linux-kernel - Re: [PATCH] selftests/nolibc: Fix up compile error for rv32

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Message-ID: <20230520141332.GB27611@1wt.eu>
Date:   Sat, 20 May 2023 16:13:32 +0200
From:   Willy Tarreau <w@....eu>
To:     Thomas Weißschuh <thomas@...ch.de>
Cc:     Zhangjin Wu <falcon@...ylab.org>, aou@...s.berkeley.edu,
        linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org,
        linux-riscv@...ts.infradead.org, palmer@...belt.com,
        paul.walmsley@...ive.com, shuah@...nel.org
Subject: Re: [PATCH] selftests/nolibc: Fix up compile error for rv32

On Sat, May 20, 2023 at 04:07:34PM +0200, Thomas Weißschuh wrote:
> Hi Willy!
> 
> On 2023-05-20 15:32:37+0200, Willy Tarreau wrote:
> > Thomas, Zhangjin,
> > 
> > I've merged your latest patches in my branch 20230520-nolibc-rv32+stkp2,
> > which was rebased to integrate the updated commit messages and a few
> > missing s-o-b from mine. Please have a look:
> > 
> >    https://git.kernel.org/pub/scm/linux/kernel/git/wtarreau/nolibc.git
> > 
> > However, Thomas, I noticed something puzzling me. While I tested with
> > gcc-9.5 (that I have here along my toolchains) I found that it would
> > systematically fail:
> > 
> >   sysroot/x86/include/stackprotector.h:46:1: warning: 'no_stack_protector' attribute directive ignored [-Wattributes]
> >      46 | {
> >         | ^
> >   !!Stack smashing detected!!
> >   qemu: uncaught target signal 6 (Aborted) - core dumped
> >   0 test(s) passed.
> > 
> > The reason is that it doesn't support the attribute "no_stack_protector".
> > Upon closer investigation, I noticed that _start() on x86_64 doens't have
> > it, yet it works on more recent compilers! So I don't understand why it
> > works with more recent compilers.
> 
> _start() not having the attribute is indeed an oversight.
> No idea how it worked before.

No problem, I preferred to mention it anyway.

> > I managed to avoid the crash by enclosing the __stack_chk_init() function
> > in a #pragma GCC optimize("-fno-stack-protector") while removing the
> > attribute (though Clang and more recent gcc use this attribute so we
> > shouldn't completely drop it either).
> 
> I would like to first align x86 to __attribute__((no_stack_protector))
> for uniformity and then figure out on how to make it nicer.

I agree.

> > I consider this non-critical as we can expect that regtests are run with
> > a reasonably recent compiler version, but if in the long term we can find
> > a more reliable detection for this, it would be nice.
> > 
> > For example I found that gcc defines __SSP_ALL__ to 1 when
> > -fstack-protector is used, and 2 when -fstack-protector-all is used.
> > With clang, it's 1 and 3 respectively. Maybe we should use that and
> > drop NOLIBC_STACKPROTECTOR, that would be one less variable to deal
> > with: the code would automatically adapt to whatever cflags the user
> > sets on the compiler, which is generally better.
> 
> That sounds great!
> 
> I explicitly looked for something like this before, dumping preprocessor
> directives and comparing.
> It seems the fact that my compilers enable this feature by default made
> me miss it.

Hmmm that's indeed possible. With -fno-stack-protector it should disappear:

  $ gcc -fno-stack-protector -dM -E -xc - < /dev/null |grep SSP
  $ gcc -fstack-protector -dM -E -xc - < /dev/null |grep SSP
  #define __SSP__ 1
  $ gcc -fstack-protector-all -dM -E -xc - < /dev/null |grep SSP
  #define __SSP_ALL__ 2
  $ clang -fstack-protector-all -dM -E -xc - < /dev/null |grep SSP
  #define __SSP_ALL__ 3

> I'll send patches.

OK thanks. Just be aware that I'll be less responsive this week-end from
now on.

Willy