Comment by ok123456
1 day ago
Versioned glibc symbols are part of the reason that binaries aren't portable across Linux distributions and time.
1 day ago
Versioned glibc symbols are part of the reason that binaries aren't portable across Linux distributions and time.
Only because people aren't putting in the effort to build their binaries properly. You need to link against the oldest glibc version that has all the symbols you need, and then your binary will actually work everywhere(*).
* Except for non-glibc distributions of course.
But to link against an old glibc version, you need to compile on an old distro, on a VM. And you'll have a rough time if some part of the build depends on a tool too new for your VM. It would be infinitely simpler if one could simply 'cross-compile' down to older symbol versions, but the tooling does not make this easy at all.
Check out `zig cc`. It let's you target specific glibc versions. It's a pretty amazing C toolchain.
https://andrewkelley.me/post/zig-cc-powerful-drop-in-replace...
It's actually doable without an old glibc as it was done by the Autopackage project: https://github.com/DeaDBeeF-Player/apbuild
That never took off though, containers are easier. Wirh distrobox and other tools this is quite easy, too.
> It would be infinitely simpler if one could simply 'cross-compile' down to older symbol versions, but the tooling does not make this easy at all.
It's definitely not easy, but it's possible: using the `.symver` assembly (pseudo-)directive you can specify the version of the symbol you want to link against.
Huh? Bullshit. You could totally compile and link in a container.
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If it requires effort to be correct, that's a bad design.
Why doesn't the glibc use the version tag to do the appropriate mapping?
I think even calling it a "design" is dubious. It's an attribute of these systems that arose out of the circumstance, nobody ever sat down and said it should be this way. Even Torvalds complaining about it doesn't mean it gets fixed, it's not analogous to Steve Jobs complaining about a thing because Torvalds is only in charge of one piece of the puzzle, and the whole image that emerges from all these different groups only loosely collaborating with each other isn't going to be anybody's ideal.
In other words, the Linux desktop as a whole is a Bazaar, not Cathedral.
I don’t understand why this is the case, and would like to understand. If I want only functions f1 and f2 which were introduced in glibc versions v1 and v2, why do I have to build with v2 rather than v3? Shouldn’t the symbols be named something like glibc_v1_f1 and glibc_v2_f2 regardless of whether you’re compiling against glibc v2 or glibc v3? If it is instead something like “compiling against vN uses symbols glibc_vN_f1 and glibc_vN_f2” combined with glibc v3 providing glibc_v1_f1, glibc_v2_f1, glibc_v3_f1, glibc_v2_f2 and glbc_v3_f2… why would it be that way?
> why would it be that way?
It allows (among other things) the glibc developers to change struct layouts while remaining backwards compatible. E.g. if function f1 takes a struct as argument, and its layout changes between v2 and v3, then glibc_v2_f1 and glibc_v3_f1 have different ABIs.
Individual functions may have a lot of different versions. They do only update them if there is an ABI change (so you may have e.g. f1_v1, f1_v2, f2_v2, f2_v3 as synbols in v3 of glibc) but there's no easy way to say 'give me v2 of every function'. If you compile against v3 you'll get f2_v3 and f1_v2 and so it won't work on v2.
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> Only because people aren't putting in the effort to build their binaries properly.
Because Linux userland is an unmitigated clusterfuck of bad design that makes this really really really hard.
GCC/Clang and Glibc make it effectively impossible almost impossible to do this on their own. The only way you can actually do this is:
1. create a userland container from the past 2. use Zig which moved oceans and mountains to make it somewhat tractable
It's awful.
> You need to link against the oldest glibc version that has all the symbols you need
Or at least the oldest one made before glibc's latest backwards incompatible ABI break.