[Bug 207898] kernel linker behaves differently on amd64 vs. i386

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=207898

--- Comment #6 from Ed Maste <emaste@freebsd.org> ---
(In reply to Don Lewis from comment #3)
> Why would the linker ignore the flag for local symbols?

I suspect there is no satisfying reason. It was probably just never
implemented, and had no (or very limited) effect on the set of (relatively
simple) in-tree kernel modules.

(In reply to Justin Cady from comment #4)
> 1. Am I correct that because of this bug, symbol names on amd64 are effectively
> required to be unique across all kernel module dependencies?

Yes, it appears that is the case.

> 2. Is there any risk to actually fixing this? I tried to understand all of the
> potential callers of link_elf_lookup_symbol(), but doing so is not
> straightforward through all the indirect calls (function pointers, macros).
> Stated differently: is there any expectation that link_elf_lookup_symbol()
> should return a local symbol?

There are two separate risks:

1) changing link_elf_lookup_symbol's beavhiour, if there are callers other than
via module loading that depend on the current behaviour

Notice that this is already covered in Kostik's patch (in the thread you
referenced). It adds link_elf_lookup_debug_symbol which continues searching
local symbols.

2) breaking in-tree or out-of-tree kernel modules that depend on this

Also in Kostik's patch there were changes to sys/modules/mii and
sys/modules/pseudofs, because they were missing entries in EXPORT_SYMS. (I did
not look to see if they have been fixed since, or whether there are other
newly-introduced cases.)

In-tree modules would have to be fixed if/when this patch goes in, but there
may well be third party modules that need similar changes.

All of that said, this is definitely a bug that should be fixed.

I propose:

1) augment Kostik's patch to add a sysctl controlling whether kernel module
symbol resolution uses local symbols or not
2) initially set it to maintain the current behaviour (use local symbols)
3) commit to main, send a call for testing
4) MFC to stable/13 (as well as stable/12 if the merge is straightforward)
maintaining current behaviour
5) fix any in-tree modules
6) toggle the the sysctl in main (i.e., no longer search local symbols in
kernel module lookup)
7) consider emitting a warning if the sysctl is changed by the user
8) wait for FreeBSD 14.0's release
9) remove the sysctl, leaving only no-local-lookup for kernel modules

-- 
You are receiving this mail because:
You are the assignee for the bug.