GNOME's new versioning scheme

Posted Sep 19, 2020 19:24 UTC (Sat) by nix (subscriber, #2304)
In reply to: GNOME's new versioning scheme by mathstuf
Parent article: GNOME's new versioning scheme

-Bsymbolic applied to (say) the link of Gtk 4 itself stops Gtk's own internal calls to its own symbols from being interposed by some other Gtk that happened to have been loaded earlier.

DF_GROUP (as usual I got this one wrong, it's a flag, not a tag) constrains symbol searches from within this library to happen only within the library and its transitive DT_NEEDED libraries: it stops global symbols or symbols in other branches of the search tree from interposing. The relevant ld flag to look for is '-Bgroup'. It is *not* the same as --start-group/--end-group or the linker script GROUP command: fairly confusing naming really. Weak symbols still work, but only if they're resolved by objects loaded by this shared library or things it loaded. (So the old pthreads trick wouldn't work, not that it's a good idea anyway in glibc 2.32+.)

GNOME's new versioning scheme

Posted Sep 22, 2020 16:33 UTC (Tue) by mathstuf (subscriber, #69389) [Link] (2 responses)

Hrm. The fact that it is a library-wide flag works most of the time, but really fails for plugins. I'd like them to load their novel dependencies directly, but those from the loading application usually should just be expected to be there. Maybe `--warn-unresolved-symbols` can help to alleviate that though.

I have a PR (that's likely fallen on deaf ears) for macOS' linker to have a "any symbol you find in this library should be looked up at runtime" behavior which is, I think, exactly the kind of behavior I'd like. https://github.com/apple-opensource/ld64/pull/1

For reference, the search term to use for the ELF flag is `DF_1_GROUP`.

GNOME's new versioning scheme

Posted Sep 22, 2020 21:38 UTC (Tue) by nix (subscriber, #2304) [Link] (1 responses)

I don't know what "looked up at runtime" means: all symbols in dynamically-linked programs are looked up at runtime, that's what dynamic linking *is*. Perhaps you mean "look for these symbols in the main program"? Because if so, that's the ELF default: the search scope for all symbols starts with the main program and goes on from there. (Well, OK, actually it starts with ld.so itself, and the main program comes right after that, usually followed by libc.)

GNOME's new versioning scheme

Posted Sep 22, 2020 21:54 UTC (Tue) by mathstuf (subscriber, #69389) [Link]

As opposed to getting "this symbol was not found" errors at link time. So if I use `-Bgroup`, symbols will be searched for in the loading chain and then *only* the DT_NEEDED libraries? Or does the library/executable providing a plugin API also need to be in the DT_NEEDED section for DF_1_GROUP to work appropriately?