The properties of ls show it is an executable.
And properties of kmod show it is a shared library.
Im trying to check for executables and hash them in ubuntu 14.04 LTS. Is there any way to differentiate executables from the other types? Thanks in advance
Executable is a Load file which executes directly in system as a program. As per your question, "ls" is a executable which is used to
list the current directory contents. The load for "ls" is placed in "/bin" or you can check using command "which ls". Shared library are the one which do some task that is commonly accessed or used by many executables. These library are loaded into the memory only once and accessed by many programs(executables) at runtime.
Related
I was going through disassembly of elf executables and understanding the elf format. In there, I saw lib64/ld-linux-x86-64.so.2 used as program interpreter in the generated executable.
My guess is: I had used printf in the source code, which had to be dynamically linked. When I checked through dynamic section, I was able to find a reference to libc.so.6 shared library (tag:DT_NEEDED). In my system, I found multiple files with that name in different directories:
sourav#ubuntu-VirtualBox:/$ sudo find / -name libc.so.6
/usr/lib/x86_64-linux-gnu/libc.so.6
find: ‘/run/user/1000/doc’: Permission denied
find: ‘/run/user/1000/gvfs’: Permission denied
/snap/snapd/13170/lib/x86_64-linux-gnu/libc.so.6
/snap/snapd/11107/lib/x86_64-linux-gnu/libc.so.6
/snap/core18/1988/lib/i386-linux-gnu/libc.so.6
/snap/core18/1988/lib/x86_64-linux-gnu/libc.so.6
/snap/core18/2128/lib/i386-linux-gnu/libc.so.6
/snap/core18/2128/lib/x86_64-linux-gnu/libc.so.6
So, I guess purpose of program interpreter is to resolve these names to the proper libraries and load them during execution. Is this correct?
It seems, we can also have executables with no program interpreter (which is the case for program interpreter itself). In that case, does system/os itself loads the shared library? If so, how does it resolves the path of library?
Is it possible to generate executable with no program interpreter using gcc? My gcc version is 'gcc version 9.3.0 (Ubuntu 9.3.0-17ubuntu1~20.04)'.
So, I guess purpose of program interpreter is to resolve these names to the proper libraries and load them during execution. Is this correct?
Yes, but that that's a bit minimalistic. Loading dynamic libraries involves locating them, loading or mapping them into memory if necessary, and resolving dynamic symbols within, possibly lazily, for multiple kinds of relocations. It involves recursively loading the libraries' own needed libraries. Also, in a dynamically linked executable, the program interpreter provides the program entry point (from the kernel's perspective), so it is also responsible for setting up and entering the program-specific entry point (for example, main() in a C or C++ program).
It seems, we can also have executables with no program interpreter (which is the case for program interpreter itself). In that case, does system/os itself loads the shared library? If so, how does it resolves the path of library?
You can have ELF executables without a program interpreter, but they are not dynamically linked, at least not in the ELF sense. There are no shared libraries to load, and certainly the system does not load any.
Is it possible to generate executable with no program interpreter using gcc? My gcc version is 'gcc version 9.3.0 (Ubuntu 9.3.0-17ubuntu1~20.04)'.
If you have static versions of all needed libraries available then you should be able to achieve that by including the -static option on the command line when you link the program. It is entirely possible, however, that you do not have the needed static libraries, even if libc is the only library you need.
In a Linux program, I need to load a shared object called libfoobar. On my system, there's libfoobar.so, libfoobar.so.0 and libfoobar.so.0.0.0. On another system with a different distribution installed, libfoobar.so does not exist, only the other two variants are available.
What is the correct filename I should be using when loading the shared object? Is it sufficient to try to load libfoobar.so.0 on any system and bail out if that fails, or should I attempt to load the other variants?
libfoobar.so.0 and libfoobar.so.0.0.0 are supposed to be symlinks which at some level point to libfoobar.so. If you found a system where only those symlinks exist but not libfoobar.so, then the installation is broken on that system.
Besides that, it is up to you whether you want to use the symlink or the original filename.
I apologize if my question is not precise because I don't have a lot
of Linux related experience. I'm currently building a Linux from
scratch (mostly following the guide at linuxfromscratch.org version
7.3). I ran into the following problem: when I build an executable it
gets a hardcoded path to something called ELF interpreter.
readelf -l program
shows something like
[Requesting program interpreter: /lib/ld-linux.so.2]
I traced this library ld-linux-so.2 to be part of glibc. I am not very
happy with this behaviour because it makes the binary very unportable
- if I change the location of /lib/ld-linux.so.2 the executable no
longer works and the only "fix" I found is to use the patchelf utility
from NixOS to change the hardcoded path to another hardcoded path. For
this reason I would like to link against a static version of the ld
library but such is not produced. And so this is my question, could
you please explain how could I build glibc so that it will produce a
static version of ld-linux.so.2 which I could later link to my
executables. I don't fully understand what this ld library does, but I
assume this is the part that loads other dynamic libraries (or at
least glibc.so). I would like to link my executables dynamically, but
I would like the dynamic linker itself to be statically built into
them, so they would not depend on hardcoded paths. Or alternatively I
would like to be able to set the path to the interpreter with
environment variable similar to LD_LIBRARY_PATH, maybe
LD_INTERPRETER_PATH. The goal is to be able to produce portable
binaries, that would run on any platform with the same ABI no matter
what the directory structure is.
Some background that may be relevant: I'm using Slackware 14 x86 to
build i686 compiler toolchain, so overall it is all x86 host and
target. I am using glibc 2.17 and gcc 4.7.x.
I would like to be able to set the path to the interpreter with environment variable similar to LD_LIBRARY_PATH, maybe LD_INTERPRETER_PATH.
This is simply not possible. Read carefully (and several times) the execve(2), elf(5) & ld.so(8) man pages and the Linux ABI & ELF specifications. And also the kernel code doing execve.
The ELF interpreter is responsible for dynamic linking. It has to be a file (technically a statically linked ELF shared library) at some fixed location in the file hierarchy (often /lib/ld.so.2 or /lib/ld-linux.so.2 or /lib64/ld-linux-x86-64.so.2)
The old a.out format from the 1990s had a builtin dynamic linker, partly implemented in old Linux 1.x kernel. It was much less flexible, and much less powerful.
The kernel enables, by such (in principle) arbitrary dynamic linker path, to have various dynamic linkers. But most systems have only one. This is a good way to parameterize the dynamic linker. If you want to try another one, install it in the file system and generate ELF executables mentioning that path.
With great pain and effort, you might make your own ld.so-like dynamic linker implementing your LD_INTERPRETER_PATH wish, but that linker still has to be an ELF shared library sitting at some fixed location in the file tree.
If you want a system not needing any files (at some predefined, and wired locations, like /lib/ld.so, /dev/null, /sbin/init ...), you'll need to build all its executable binaries statically. You may want (but current Linux distributions usually don't do that) to have a few statically linked executables (like /sbin/init, /bin/sash...) that will enable you to repair a system broken to the point of not having any dynamic linker.
BTW, the /sbin/init -or /bin/sh - path is wired inside the kernel itself. You may pass some argument to the kernel at boot load time -e.g. with GRUB- to overwrite the default. So even the kernel wants some files to be here!
As I commented, you might look into MUSL-Libc for an alternative Libc implementation (providing its own dynamic linker). Read also about VDSO and ASLR and initrd.
In practice, accept the fact that modern Linuxes and Unixes are expecting some non-empty file system ... Notice that dynamic linking and shared libraries are a huge progress (it was much more painful in the 1990s Linux kernels and distributions).
Alternatively, define your own binary format, then make a kernel module or a binfmt_misc entry to handle it.
BTW, most (or all) of Linux is free software, so you can improve it (but this will take months -or many years- of work to you). Please share your improvements by publishing them.
Read also Drepper's Hwo to Write Shared Libraries paper; and this question.
I ran into the same issue. In my case I want to bundle my application with a different GLIBC than comes system installed. Since ld-linux.so must match the GLIBC version I can't simply deploy my application with the according GLIBC. The problem is that I can't run my application on older installations that don't have the required GLIBC version.
The path to the loader interpreter can be modified with --dynamic-linker=/path/to/interp. However, this needs to be set at compile time and therefore would require my application to be installed in that location (or at least I would need to deploy the ld-linux.so that goes with my GLIBC in that location which goes against a simple xcopy deployment.
So what's needed is an $ORIGIN option equivalent to what the -rpath option can handle. That would allow for a fully dynamic deployment.
Given the lack of a dynamic interpreter path (at runtime) leaves two options:
a) Use patchelf to modify the path before the executable gets launched.
b) Invoke the ld-linux.so directly with the executable as an argument.
Both options are not as 'integrated' as a compiled $ORIGIN path in the executable itself.
I am working on a project to load and run a custom binary format executable (PE, in my case) on a Linux platform. I've done this pretty successfully so far by first loading the executable and then loading a small ELF shared library that calls the start address of the executable and then exits safely.
I would really like not doing the ELF loading myself for a few reasons, though. First, the shared library I use is written in assembly (I can't use anything else because I'm not linking to libc, etc.), which will be very platform-specific, and I'd like to move away from that and use C so I can compile for any platform. Also, it will be easier and safer to use Linux's native ELF loader instead of my own simplified version.
I'm wondering if there is a way to use my binfmt handler, an installed kernel module, to load my executable and then ask Linux to load my shared library (and its dependencies) into the same address space without overwriting my executable code. I first thought that the uselib syscall might be useful, but the description on the man page is unclear about whether or not this will serve my purposes:
From libc 4.4.4 on only the library "/lib/ld.so" is loaded, so that
this dynamic library can load the remaining libraries needed (again
using this call). This is also the state of affairs in libc5.
glibc2 does not use this call.
I've also never seen an example of its use, and I'm always wary of using syscalls that I don't understand.
Is there a good way to achieve what I've described? Can I use Linux's existing capabilities to load a shared library (written in C) into an address space already containing executable code, and, if so, how can I use that library without knowing where it has been loaded?
there is already a project like this called binfmt_pe (by me!) which is a kernel module and will have it's own linker (similar to /lib/ld). check it out here.
As for your question about making modules and the loader/linker, there are links below. I also included links with info about ELF and PE executables.
I hope this helps. :)
Useful information for making a Linux Kernel Module
The Linux Kernel Module Programming Guide
Writing Your Own Loadable Kernel Module
Linux Data Structures
The Linux kernel: The kernel source
Kernel Support for miscellaneous Binary Formats
binfmt_elf.c
binfmt_misc.c
Information About Dynamnic Loading/Linking
Understanding ld-linux.so.2
ld.so : Dynamic-Link Library support
How To Write Shared Libraries - PDF
ld-linux(8) - Linux man page
rpath
Listing Shared Library Dependencies
Information About ELF and PE Formats
OSRC: Executable File Formats
EXE Format
How Windows NT Recognizes MS-DOS - Based Applications
Common Object File Format (COFF)
Peering Inside the PE: A Tour of the Win32 Portable Executable File Format
Executable and Linkable Format
An In-Depth Look into the Win32 Portable Executable File Format
An In-Depth Look into the Win32 Portable Executable File Format, Part 2
Injective Code Inside an Import Table
The PE Format | Hackers Library
IMAGE_NT_HEADERS structure (Windows)
x86 Disassembly/Windows Executable Files
the Portable Executable Format on Windows
I am trying to modify this shared library (with .so) extension on Linux. I am inserting some printf statement and fprintf statement to debug, and it has no effect. I removed the .so file and realized that the the program still runs fine. Does it mean that the program is loaded into memory?? (But I'm sure only the program I'm testing for uses that .so file though)
How do I get it to unload so I can make sure my program is loading the modified one?
No, shared libraries are not cached in memory. If you have deleted the .so file and your program still runs, then either:
the program is loading an .so of the same name from a different location, or
the program can run without loading the .so
If the .so is supposed to be loaded at program startup, then you can use ldd to find out where your OS thinks the .so actually is.
If the .so is loaded dynamically at runtime, then perhaps strace will be able to help pinpoint what is happening.
You can read /proc/1234/maps to find out the memory map of process 1234. This also shows the dynamically loaded shared objects.
You may use the LD_LIBRARY_PATH environment variable to change the path of shared libraries and ldconfig to upgrade its cache. Look also in /etc/ld.so.conf etc.
Of course, you have to restart the program loading your shared library.