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

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Message-ID: <20230520133237.GA27501@1wt.eu>
Date:   Sat, 20 May 2023 15:32:37 +0200
From:   Willy Tarreau <w@....eu>
To:     Zhangjin Wu <falcon@...ylab.org>, linux@...ssschuh.net
Cc:     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

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.

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 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.

Regards,
Willy