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

From: Arvind Sankar <nivedita@alum.mit.edu>
To: Fangrui Song <maskray@google.com>
Cc: Arvind Sankar <nivedita@alum.mit.edu>,
	Borislav Petkov <bp@alien8.de>,
	Nathan Chancellor <natechancellor@gmail.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, "H. Peter Anvin" <hpa@zytor.com>,
	x86@kernel.org, linux-kernel@vger.kernel.org,
	clang-built-linux@googlegroups.com, Michael Matz <matz@suse.de>
Subject: Re: [PATCH] x86/boot/compressed: Fix compressed kernel linking with lld
Date: Sat, 22 Feb 2020 15:17:16 -0500	[thread overview]
Message-ID: <20200222201715.GA3674682@rani.riverdale.lan> (raw)
In-Reply-To: <20200222185806.ywnqhfqmy67akfsa@google.com>

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@alum.mit.edu>
> >> ---
> >> 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@gmail.com>
> 
> 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.