machine and when I dump the section headers with readelf -S it shows PLT and GOT sections.
This is an accident of implementation. The sections come from crt1.o, and there isn't a separate crt1s.o for fully-static linking, so you end up with .plt and .got entries from there.
You can strip these sections, and the binary will still work:
objcopy -R.got -R.plt a.out a.out2
Note: do not strip .rela.plt, as that section is still needed to implement IFUNCs.
I found that gcc generates a .got and .got.lpt when generating position independent code and taking the address of a function defined in another source file.
My test files were:
part1.c:
extern void afunc();
int _start()
{
return 0x55 & (__SIZE_TYPE__) afunc;
}
part2.c:
void afunc() {}
My test was (substitute your own gcc version):
for o in s 4 3 2 1 0
do
aarch64-linux-gnu-gcc-10 -fPIC part1.c part2.c -o static.elf -static -nostdlib -O$o &&
aarch64-linux-gnu-objdump -x static.elf | grep 'GLOBAL_OFFSET'
done
I get the following output for all optimization levels:
0000000000410fd8 l O .got 0000000000000000 _GLOBAL_OFFSET_TABLE_
Replacing -fPIC with -fno-PIC and the segment goes away.
You can tell if your compiler defaults to -fPIC by running this:
aarch64-linux-gnu-gcc-10 -mcmodel=large -x c - < /dev/null
From which, I get the error, if it does:
cc1: sorry, unimplemented: code model ‘large’ with ‘-fPIC’
Related
I am trying to follow the exercise in the book PC Assembly by Paul Carter. http://pacman128.github.io/pcasm/
I'm trying to run the program from 1.4 page 23 on Ubuntu 18. The files are all available on the github site above.
Since original code is for 32bit I compile using
nasm -f elf32
for first.asm and asm_io.asm to get the object files. I also compile driver.c
I use the linker from gcc and run
gcc -m32 -o first first.o asm_io.o driver.o
but it keeps giving me a bun of errors like
undefined reference to '_scanf'
undefined reference to '_printf'
(note _printf appears instead of printf because some conversion is done in the file asm_io.asm to maintain compatibility between windows and linux OS's)
I don't know why these errors are appearing. I also try running using linker directly
ld -m elf_i386 -e main -o first -first.o driver.o asm_io.o -I /lib/i386-linux-gnu/ld-linux.so.2
and many variations since it seems that its not linking with the C libraries.
Any help? Stuck on this for a while and couldn't find a solution on similar questions
Linux doesn't prepend _ to names when mapping from C to asm symbol names in ELF object files1.
So call printf, not _printf, because there is no _printf in libc.
Whatever "compatibility" code did that is doing it wrong. Only Windows and OS X use _printf, Linux uses printf.
So either you've misconfigured something or defined the wrong setting, or it requires updating / porting to Linux.
Footnote 1: In ancient history (like over 20 years ago), Linux with the a.out file format did use leading underscores on symbol names.
Update: the library uses the NASM preprocessor to %define _scanf scanf and so on, but it requires you to manually define ELF_TYPE by assembling with nasm -d ELF_TYPE.
They could have detected ELF32 or ELF64 output formats on their own, because NASM pre-defines __OUTPUT_FORMAT__. Someone should submit a pull-request to make this detection automatic with code something like this:
%ifidn __OUTPUT_FORMAT__, elf32
%define ELF_TYPE 32
%elifidn __OUTPUT_FORMAT__, elf64
%define ELF_TYPE 64
%endif
%ifdef ELF_TYPE
...
%endif
This question already has answers here:
Assembling 32-bit binaries on a 64-bit system (GNU toolchain)
(2 answers)
Closed 6 years ago.
I tried to write simple program without main
segment .data
fmt db "test", 0xa, 0
segment .text
global _start
extern printf
_start:
lea rdi, [fmt] ; print simple string
xor eax, eax
call printf
mov eax, 60 ; exit successfully
xor edi, edi
syscall
Compile:
yasm -f elf64 main.s; ld -o main main.o
Got
main.o: In function `_start':
main.s:(.text+0xb): undefined reference to `printf'
How should one fix this?
Cause of Your Error
The reason you get undefined reference to printf while linking is because you didn't link against the C library. As well, when using your own _start label as an entry point there are other issues that must be overcome as discussed below.
Using LIBC without C Runtime Startup Code
To make sure you are using the appropriate dynamic linker at runtime and link against the C library you can use GCC as a frontend for LD. To supply your own _start label and avoid the C runtime startup code use the -nostartfiles option. Although I don't recommend this method, you can link with:
gcc -m64 -nostartfiles main.o -o main
If you wish to use LD you can do it by using a specific dynamic linker and link against the C library with -lc. To link x86_64 code you can use:
ld -melf_x86_64 --dynamic-linker=/lib64/ld-linux-x86-64.so.2 main.o -lc -o main
If you had been compiling and linking for 32-bit, then the linking could have been done with:
ld -melf_i386 --dynamic-linker=/lib/ld-linux.so.2 main.o -lc -o main
Preferred Method Using C Runtime Startup Code
Since you are using the C library you should consider linking against the C runtime. This method also works if you were to create a static executable. The best way to use the C library is to rename your _start label to main and then use GCC to link:
gcc -m64 main.o -o main
Doing it this way allows you to exit your program by returning (using RET) from main the same way a C program ends. This also has the advantage of flushing standard output when the program exits.
Flushing output buffers / exit
Using syscall with EAX=60 to exit won't flush the output buffers. Because of this you may find yourself having to add a newline character on your output to see output before exiting. This is still not a guarantee you will see what you output with printf. Rather than the sys_exit SYSCALL you might consider calling the C library function exit. the MAN PAGE for exit says:
All open stdio(3) streams are flushed and closed. Files created by tmpfile(3) are removed.
I'd recommend replacing the sys_exit SYSCALL with:
extern exit
xor edi, edi ; In 64-bit code RDI is 1st argument = return value
call exit
The printf function is provided by the C standard library, libc. To use it, you need to pass -lc to the linker:
ld -o main main.o -lc
I recommend you to use the C compiler as a frontend for the linker to get possible other flags right, too:
cc -o main -nostartfiles main.o
Note that since your program doesn't initialize the C standard library, e parts of it may not work as expected. If you want to use the C standard library in your assembly program, I recommend you to make your assembly program start from main and to link it by invoking the C compiler:
yasm -f elf64 main.s
cc -o main main.o
Is there any way we can get gcc to detect a duplicate symbol in static libraries vs the main code (Or another static library ?)
Here's the situation:
main.c erroneously contained a function definition, e.g. with the signature uint foohash(const char*)
foo.c also contains a function definition with the signature uint foohash(const char*)
foo.c and other source files are compiled to a static util library, which the main program links in, i.e. something like:
gcc -o main main.o util.o -L ./libs -lfooutils
So, now main.o and libs/libfooutils.a both contain a foohash function. Presumably the linker found that symbol in main.o and doesn't bother looking for it elsewhere.
Is there any way we can get gcc to detect such a situation ?
Indeed as Simon Richter stated, --whole-archive option can be useful. Try to change your command-line to:
gcc -o main main.o util.o -L ./libs -Wl,--whole-archive -lfooutils -Wl,--no-whole-archive
and you'll see a multiple definition error.
gcc calls the ld program for linking. The relevant ld options are:
--no-define-common
--traditional-format
--warn-common
See the man page for ld. These should be what you need to experiment with to get the warnings sought.
Short answer: no.
GCC does not actually do anything with libraries. It is the task of ld, the linker (called automatically by GCC) to pull in symbols from libraries, and that's really a fairly dumb tool.
The linker has lots of complex jiggery pokery for combining different types of data from different sources, and supporting different file formats, and all the evil little details of binary executables, but in the end, all it really does is look for undefined symbols and find the definitions.
What you can do is a link trace (pass -t to gcc) to see what comes from where. Or else run nm on all the object files and libraries in your system, and write a script to detect duplicates.
I am trying to understand how to dynamically create and link a shared library in Fortran under Linux.
I have two files: The first one, liblol.f90, looks like this:
subroutine func()
print*, 'lol!'
end subroutine func
I compile it with gfortran -shared -fPIC -o liblol.so liblol.f90
The second file, main.f90, looks like this:
program main
call func()
end program main
When I now try to compile that with the command gfortran -L. -llol main.f90 -o main, I get the following error:
/tmp/ccIUIhcE.o: In function `MAIN__':
main.f90:(.text+0xa): undefined reference to `func_'
collect2: ld returned 1 exit status
I do not understand why it says "undefined reference", since the output of nm -D liblol.so gives me this:
w _Jv_RegisterClasses
0000000000201028 A __bss_start
w __cxa_finalize
w __gmon_start__
0000000000201028 A _edata
0000000000201038 A _end
0000000000000778 T _fini
U _gfortran_st_write
U _gfortran_st_write_done
U _gfortran_transfer_character_write
0000000000000598 T _init
00000000000006cc T func_
Is there any other parameter needed?
The only thing that has to be changed is the order of the arguments, as in
gfortran -L. main.f90 -llol -o main
Yes, only main.f90 and -llol are reversed. I hope this saves someone the year of his life I just lost on this. On a related note, if you are trying to compile a program which uses LAPACK or BLAS (which did not work for me and is why in the first place I tried to create a shared library myself), the same also applies. Write the name of the source file first:
gfortran mylapack.f90 -llapack -lblas -o mylapack
The reason for this can be found in the manual pages, see the top of http://gcc.gnu.org/onlinedocs/gcc/Link-Options.html for the option -l:
It makes a difference where in the command you write this option; the
linker searches and processes libraries and object files in the order
they are specified. Thus, foo.o -lz bar.o searches library z after
file foo.o but before bar.o. If bar.o refers to functions in `z',
those functions may not be loaded.
This question is related to this one as well as its answer.
I just discovered some ugliness in a build I'm working on. The situation looks somewhat like the following (written in gmake format); note, this specifically applies to a 32-bit memory model on sparc and x86 hardware:
OBJ_SET1 := some objects
OBJ_SET2 := some objects
# note: OBJ_SET2 doesn't get this flag
${OBJ_SET1} : CCFLAGS += -PIC
${OBJ_SET1} ${OBJ_SET2} : %.o : %.cc
${CCC} ${CCFLAGS} -m32 -o ${#} -c ${<}
obj1.o : ${OBJ_SET1}
obj2.o : ${OBJ_SET2}
sharedlib.so : obj1.o obj2.o
obj1.o obj2.o sharedlib.so :
${LINK} ${LDFLAGS} -m32 -PIC -o ${#} ${^}
Clearly it can work to mix objects compiled with and without PIC in a shared object (this has been in use for years). I don't know enough about PIC to know whether it's a good idea/smart, and my guess is in this case it's not needed but rather it's happening because someone didn't care enough to find out the right way to do it when tacking on new stuff to the build.
My question is:
Is this safe
Is it a good idea
What potential problems can occur as a result
If I switch everything to PIC, are there any non-obvious gotchas that I might want to watch out for.
Forgot I even wrote this question.
Some explanations are in order first:
Non-PIC code may be loaded by the OS into any position in memory in [most?] modern OSs. After everything is loaded, it goes through a phase that fixes up the text segment (where the executable stuff ends up) so it correctly addresses global variables; to pull this off, the text segment must be writable.
PIC executable data can be loaded once by the OS and shared across multiple users/processes. For the OS to do this, however, the text segment must be read-only -- which means no fix-ups. The code is compiled to use a Global Offset Table (GOT) so it can address globals relative to the GOT, alleviating the need for fix-ups.
If a shared object is built without PIC, although it is strongly encouraged it doesn't appear that it's strictly necessary; if the OS must fix-up the text segment then it's forced to load it into memory that's marked read-write ... which prevents sharing across processes/users.
If an executable binary is built /with/ PIC, I don't know what goes wrong under the hood but I've witnessed a few tools become unstable (mysterious crashes & the like).
The answers:
Mixing PIC/non-PIC, or using PIC in executables can cause hard to predict and track down instabilities. I don't have a technical explanation for why.
... to include segfaults, bus errors, stack corruption, and probably more besides.
Non-PIC in shared objects is probably not going to cause any serious problems, though it can result in more RAM used if the library is used many times across processes and/or users.
update (4/17)
I've since discovered the cause of some of the crashes I had seen previously. To illustrate:
/*header.h*/
#include <map>
typedef std::map<std::string,std::string> StringMap;
StringMap asdf;
/*file1.cc*/
#include "header.h"
/*file2.cc*/
#include "header.h"
int main( int argc, char** argv ) {
for( int ii = 0; ii < argc; ++ii ) {
asdf[argv[ii]] = argv[ii];
}
return 0;
}
... then:
$ g++ file1.cc -shared -PIC -o libblah1.so
$ g++ file1.cc -shared -PIC -o libblah2.so
$ g++ file1.cc -shared -PIC -o libblah3.so
$ g++ file1.cc -shared -PIC -o libblah4.so
$ g++ file1.cc -shared -PIC -o libblah5.so
$ g++ -zmuldefs file2.cc -Wl,-{L,R}$(pwd) -lblah{1..5} -o fdsa
# ^^^^^^^^^
# This is the evil that made it possible
$ args=(this is the song that never ends);
$ eval ./fdsa $(for i in {1..100}; do echo -n ${args[*]}; done)
That particular example may not end up crashing, but it's basically the situation that had existed in that group's code. If it does crash it'll likely be in the destructor, usually a double-free error.
Many years previous they added -zmuldefs to their build to get rid of multiply defined symbol errors. The compiler emits code for running constructors/destructors on global objects. -zmuldefs forces them to live at the same location in memory but it still runs the constructors/destructors once for the exe and each library that included the offending header -- hence the double-free.