4 ms·

It kinda work, but not in a source based distro. If you can atomically rebuild @world with changed definition of off_t, then there will be no problem. But sourc

• codys 2y ago
  > there would be inconveniences like libc.so has 64-bit off_t glibc specifically has support for the 32-bit and 64-bit time_t abi simultaneously. From the post: > What’s important here is that a single glibc build remains compatible with all three variants. However, libraries that use these types in their API are not.
  • ordu 2y ago
    Yeah, glibc was a bad example. Probably libz.so or something would be better.
    • codys 2y ago
      Yep, agreed. Though it does expose an option here which would be "have glibc provide a mechanism for other libraries (likely more core/widely used libs) to support both ABIs simultaneously. Presumably if that had been done in glibc when 64-bit time_t support was added, we could have had multi-size-time ABI support in things like zlib by now. Seems like a mistake on glibc's part not to create that initially (years ago). Though if other distros have already switched, I'd posit that perhaps Gentoo needs to rethink its design a bit so it doesn't run into this issue instead.
      • jasomill 2y ago
        This sounds like something similar to NeXTSTEP/macOS fat binaries, only with the possibility of code sharing between "architectures". I like it, though it sounds like something that'd be unlikely to see adoption in the Linux world without the endorsement of multiple major distros.
        • codys 2y ago
          Fat binaries (packing together 2 entirely different objects and picking one of them) could potentially work here, though I suspect we'd run into issues of mixed 32-bit/64-bit time_t things. Another option that's closer to how things work with LFS (large file support) (mostly in the past now) is to use create different interfaces to support 64-bit time_t, and pick defaults for time_t at compile time with a macro that picks the right impl. Also possible could be having something like a version script (https://sourceware.org/binutils/docs/ld/VERSION.html https://sourceware.org/binutils/docs/ld/VERSION.html) to tell the linker what symbols to use when 64-bit time_t was enabled. While this one might have some benefits, generally folks avoid using version scripts when possible, and would require changes in glibc, making it unlikely. Both of those options (version script extention, and LFS-like pattern) could allow re-using the same binary (ie: smaller file size, no need to build code twice in general), and potentially enable mixing 32-bit time_t and 64-bit time_t code together in a single executable (not desirable, but does remove weird link issues).
          • jasomill 2y ago
            I was thinking along the lines of a simple extension to the Mach-O fat binary concept to allow each member of the fat binary archive to be associated with one or more ABIs rather than just a single architecture. Then all the time_t size-independent code would go into a member associated with both 32-bit and 64-bit time_t ABIs, and all the time_t size-dependent code would be separately compiled into single-ABI members, one for each ABI. For both executables and libraries, name conflicts between members associated with the same ABI would be prohibited at compile/link time. The effective symbol table for both compile time and runtime linking would then be the union of symbol tables defined by all members associated with the active ABI. Language-level mechanisms that allow compilers to recognize and separately compile time_t-dependent symbols would be required, ideally implemented in ways that allow such dependence to be inferred in most if not all cases. While compilers would be free to size-optimize code by splitting functions into dependent and independent subparts, I see no immediate reason why this needs to be explicitly supported at link level. Finally, as I imagine it, mixing 32- and 64-bit time_t ABIs would be prohibited — implicitly, as 64-bit time_t symbols aren't ever visible to 32-bit time_t code and vice versa — with code that needs to support non-native time_t values (for I/O, IPC, etc.) left its own devices, just like code dealing with, e.g., non-native integer and floating-point formats today. Admittedly this sounds like a lot of unnecessary work vs. similar alternatives built on existing multi-arch mechanisms, but it's still an interesting idea to contemplate.
      • o11c 2y ago
        > have glibc provide a mechanism for other libraries (likely more core/widely used libs) to support both ABIs simultaneously The linked article is wrong to imply this isn't possible - and it really doesn't depend on "GLIBC provide a mechanism". All you have to do is: * Compile the library itself with traditional time_t (32-bit or 64-bit depending on platform), but convert and call time64 APIs internally. * Make a copy of every public structure that embeds a time_t (directly or indirectly), using time64_t (or whatever contains it) instead. * Make a copy of every function that takes any time-using type, to use the time64 types. * In the public headers, check if everything is compiled with 64-bit time_t, and if so make all the traditional types/functions aliases for the time64 versions. * Disable most of this on platforms that already used 64-bit time_t. Instead (for convenience of external callers) make all the time64 names aliases for the traditional names. It's just that this is a lot of work, and little benefit to any particular library. the GCC 5 std::string transition is probably a better story than LFS, in particular the compiler-supported `abi_tag` to help detect errors (but I think that's only for C++, ugh - language without room for automatic mangling suck). (minor note: using typedefs rather than struct tags for your API makes this easier)
        • AshamedCaptain 2y ago
          This is a nightmare to implement (unless you go library by library and do it by hand, or you have significant compiler help), as e.g. functions may not directly use a struct with time_t but indirectly assume something about the size of the struct. Note that for example the std::string transition did not do this.
• Athas 2y ago
  Why not essentially treat it as a cross compilation scenario? NixOS is also source based, but I don't think such a migration would be particularly difficult. You'd use the 32-bit off_t gcc to compile a glibc with 64-bit off_t, then compile a 64-bit off_t gcc linked against that new glibc, and so on. The host compiler shouldn't matter. I always understood the challenge as binary compatibility, when you can't just switch the entire world at once.
  • codys 2y ago
    Nixos has it easier here because they don't require packages to be "installed" before building code against them. For Gentoo, none of their build scripts (ebuilds) are written to support that. It's plausible that they might change the embuild machinery so that this kind of build (against non-installed packages) could work, but it would need investigation and might be a difficult lift to get it working for all packages. "treat it as a cross compilation scenario" is essentially what the post discusses when they mention "use a different CHOST". A CHOST is a unique name identifying a system configuration, like "x86_64-unknown-linux-gnu" (etc). Gentoo treats building for different CHOSTs as cross compiling.
  • andrewaylett 2y ago
    NixOS isn't the same kind of source-based. At some level, even Debian could be said to be source based: there's nothing stopping you from deciding to build every package from source before installing it, and obviously the packages are themselves built from source at some point. NixOS sits between Debian and Gentoo, as it maintains an output that's capable of existing independently of the rest of the system (like Debian) but is designed to use the current host as a builder (like Gentoo). Gentoo doesn't have any way to keep individual builds separate from the system as a whole, as intimated in the article, so you need to work out how to keep the two worlds separate while you do the build. I think what they're suggesting winds up being pretty similar to what you suggest, just with the right plumbing to make it work in a Gentoo system. NixOS would need different plumbing, I'm not sure whether they've done it yet or how but I can easily imagine it being more straightforward than what Gentoo is needing to do.
    • 2y ago
• ArsenArsen 2y ago
  this is just as much of a problem on binary distros mind you. just because there might be a smaller time delta does not mean that the problem is solved
• codys 2y ago
  nixos is an example of a distro that is source based and can do the atomic rebuild here because it has a way to build packages against other packages that aren't "installed". In nixos, this is because there are very few things that get "installed" as the global, only thing to use. But one could imagine that Gentoo could build something that would allow them to at least build up one set of new packages without installing them and then install the files all at once.
• jasomill 2y ago
  As someone whose nearest exposure to a "source based distro" is FreeBSD, this sounds like madness, as it means a broken build could not only impair attempts to repair the build, but render the system unbootable and/or unusable. And as someone who regularly uses traditional Linux distros like Debian and Fedora, the idea that a package management system would allow a set of packages known to be incompatible with one another to be installed without force, or that core package maintainers would knowingly specify incorrect requirements, is terrifying. While I'm not familiar with Gentoo, my reading of this article suggests that its maintainers are well aware of these sorts of problems and that Gentoo does not, in fact, suffer from them (intentionally; inevitably mistakes happen just as they occasionally do on the bleeding edge of FreeBSD and other Linux distros).
  • AshamedCaptain 2y ago
    FreeBSD does not use package management for the core system (i.e. freebsd itself, not ports). Gentoo does.