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

[Sebastian Ott]

On Wed, 13 Sep 2023, Willy Tarreau wrote:
> On Wed, Sep 13, 2023 at 10:19:00PM +0200, Thomas Weißschuh wrote:
> > > 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?
>
> I tried here with gcc-11.4.0 native on an ubuntu-22.04 and using a
> cross gcc-9.5 executed on another box and couldn't reproduce the issue
> at all. It could be that the compiler inserts whatever, did someone
> try to disassemble de resulting program to see what it looks like ?
> Maybe we're even dealing with issues related to random stack alignment
> that causes issues past a function call due to some garbage being placed
> at the wrong place in the stack. Also, dmesg should generally report
> what (and where) the segv happened. Similarly, gdb with "info reg"
> and "disassemble $pc" should report some info.

Sadly there is no kernel output for this I guess this happens just too
early. gdb just reports "The program has no registers now."

Sebastian