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Message-ID: <20250512174733.GH1827@brightrain.aerifal.cx>
Date: Mon, 12 May 2025 13:47:33 -0400
From: Rich Felker <dalias@...c.org>
To: Andy Caldwell <andy.m.caldwell@...glemail.com>
Cc: musl@...ts.openwall.com
Subject: Re: [PATCH] force -fno-lto for CRT files
On Mon, May 12, 2025 at 04:15:34PM +0100, Andy Caldwell wrote:
> Hey,
>
> An alternative approach would be to mark the function as `used` which the
> attached patch does. Sadly this attribute appears to maybe be a GNU-ism (also
> supported in all versions of clang I could find) so maybe it would need a
> feature check too to handle other compilers, but this feels like the correct
> fix - telling the compiler that the symbol is used rather than disabling
> compiler features that misbehave because the compiler can't see the indirect
> calls.
>
>
> From 988328716d269005ebb55095d74576bf88f5154f Mon Sep 17 00:00:00 2001
> From: Andy Caldwell <andy.m.caldwell@...glemail.com>
> Date: Mon, 12 May 2025 16:01:52 +0100
> Subject: [PATCH] Mark `__dls2` as `__attribute__((used))`
>
> This function is only referenced indirectly (though an explicit symbol
> lookup on inline assembly) which causes LTO to strip it, causing linker
> errors. The `used` attribute tells the compiler (including the LTO
> phase) that the function is referenced, even if the compiler can't see
> that, preventing the function getting dropped.
> ---
> ldso/dynlink.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/ldso/dynlink.c b/ldso/dynlink.c
> index 715948f4..ab78274e 100644
> --- a/ldso/dynlink.c
> +++ b/ldso/dynlink.c
> @@ -1704,7 +1704,7 @@ static void install_new_tls(void)
> * symbols. Its job is to perform symbolic relocations on the dynamic
> * linker itself, but some of the relocations performed may need to be
> * replaced later due to copy relocations in the main program. */
> -
> +__attribute__((used))
> hidden void __dls2(unsigned char *base, size_t *sp)
> {
> size_t *auxv;
> --
> 2.49.0
Is that even the only problem? My understanding is that this bug in
LTO affects all the crt start files as well, and in those it's more
explicit: it's ignoring a top-level asm root not just a reference via
inline asm inside a function.
On top of that, LTO-type transformations are wrong/unsafe for code
that executes prior to relocations and passing control to the entry
point, since at least in theory they could lift things without side
effects to somehow run before relocations or other initialization they
depend upon has happened. In some sense, this code runs in a separate,
lower-level execution environment operating *on* the memory of the
progam that will run rather than *as part of* the program. So I think
it makes sense to suppress LTO.
I kinda don't like introducing a second parallel set of "NOLTO" rules
that are identical to the "NOSSP" ones though, when the motivation for
both (that the code runs prior to the program/runtime being ready) is
the same. Maybe we could rename "NOSSP" to something more descriptive
and use the same CFLAGS & makefile rules for both? OTOH, I'd kinda
rather not change/remove CFLAGS_NOSSP in case it breaks folks' custom
config.mak's.. but maybe I'm the only one with one of those anyway.
Rich
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