Re: [musl] Can’t build musl with lto=thin

[Rich Felker <dalias@libc.org>]

On Sat, Jan 30, 2021 at 11:04:32PM +0000, Jiahao XU wrote:
> > So something like (in config.mak):
> >
> > obj/ldso/dlstart.lo: CFLAGS_ALL += -fno-lto
> 
> Thanks, with this I was able to build libc.so successfully with clang and created a 3.5 KB hello world program using clang and lld.
> 
> However, I still wasn’t able to statically linked with libc.
> 
> Once I added ‘-static’ to the compiler flags, the executable failed with ‘Segmentation fault (core dumped)’.

It's libc.a, not libc.so, that will be involved in making a
static-linked binary. It's hard to know what's going wrong without
more information. Can you run under a debugger and provide a
backtrace, disassembly, and register dump for where the crash occurs?

Rich


> ________________________________
> From: Rich Felker <dalias@libc.org>
> Sent: Sunday, January 31, 2021 7:12:31 AM
> To: Jiahao XU <Jiahao_XU@outlook.com>
> Cc: musl@lists.openwall.com <musl@lists.openwall.com>
> Subject: Re: [musl] Can’t build musl with lto=thin
> 
> On Fri, Jan 29, 2021 at 12:19:42PM +0000, Jiahao XU wrote:
> > musl-1.2.2 compilation with clang-11 failed to build libc.so at the final linking stage:
> >
> >     ld.lld: error: undefined hidden symbol: __dls2
> >     >>> referenced by ld-temp.o
> >     >>>                          lto.tmp:(_dlstart_c)
> >     >>> did you mean: __dls3
> >     >>> defined in: lto.tmp
> >
> > I am using CFLAGS=‘-march=native -mtune=native -Oz -flto
> > -fmerge-all-constants -fomit-frame-pointer’ and LDFLAGS=‘-flto
>   ^^^^^^^^^^^^^^^^^^^^^
> 
> The -fmerge-all-constants option gives non-conforming language
> semantics and should not be used, but that's a separate issue.
> 
> > -fuse-ld=lld -Wl,—plugin-opt=O3,-O3,—icf=safe’.
> 
> > No configure option is supplied.
> 
> Otherwise, it's a known issue that LTO misses references from asm
> (both top-level and in functions). I think dlstart.lo and a few other
> files should just be built with LTO disabled; any LTO-type
> optimization in code that runs at this stage is inherently invalid,
> anyway. So something like (in config.mak):
> 
> obj/ldso/dlstart.lo: CFLAGS_ALL += -fno-lto
> 
> Rich