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

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Date:   Sat, 22 Feb 2020 13:01:01 -0800
From:   Fangrui Song <maskray@...gle.com>
To:     Arvind Sankar <nivedita@...m.mit.edu>
Cc:     Borislav Petkov <bp@...en8.de>,
        Nathan Chancellor <natechancellor@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org, clang-built-linux@...glegroups.com,
        Michael Matz <matz@...e.de>
Subject: Re: [PATCH] x86/boot/compressed: Fix compressed kernel linking with
 lld

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@...m.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@...il.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.

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.