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

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Date:   Thu, 14 Sep 2023 01:02:30 +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 22:19:00+0200, Thomas Weißschuh wrote:
> On 2023-09-13 20:44:59+0200, Sebastian Ott wrote:
> > 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 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?

Callchain for the failure:

load_elf_binary()
  -> if (likely(elf_bss != elf_brk) && unlikely(padzero(elf_bess)))
    -> padzero()
      -> clear_user()
        -> __arch_clear_user()
	  -> failure in arch/arm64/lib/clear_user.S

Resulting in a EFAULT which gets translated to SIGSEGV somewhere.


The following patch, which seems sensible to me, fixes it for me.
But as this is really old, heavily used code I'm a bit hesitant.

diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c
index 7b3d2d491407..13f71733ba63 100644
--- a/fs/binfmt_elf.c
+++ b/fs/binfmt_elf.c
@@ -112,7 +112,7 @@ static struct linux_binfmt elf_format = {
 
 static int set_brk(unsigned long start, unsigned long end, int prot)
 {
-	start = ELF_PAGEALIGN(start);
+	start = ELF_PAGESTART(start);
 	end = ELF_PAGEALIGN(end);
 	if (end > start) {
 		/*