linux-kernel - Re: [PATCH v3 02/11] tools/nolibc: add new crt.h with _start

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Date:   Sat, 15 Jul 2023 17:23:01 +0800
From:   Zhangjin Wu <falcon@...ylab.org>
To:     thomas@...ch.de
Cc:     arnd@...db.de, falcon@...ylab.org, linux-kernel@...r.kernel.org,
        linux-kselftest@...r.kernel.org, w@....eu
Subject: Re: [PATCH v3 02/11] tools/nolibc: add new crt.h with _start_c

> On 2023-07-14 17:47:23+0800, Zhangjin Wu wrote:
> > > On 2023-07-14 13:58:13+0800, Zhangjin Wu wrote:
> 
> > [..]
> 
> > > I was also not able to reproduce the issue.
> > >
> > 
> > Thanks very much for your 'reproduce' result, It is so weird, just
> > rechecked the toolchain, 13.1.0 from https://mirrors.edge.kernel.org/ is
> > ok, gcc 9, gcc 10.3 not work.
> > 
> > But even in the page of 13.1.0 [1], we still see this line:
> > 
> >     Most optimizations are completely disabled at -O0 or if an -O level is not set on the command line, even if individual optimization flags are specified.
> > 
> > Not sure if "individual optimization flags" also means the optimize()
> > flags in gcc attributes. or the doc is not updated yet?
> > 
> > And further found gcc 11.1.0 is ok, gcc 10.4 still not work, so, gcc
> > 11.1.0 may changed something to let the "individual optimization flags"
> > work with -O0.
> > 
> > We may need to at least document this issue in some files, -O0 is not such a
> > frequently-used option, not sure if we still need -O0 work with the older gcc <
> > 11.1.0 ;-)
> 
> It seems we can avoid the issue by enforcing optimizations for _start:
> 
> diff --git a/tools/include/nolibc/arch-x86_64.h b/tools/include/nolibc/arch-x86_64.h
> index f5614a67f05a..b9d8b8861dc4 100644
> --- a/tools/include/nolibc/arch-x86_64.h
> +++ b/tools/include/nolibc/arch-x86_64.h
> @@ -161,12 +161,9 @@
>   * 2) The deepest stack frame should be zero (the %rbp).
>   *
>   */
> -void __attribute__((weak, noreturn, optimize("omit-frame-pointer"))) __no_stack_protector _start(void)
> +void __attribute__((weak, noreturn, optimize("Os", "omit-frame-pointer"))) __no_stack_protector _start(void)
>

Great, it works and it is minimal enough ;-)

Thanks very much.

> > 
> > Willy, I'm not sure if the issues solved by the commit 7f8548589661
> > ("tools/nolibc: make compiler and assembler agree on the section around
> > _start") still exist after we using _start_c()?
> > 
> > Thomas, because we plan to move the stackprotector init to _start_c(), If using
> > pure assembly _start, we may also not need the __no_stack_protector macro too?
> 
> It would probably not needed anymore in this case.
>

Yeah, but let's reserve it as-is for we have the working
omit-frame-pointer now.

Best regards,
Zhangjin

> Thomas