alt Hacker NewsThis article was painful to read because of all the misconceptions. A cpio archive is not a filesystem. Author uses initramfs, which is based on tmpfs. Linux can extract cpio to tmpfs. An archive of files and directories is in itself not a program.
Just because something looks similar doesn't mean it's equivalent. Binary programs are executed on the CPU, so if there's an interpreter involed it's hiding in the hardware environment. That's outside the scope of an OS kernel.
If you have a shell script in your filesystem and run it, you need to also provide the shell that interprets the script. Author omits this detail and confuses the kernel with the shell program.
Linux can easily be compiled without support for initramfs and ramdisk. It can still boot and run whatever userland sits in the filesystem.
"Linux initrd interpreter" hurts my brain. That's not how it works.
Edit: should've read further. Still a backwards way of explaining things imho.
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It's the init in the cpio which is the interpeted program, and the rest of the cpio is memory for this interpeted progam.
Binary programs are executed on the CPU but the program file is an archive with sections, and only one of them is the program, usually, while the others are all metadata. The CPU isn't capable of understanding the program file at all. Linux has to establish the conditions under which the program runs, that means at a minimum establishing the address space in which the program counter lies then jumping to that address. The instructions for how to do that are in the metadata sections of the ELF executable.
> An archive of files and directories is in itself not a program.
Okay, but you can make the same argument to say that ELF files aren't programs in and of themselves either. In fact, some ELF files are dynamic libraries without an entrypoint, and therefore not actually executable in any meaningful way unless connected to yet another program.
If you can accept that some ELF files are executables and some aren't, then you can also accept that some CPIOs are executables and some aren't. What's the difference between ld.so unpacking an ELF file into RAM and running its entrypoint, and the Linux kernel unpacking an initramfs into RAM and running its entrypoint?