Re: [PATCH 00/46] gcc-LTO support for the kernel

[Peter Zijlstra <peterz@infradead.org>]

On Thu, Nov 17, 2022 at 01:55:07PM +0000, Richard Biener wrote:

> > > I'm not sure what you're on about; only symbols annotated with
> > > EXPORT_SYMBOL*() are accessible from modules (aka DSOs) and those will
> > > have their address taken. You can feely eliminate any unused symbol.
> 
> But IIRC that's not reflected on the ELF level by making EXPORT_SYMBOL*()
> symbols public and the rest hidden - instead all symbols global in the C TUs
> will become public and the module dynamic loader details are hidden from
> GCCs view of the kernel image as ELF relocatable object.

It is reflected by keeping their address in __ksymtab_$foo sections, as
such their address 'escapes'.

> > We have an __ADDRESSABLE() macro and asmlinkage modifier to annotate
> > symbols that may appear to the compiler as though they are never
> > referenced.
> > 
> > Would it be possible to repurpose those so that the LTO code knows
> > which symbols it must not remove?
> 
> I find
> 
> /*
>  * Force the compiler to emit 'sym' as a symbol, so that we can reference
>  * it from inline assembler. Necessary in case 'sym' could be inlined
>  * otherwise, or eliminated entirely due to lack of references that are
>  * visible to the compiler.
>  */
> #define ___ADDRESSABLE(sym, __attrs) \
> 	static void * __used __attrs \
> 		__UNIQUE_ID(__PASTE(__addressable_,sym)) = (void *)&sym;
> #define __ADDRESSABLE(sym) \
> 	___ADDRESSABLE(sym, __section(".discard.addressable"))
> 
> that should be enough to force LTO keeping 'sym' - unless there's
> a linker script that discards .discard.addressable which I fear LTO
> will notice, losing the effect.

The initial LTO link pass will not discard .discard sections in order to
generate a regular ELF object file. This object file is then fed to
objtool and the kallsyms tool and eventually linked with the linker
script in a multi-stage link pass.

Also see scripts/link-vmlinux.sh for all the horrible details.

> The folks who worked on LTO enablement of the kernel should know the
> real issue better - I understand asm()s are a pain because GCC
> refuses to parse the assembler string heuristically for used
> symbols (but it can never be more than heuristics). 

I don't understand why it can't be more than heuristics; eventually the
asm() contents end up in a real assembler and it has to make sense.

Might as well parse it directly -- isn't that what clang-ias does?

> The issue with asm()s is not so much elimination (__used solves that)
> but that GCC can end up moving the asm() and the refered to symbols to
> different link-time units causing unresolved symbols for non-global
> symbols.  -fno-toplevel-reorder should fix that at some cost.

I thought the whole point of LTO was that there was only a single link
time unit, translate all the tus into intermadiate gunk and then collect
the whole lot in one go.