Where to embed AppImage magic bytes in the future?

[TheAssassin]

I've been thinking about the magic bytes problem we have with the current type 2 a bit. The only solution is to move these magic bytes to another location. This location must also be at a specific offset and provide enough space for them.

One solution I find especially appealing is the following: AppImage type 1 used ISO9660 whose magic number may be embedded at 3 specific locations: 0x8001, 0x8801, 0x9001. Obviously these locations didn't interfere with the ELF binary. The offsets are (in decimal numbers) around 32-27kiB, our runtime is larger (~150-200kiB). So, the question is, why did ELF and ISO9660 work together in that scenario. And more interestingly, can we just place our own magic bytes where, in case of type 1, the ISO9660 bytes would be expected?

[TheAssassin]

@azubieta also had a few suggestions. Can you please put them in here, too? Just make sure they're at constant offsets, this is an absolute requirement.

[probonopd]

There are limits on how far into the file magic bytes should be located (for performance reasons), and what we did with type-1 was outside of what is allowable iirc.

[TheAssassin]

You mean performance wise? Modern filesystems use (at least!) 4kiB blocks, and with caching and everything it doesn't make a large difference how much you read from the file beginning; with HDDs for instance, you lose much more performance by having to relocate the actuator. As the files have most likely been written sequentially, reading 8 or more 4kiB blocks sequentially doesn't really affect performance.

[probonopd]

I don't mean anything, I am reciting from the top of my mind what we had learned from shared-mime-info when we were putting together the type-2 spec draft. I think it has to do with network shares. cc @elvisangelaccio

[TheAssassin]

Interesting thought. Do you have a reference?

---

Here is what @azubieta was explaining to me last week:

The ELF format is, to some extent, flexible. The only part which is really at a static offset (0) is the ELF "header". The ELF header has a fixed size. Figure 1-3 in the ELF specification uses a struct of fixed size data structures to describe it.

#define EI_NIDENT 16

typedef struct {
    unsigned char    e_ident[EI_NIDENT];
    Elf32_Half       e_type;
    Elf32_Half       e_machine;
    Elf32_Word       e_version;
    Elf32_Addr       e_entry;
    Elf32_Off        e_phoff;
    Elf32_Off        e_shoff;
    Elf32_Word       e_flags;
    Elf32_Half       e_ehsize;
    Elf32_Half       e_phentsize;
    Elf32_Half       e_phnum;
    Elf32_Half       e_shentsize;
    Elf32_Half       e_shnum;
    Elf32_Half       e_shstrndx;
} Elf32_Ehdr;

Figure 1-3, ELF specification.

Figure 1-1 provides an overview of the ELF format. There are more structures like the (optional) program header table, the sections' location etc., whose locations are described by some sort of "pointers" in the ELF header. e_phoff for instance holds the offset in bytes at which the program header table starts (if there is none, it's 0), e_shoff contains the offset of the sections, etc. e_ehsize contains the total size of the entire ELF header.

Figure 1-1, ELF specification.

The only entity we know which is static size is the ELF header. What we could do is insert our magic bytes right after the ELF header (e.g., by defining "we use an additional byte there" to achieve some alignment), then append the rest of the ELF header and add <size of magic bytes area, e.g., 4, 8> to all the offsets and sizes where applicable.

This shouldn't be overly hard to implement, we can implement this as a C program which we build before the runtime and run on the runtime as a post-build command to insert our magic bytes.

We should document this thoroughly, but it's not too hard to understand, the ELF specification is well written and easy to understand for anyone with a basic understanding of file formats.

[probonopd]

Sounds like a good idea to insert a section for our use (e.g., magic bytes) right after the ELF header. I have no clue on how to do it though. What happens if the ELF header grows?

[TheAssassin]

I would not use the word "section" for this approach, since it may be mixed up with real "ELF sections" (if you meant those: they're unsuitable, we need a static offset but you can't predict their location, lookups to them require at least one lookup to the ELF header in advance, which is unsuitable for many applications, e.g., binfmt_misc).

Define "grow". The order is basically, ELF header -> our stuff -> PHT, sections etc. Whatever would be added which could make the header grow would grow after the ELF header and our magic bytes section. As long as applications don't tamper with the offsets, things will continue to work.

Of course this time we should test our new ELF runtime with all platforms we know where things are broken. Basically, testrun current runtime, and if it breaks, try the new binary. I can well imagine some tools ignore e.g., the offsets and make assumptions not mentioned in the standard. But, to be honest, the old magic bytes were doing the same (assuming that interpreters will ignore our magic bytes). This approach has the advantage that if an implementation doesn't like the approach, we have a very good argument why the other projects need to fix their implementation: they violate/ignore the ELF specification (much better than saying "but the ELF spec says it's an optional thing, and you don't have to implement it" or "all the other implementations ignore it, too").

[probonopd]

I meant "section" as in ELF section. Can't we make an ELF section at a defined offset?

[TheAssassin]

No, see what I described above. Most files don't have a PHT, but what if they do? Sections are all the same size, but that size differs, so how to make guarantees? We've thoroughly analyzed this, and there's no way to make this predictable.

What @azubieta (and now me, too) suggest is also fully compatible with the ELF specification.

[elvisangelaccio]

I don't mean anything, I am reciting from the top of my mind what we had learned from shared-mime-info when we were putting together the type-2 spec draft. I think it has to do with network shares. cc @elvisangelaccio

Basically yes. Quoting from shared-mime-info's hacking file:

Magic offset must be as small as possible. For example, the worst case
scenario for ISO images is 32k inside the file. This is too big for a sniff
buffer, especially on remote locations. Avoid those.

[TheAssassin]

@elvisangelaccio yeah, that makes sense. "sniff buffers"... hadn't heard about those for a while, but I remember what they are for. The ISO9660 thing I suggested above is only interesting and relevant for us because we know it works.

I would claim that anything < 1k (or even 4k) is okay.

[TheBrokenRail]

I wrote a small program that inserts empty space between the ELF header and everything else: https://gitea.thebrokenrail.com/TheBrokenRail/elf-padder.

Biggest problem is that this empty space must be rounded to a page boundary (usually 4KB) or the ELF will fail to load.

Second biggest problem is that I've barely tested this at all. I was able to add 4KB of empty space to both /bin/sh and qemu-arm-static and both successfully loaded. I've only tested with 64-bit files, but 32-bit should work too.

But it is able to insert 4KB (or more) of empty space to a constant location in an ELF file.

Usage:

elf-padder <minimum-padding> <file> > <output-file>

Example:

$ ./elf-padder 4 /usr/bin/python3.8 > ./new-python
[INFO]: Aligning Padding To Page Boundary: 4096
$ chmod +x new-python 
$ ./new-python 
Python 3.8.10 (default, Nov 26 2021, 20:14:08) 
[GCC 9.3.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>>

I hope this is helpful!

[probonopd]

Thanks @TheBrokenRail. Do you think this kind of functionality could/should be added to https://github.com/NixOS/patchelf?

[TheBrokenRail]

Thanks @TheBrokenRail. Do you think this kind of functionality could/should be added to https://github.com/NixOS/patchelf?

The code is pretty simple, so I don't think it would too difficult to adapt to patchelf's code-base.

However, I think adding empty space after the ELF header is a bit out-of-scope for patchelf, since its main purpose is modifying binaries to work on systems with different library names and/or locations. The real decision would be up to the patchelf maintainers of course.

[azubieta]

@TheBrokenRail agreed that this feature have no room in the patchelf project scope. Also we have a way of doing it at build time using linker scripts. Please check: https://github.com/AppImageCrafters/appimage-runtime