linux-kernel - Re: aarch64 binaries using nolibc segfault before reaching the entry point

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Date:   Wed, 13 Sep 2023 22:19:00 +0200
From:   Thomas Weißschuh <linux@...ssschuh.net>
To:     Sebastian Ott <sebott@...hat.com>
Cc:     Mark Brown <broonie@...nel.org>, Willy Tarreau <w@....eu>,
        linux-kernel@...r.kernel.org
Subject: Re: aarch64 binaries using nolibc segfault before reaching the entry
 point

On 2023-09-13 20:44:59+0200, Sebastian Ott wrote:
> Hi,
> 
> the tpidr2 selftest on an arm box segfaults before reaching the entry point.
> I have no clue what is to blame for this or how to debug it but for a
> statically linked binary there shouldn't be much stuff going on besides the
> elf loader?
> 
> I can reproduce this with a program using an empty main function. Also checked
> for other nolibc users - same result for init.c from rcutorture.
> 
> tools/testing/selftests/arm64/fp/za-fork is working though - the only
> difference I could spot here is that it is linked together with another object
> file. I also looked at the elf headers but didn't find anything obvious (but
> I'm a bit out of my comfort zone here..)
> 
> After playing around with linker options I found that using -static-pie
> lets the binaries run successful.
> 
> [root@arm abi]# cat test.c
> int main(void)
> {
>         return 1;
> }
> [root@arm abi]# gcc -Os -static -Wall -lgcc -nostdlib -ffreestanding  -include ../../../../include/nolibc/nolibc.h  test.c
> [root@arm abi]# ./a.out Segmentation fault
> [root@arm abi]# gcc -Os -static -Wall -lgcc -nostdlib -ffreestanding -static-pie -include ../../../../include/nolibc/nolibc.h  test.c
> [root@arm abi]# ./a.out [root@arm abi]#
> 
> All on aarch64 running fedora37 + upstream kernel. Any hints on what could
> be borken here or how to actually fix it?

I reduced it to the following reproducer:

    $ cat test.c
    int foo;  /* It works when deleting this variable */
    
    void __attribute__((weak, noreturn, optimize("Os", "omit-frame-pointer"))) _start(void)
    {
    	__asm__ volatile (
    		"mov x8, 93\n"       /* NR_exit == 93 */
    		"svc #0\n"
    	);
    	__builtin_unreachable();
    }
    
    $ aarch64-linux-gnu-gcc -Os -static -fno-stack-protector -Wall -nostdlib test.c
    $ ./a.out
    Segmentation fault

Also when running under gdb the error message is:

    During startup program terminated with signal SIGSEGV, Segmentation fault.

So it seems the error already happens during loading.

Could be a compiler or kernel bug?

Thomas