[roblabla]

Not with NixOS. ld.so is tied to a version of glibc, in ways that can be subtly incompatible. And nixos can have multiple glibc version installed on a single machine.

Besides, it allows for upgrades/downgrades to be done in a way that's much less error-prone.

[mort96]

"ld.so is tied to a version of glibc" is such a horrible GNUism.

[myrmidon]

Isn't that kinda to be expected if you want to provide dynamic loading functionality (dlopen)?

Is the windows situation really all that different/better (with GetProcAddress in kernel32.dll)?

[roblabla]

In theory, ld.so could provide a stable interface to its dynamic loading capabilities independently of glibc. Then glibc would not have to be updated in concert with ld.so. There's no inherent reason that glibc should be the library shipping a dynamic linker - we could easily be in a world where libld was developed independently from libc.

On Windows, things are less modular, so it's less of an issue. That said, there's also weird shenanigans when it comes to the CRT (which can be statically linked) vs ntdll (which provides the actual linker implementation), that can make certain niche features of the linker misbehave (delay loading in particular is weird).

[kazinator]

In practice, ld.so is a piece of glibc. If you want to build ld.so from scratch, you have to clone glibc or get a tarball of glibc, and configure and build glibc.

You could make the argument that the glibc developers should split ld.so into its own subprojects. What for? That would just bring the extra responsibility of making sure that variations in glibc version work with variations in ld.so version for whatever reason.

What would happen in practice is that they would keep their version numbers in lock step, and systems integrators would use the same version. While the upstream glibc has to go through a dance of pretending that someone cares about their independence.

[pas]

still, eventually there's a need to support on one platform different ld.so/glibc pairs (even if they are API/ABI compatible)

it seems nixos could set up a wrapper that invokes the right ld.so based on the executable. though at this point they could probably edit the ELF binaries and patch the fixed path to ld.so when nix is installing the program.

yeah, it seems strange that this needs kernel support. but more eBFP extension points are usually welcome, so sure, why not?

[roblabla]

> though at this point they could probably edit the ELF binaries and patch the fixed path to ld.so when nix is installing the program.

That's exactly what they're doing right now - though instead of being "when installing the program", it's "when compiling the program". The problem with hardcoding the path is that it pins the "nix store" (where nix installs all of its programs) to a hardcoded location. If you want to move it, you have to rebuild all your packages - which is suboptimal.

In theory, nix could have a system in place to just patch all binaries when moving the nix store, but that would be incompatible with content-addressed derivation and otherwise break some other nice properties of the nix store.

[setheron]

(author) You did a great job articulating the points!

[jcgl]

Not my area, but isn't it really only because glibc doesn't maintain stable interfaces across versions? If it did, you absolutely could use the same ld.so with different glibc versions. But it doesn't, so here we are.

[geocar]

The C standard isn’t stable across versions.

extern int errno;

[mort96]

The Windows situation is way different: every process is supposed to link against kernel32.dll, that's the public interface to the kernel. In Linux, glibc is just one of many C stdlib implementations, you can have many versions of glibc on the same system, etc.

[myrmidon]

Sure, but if you want dynamic loading from your c stdlib (which is defensible IMO), and you want the behavior/implementation to match with the loader, then you need some kind of coupling somewhere no?

You could have a very slim libdlopen that is used by both loader and libc, but I don't really see how that's any improvement/much different.

[p_l]

That's how Windows handles it - the OS libraries are language runtime agnostic and do not export language specific symbols nor need them, and language runtimes are distributed often separately.

So your C stdlib does not have dlopen() (in fact, it's not really part of C stdlib in POSIX anyway!) but you can link with standard OS-level ABI to OS-provided runtime services like "find a library and symbol from it" - AmigaOS did something similar.

[kazinator]

ld.so is a component of glibc!

  $ /lib64/ld-linux-x86-64.so.2
  /lib64/ld-linux-x86-64.so.2: missing program name
  Try '/lib64/ld-linux-x86-64.so.2 --help' for more information.
  !1!
  $ /lib64/ld-linux-x86-64.so.2 --version
  ld.so (GNU libc) stable release version 2.34.
  ^^^^^^^^^^^^^^^^
  Copyright (C) 2021 Free Software Foundation, Inc.
  This is free software; see the source for copying conditions.
  There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A
  PARTICULAR PURPOSE.

[charcircuit]

>in ways that can be subtly incompatible

There is much more value to be had in making glibc properly backward compatible so you can have a single one that can be used with everything than trying to make it so that you can swap everything around, creating extra complexity and compatibility risks.

[dspillett]

> […] in making glibc properly backward compatible […]

You could only do that going forward though, and would be stuck, at least for a time, with the historic versions that still need extra handling. In an ideal world that wouldn't be needed, but in an ideal world you'd not need multiple versions of glibc at all so we aren't there.

Support in the kernel means that it will work even if applied to old packages that for some other reason need a held-back version of glibc. As mentioned elsewhere it also gives the feature to #! directives in scripts too.

[roblabla]

How is that more valuable? It comes with two big pitfalls:

- It would still not allow downgrades to work properly

- It would cause glibc/ld.so to have a harder time adding new features, as they now need to worry about incompatible versions being used together

Meanwhile, having different ld.so has many good use-cases, like simplifying development of ld.so itself, allowing them to swapped during updates in ways that are safer, etc...

And the eBPF binfmt support is a rather simple, generic mechanism that is likely to have many other use-cases beyond ld.so. So it's overall a pretty good resolution to the issue?

[charcircuit]

>It would still not allow downgrades to work properly

If you really need this, then there are options like swapping the linker with an older version or making a hardcoded linker that now points to an older one.

>It would cause glibc/ld.so to have a harder time adding new features, as they now need to worry about incompatible versions being used together

Every language runtime has to care about not breaking compatibility with apps that have been released already. This is not a new or unique problem.