Re: [PATCH] aarch64: Check PIC instead of SHARED in start.S

["H.J. Lu" <hjl dot tools at gmail dot com>]

On 10/3/17, Szabolcs Nagy <szabolcs.nagy@arm.com> wrote:
> On 02/10/17 12:20, H.J. Lu wrote:
>> On 10/2/17, Szabolcs Nagy <szabolcs.nagy@arm.com> wrote:
>>> On 29/09/17 22:32, H.J. Lu wrote:
>>>> Since start.o may be compiled as PIC, we should check PIC instead of
>>>> SHARED.
>>>>
>>>> OK for master?
>>>>
>>>
>>> i believe that the compile/link tests worked..
>>
>> Does static PIE of hjl/pie/static branch run on arm and aarch64?
>>
>
> no, if i build with --enable-static-pie the install step
> fails when the static linked sln runs.
>
> there are relative relocs against the func ptrs that are
> loaded from GOT in the startup code, but execution fails
> even before those are used because there are R*_JUMP_SLOT
> and R*_GLOB_DAT relocs which are not processed correctly.
>
> in particular in
>   if (__pthread_initialize_minimal != NULL)
>     __pthread_initialize_minimal ();
> the symbol value loaded from GOT is non-NULL even though
> there is no pthread linked in, that is probably a linker bug.
>
>>> ..but i still don't understand how the GOT entries
>>> of the startup code get initialized in PIE executable
>>> at runtime.
>>
>> You just avoid GOT entries in start.S for static PIE by using
>> PC relative relocations.
>>
>
> i don't see how can you do that when you have to pass
> absolute addresses as arguments to __libc_start_main
> and the base address is not yet computed.

Does ARM support PC relative relocation for local function address?
All functions are local in static PIE.  In i386/start.S, there are

	/* Load PIC register.  */
	call 1f
	addl $_GLOBAL_OFFSET_TABLE_, %ebx

	/* Push address of our own entry points to .fini and .init.  */
	leal __libc_csu_fini@GOTOFF(%ebx), %eax
	pushl %eax
	leal __libc_csu_init@GOTOFF(%ebx), %eax
	pushl %eax

	pushl %ecx		/* Push second argument: argv.  */
	pushl %esi		/* Push first argument: argc.  */

# ifdef SHARED
	pushl main@GOT(%ebx)
# else
	/* Avoid relocation in static PIE since _start is called before
	   it is relocated.  */
	leal main@GOTOFF(%ebx), %eax
	pushl %eax
# endif

GOTOFF can be resolved by linker to avoid dynamic relocations.

[hjl@gnu-efi-2 gcc]$ cat x.c
extern void foo (void) __attribute__ ((visibility("hidden")));
extern void bar (void*);

void
xxx (void)
{
  bar (foo);
}
[hjl@gnu-efi-2 gcc]$ cat x.s
	.arch armv8-a
	.file	"x.c"
	.text
	.align	2
	.align	3
	.global	xxx
	.type	xxx, %function
xxx:
	adrp	x0, foo
	add	x0, x0, :lo12:foo
	b	bar
	.size	xxx, .-xxx
	.hidden	foo
	.ident	"GCC: (GNU) 8.0.0 20171002 (experimental)"
	.section	.note.GNU-stack,"",@progbits
[hjl@gnu-efi-2 gcc]$

Does this need GOT?

-- 
H.J.