Re: [PATCH] x86/boot/compressed: Fix compressed kernel linking with lld

[Fangrui Song]

On 2020-02-22, Arvind Sankar wrote:
> On Sat, Feb 22, 2020 at 10:58:06AM -0800, Fangrui Song wrote:
> > On 2020-02-22, Nathan Chancellor wrote:
> > >On Sat, Feb 22, 2020 at 12:18:59PM -0500, Arvind Sankar wrote:
> > >> Commit TBD ("x86/boot/compressed: Remove unnecessary sections from
> > >> bzImage") discarded unnecessary sections with *(*). While this works
> > >> fine with the bfd linker, lld tries to also discard essential sections
> > >> like .shstrtab, .symtab and .strtab, which results in the link failing
> > >> since .shstrtab is required by the ELF specification. .symtab and
> > >> .strtab are also necessary to generate the zoffset.h file for the
> > >> bzImage header.
> > >>
> > >> Since the only sizeable section that can be discarded is .eh_frame,
> > >> restrict the discard to only .eh_frame to be safe.
> > >>
> > >> Signed-off-by: Arvind Sankar <nivedita@xxxxxxxxxxxx>
> > >> ---
> > >> Sending as a fix on top of tip/x86/boot.
> > >>
> > >>  arch/x86/boot/compressed/vmlinux.lds.S | 4 ++--
> > >>  1 file changed, 2 insertions(+), 2 deletions(-)
> > >>
> > >> diff --git a/arch/x86/boot/compressed/vmlinux.lds.S b/arch/x86/boot/compressed/vmlinux.lds.S
> > >> index 12a20603d92e..469dcf800a2c 100644
> > >> --- a/arch/x86/boot/compressed/vmlinux.lds.S
> > >> +++ b/arch/x86/boot/compressed/vmlinux.lds.S
> > >> @@ -74,8 +74,8 @@ SECTIONS
> > >>  	. = ALIGN(PAGE_SIZE);	/* keep ZO size page aligned */
> > >>  	_end = .;
> > >>
> > >> -	/* Discard all remaining sections */
> > >> +	/* Discard .eh_frame to save some space */
> > >>  	/DISCARD/ : {
> > >> -		*(*)
> > >> +		*(.eh_frame)
> > >>  	}
> > >>  }
> > >> --
> > >> 2.24.1
> > >>
> > >
> > >FWIW:
> > >
> > >Tested-by: Nathan Chancellor <natechancellor@xxxxxxxxx>
> >
> > I am puzzled. Doesn't -fno-asynchronous-unwind-tables suppress
> > .eh_frame in the object files? Why are there still .eh_frame?
> >
> > Though, there is prior art: arch/s390/boot/compressed/vmlinux.lds.S also discards .eh_frame
>
> The compressed kernel doesn't use the regular flags and it seems it
> doesn't have that option. Maybe we should add it in to avoid generating
> those in the first place.
>
> The .eh_frame discard in arch/x86/kernel/vmlinux.lds.S does seem
> superfluous though.

Yes, please do that. I recommend suppressting unneeded sections at
compile time, instead of discarding them at link time.

https://github.com/torvalds/linux/commit/83a092cf95f28696ddc36c8add0cf03ac034897f
added -Wl,--orphan-handling=warn to arch/powerpc/Makefile .
x86 can follow if that is appropriate.

I don't recommend -Wl,--orphan-handling=error, which can unnecessarily
break the builds.