Bug 236647

Description Alan Somers 2019-03-19 21:28:19 UTC

When a module gets renamed, it's customary to hard link the new module name to the old one so newly upgraded systems can still boot before they've been reconfigured.  However, sometimes "kldload newname" will register the module with the old name, as shown below:

root@freebsd:/boot/kernel # ls -li fuse*.ko
2328482 -r-xr-xr-x  2 root  wheel  113928 Mar 19 20:00 fuse.ko
2328482 -r-xr-xr-x  2 root  wheel  113928 Mar 19 20:00 fusefs.ko
root@freebsd:/boot/kernel # kldload fusefs
fuse-freebsd: version 0.4.4, FUSE ABI 7.8
root@freebsd:/boot/kernel # kldstat
Id Refs Address                Size Name
 1    3 0xffffffff80200000  25140f8 kernel
 2    1 0xffffffff82811000     a270 fuse.ko

There are four separate functions that cause this bug.  Taken individually, each one of them behaves reasonably.  But the combination is problematic:

1) kldxref(8), which generates /boot/linker.hints, examines each file in turn and independently writes an entry for it into the hints file.  In this case, it legitimately writes a "fusefs" => "fuse.ko" entry.  It writes it before the "fusefs" -> "fusefs.ko" entry probably because fuse.ko sorts lexicographically before fusefs.ko.  This behavior is reasonable, because fuse.ko _does_ technically implement the fusefs module.

2) In the kernel, linker_hints_lookup chooses the first filename from the hints file that matches the desired module name.  This behavior is reasonable, because it would be slower to read the entire hints file, looking for the "best" matching filename.

3) linker_hints_lookup accepts filenames that don't exactly match module names.  This behavior is also reasonable, because a user might legitimately have multiple filenames implementing the same module, if they're at different versions or something.

4) linker_load_file registers the module by its filename, not its module name.  This might also be reasonable for the case described above where multiple files implement different versions of the same module.

What's the best solution?  I can think of two:
1) Change linker_load_file to register by module name, not filename.

2) Add special logic to kldxref for handling multiple-linked files.  Only generate one hint instead of many.