linux-kernel - Re: [PATCH v3 3/5] RISC-V: Align the .init.text section

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Message-ID: <mhng-5b9a6df7-f6a2-4db8-aaf9-b4e89115522d@palmerdabbelt-glaptop>
Date:   Tue, 15 Dec 2020 22:02:54 -0800 (PST)
From:   Palmer Dabbelt <palmer@...belt.com>
To:     Atish Patra <Atish.Patra@....com>
CC:     linux-kernel@...r.kernel.org, Atish Patra <Atish.Patra@....com>,
        Jim Wilson <jimw@...ive.com>, aou@...s.berkeley.edu,
        akpm@...ux-foundation.org, anup@...infault.org, ardb@...nel.org,
        greentime.hu@...ive.com, ren_guo@...ky.com,
        linux-riscv@...ts.infradead.org, walken@...gle.com,
        ojeda@...nel.org, rppt@...nel.org,
        Paul Walmsley <paul.walmsley@...ive.com>, zong.li@...ive.com
Subject:     Re: [PATCH v3 3/5] RISC-V: Align the .init.text section

On Wed, 04 Nov 2020 16:04:37 PST (-0800), Atish Patra wrote:
> In order to improve kernel text protection, we need separate .init.text/
> .init.data/.text in separate sections. However, RISC-V linker relaxation
> code is not aware of any alignment between sections. As a result, it may
> relax any RISCV_CALL relocations between sections to JAL without realizing
> that an inter section alignment may move the address farther. That may
> lead to a relocation truncated fit error. However, linker relaxation code
> is aware of the individual section alignments.
>
> The detailed discussion on this issue can be found here.
> https://github.com/riscv/riscv-gnu-toolchain/issues/738
>
> Keep the .init.text section aligned so that linker relaxation will take
> that as a hint while relaxing inter section calls.
> Here are the code size changes for each section because of this change.
>
> section         change in size (in bytes)
>   .head.text      +4
>   .text           +40
>   .init.text      +6530
>   .exit.text      +84
>
> The only significant increase in size happened for .init.text because
> all intra relocations also use 2MB alignment.
>
> Suggested-by: Jim Wilson <jimw@...ive.com>
> Signed-off-by: Atish Patra <atish.patra@....com>
> ---
>  arch/riscv/kernel/vmlinux.lds.S | 8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
>
> diff --git a/arch/riscv/kernel/vmlinux.lds.S b/arch/riscv/kernel/vmlinux.lds.S
> index 3ffbd6cbdb86..cacd7898ba7f 100644
> --- a/arch/riscv/kernel/vmlinux.lds.S
> +++ b/arch/riscv/kernel/vmlinux.lds.S
> @@ -30,7 +30,13 @@ SECTIONS
>  	. = ALIGN(PAGE_SIZE);
>
>  	__init_begin = .;
> -	INIT_TEXT_SECTION(PAGE_SIZE)
> +	__init_text_begin = .;
> +	.init.text : AT(ADDR(.init.text) - LOAD_OFFSET) ALIGN(SECTION_ALIGN) { \
> +		_sinittext = .;						\
> +		INIT_TEXT						\
> +		_einittext = .;						\
> +	}
> +
>  	. = ALIGN(8);
>  	__soc_early_init_table : {
>  		__soc_early_init_table_start = .;

Not sure what's going on here (or why I wasn't catching it earlier), but this
is breaking boot on one of my test configs.  I'm not getting any Linux boot
spew, so it's something fairly early.  I'm running defconfig with

    CONFIG_PREEMPT=y
    CONFIG_DEBUG_PREEMPT=y
    CONFIG_PROVE_LOCKING=y

It looks like that's been throwing a bunch of warnings for a while, but it did
at least used to boot.  No idea what PREEMPT would have to do with this, and
the other two don't generally trigger issues that early in boot (or at least,
trigger halts that early in boot).

There's a bunch of other stuff that depends on this that's on for-next so I
don't want to just drop it, but I also don't want to break something.  I'm just
running QEMU's virt board.

I'll take a look again tomorrow night, but if anyone has some time to look
that'd be great!