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Date:      Thu, 12 Jan 2017 00:37:53 -0800
From:      Mark Millard <markmi@dsl-only.net>
To:        Ed Maste <emaste@freebsd.org>, Roman Divacky <rdivacky@vlakno.cz>
Cc:        FreeBSD Toolchain <freebsd-toolchain@freebsd.org>
Subject:   Re: /usr/bin/ld.lld on powerpc64: produces a.out for which: ld-elf.so.1: assert failed: /usr/src/libexec/rtld-elf/powerpc64/reloc.c:374
Message-ID:  <EF97071B-AE4A-4520-A997-52249B8DAB5A@dsl-only.net>
In-Reply-To: <CAPyFy2DG3ucUkxDCwRJ10a-nhC1=YvVrwR7v0dw6LJk=e61nvQ@mail.gmail.com>
References:  <7139F615-8F18-4EDC-9051-5FFEC0C4057F@dsl-only.net> <CAPyFy2CSmNyUjQcXyq4qUWp_A=Qr81O7fpRbZ5GcfFHFhdSntw@mail.gmail.com> <CF2D86C8-8EEB-4536-8D83-6F8C676EFEF6@dsl-only.net> <20170111194844.GA16135@vlakno.cz> <8242A7B9-7ED3-4861-8209-F3728113D188@dsl-only.net> <20170111210658.GA20265@vlakno.cz> <CAPyFy2DG3ucUkxDCwRJ10a-nhC1=YvVrwR7v0dw6LJk=e61nvQ@mail.gmail.com>

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On 2017-Jan-11, at 1:23 PM, Ed Maste <emaste at freebsd.org> wrote:

> On 11 January 2017 at 21:06, Roman Divacky <rdivacky at vlakno.cz> =
wrote:
>> Looks like a progress :) Three questions...
>>=20
>> Is the readelf -a reasonable now?
>=20
> FYI, I just committed an ELF Tool Chain fix (r311941) so readelf
> should display the relocation types properly now.

Thanks. I updated to -r311950 to pick this up.

>> If you compile with -g, does the
>> backtrace make a bit more sense? And finally, can you try to =
"nexti/stepi" in gdb from
>> _start to see where things go wrong? Possibly doing it both for ld =
linked a.out
>> and lld linked a.out and compare where things differ.

I had compiled with -g. It never gets to main. . .

# /usr/local/bin/gdb a.out
. . .
Reading symbols from a.out...done.
(gdb) start
Temporary breakpoint 1 at 0x1001045c: file main.c, line 3.
Starting program: /root/c_tests/a.out=20

Program received signal SIGSEGV, Segmentation fault.
0x000000001001056c in ?? ()

Note that the temporary breakpoint is never hit.

(gdb) bt
#0  0x000000001001056c in ?? ()
#1  0x00000000100100d8 in ?? ()
#2  0x00000000500279b0 in ._rtld_start () at =
/usr/src/libexec/rtld-elf/powerpc64/rtld_start.S:104
Backtrace stopped: frame did not save the PC

(gdb) up 2
#2  0x00000000500279b0 in ._rtld_start () at =
/usr/src/libexec/rtld-elf/powerpc64/rtld_start.S:104
104		blrl	/* _start(argc, argv, envp, obj, cleanup, =
ps_strings) */
(gdb) disass
Dump of assembler code for function ._rtld_start:
   0x0000000050027930 <+0>:	stdu    r1,-144(r1)
   0x0000000050027934 <+4>:	std     r3,96(r1)
   0x0000000050027938 <+8>:	std     r4,104(r1)
   0x000000005002793c <+12>:	std     r5,112(r1)
   0x0000000050027940 <+16>:	std     r8,136(r1)
   0x0000000050027944 <+20>:	bl      0x50027950 <._rtld_start+32>
   0x0000000050027948 <+24>:	.long 0x0
   0x000000005002794c <+28>:	.long 0x30e40
   0x0000000050027950 <+32>:	mflr    r3
   0x0000000050027954 <+36>:	ld      r4,0(r3)
   0x0000000050027958 <+40>:	add     r3,r4,r3
   0x000000005002795c <+44>:	ld      r4,-32768(r2)
   0x0000000050027960 <+48>:	subf    r4,r4,r2
   0x0000000050027964 <+52>:	bl      0x50027c64 <reloc_non_plt_self>
   0x0000000050027968 <+56>:	nop
   0x000000005002796c <+60>:	ld      r4,104(r1)
   0x0000000050027970 <+64>:	addi    r3,r4,-8
   0x0000000050027974 <+68>:	addi    r4,r1,128
   0x0000000050027978 <+72>:	addi    r5,r1,120
   0x000000005002797c <+76>:	bl      0x50028608 <_rtld>
   0x0000000050027980 <+80>:	nop
   0x0000000050027984 <+84>:	ld      r2,8(r3)
   0x0000000050027988 <+88>:	ld      r11,16(r3)
   0x000000005002798c <+92>:	ld      r3,0(r3)
   0x0000000050027990 <+96>:	mtlr    r3
   0x0000000050027994 <+100>:	ld      r3,96(r1)
   0x0000000050027998 <+104>:	ld      r4,104(r1)
   0x000000005002799c <+108>:	ld      r5,112(r1)
   0x00000000500279a0 <+112>:	ld      r6,120(r1)
   0x00000000500279a4 <+116>:	ld      r7,128(r1)
   0x00000000500279a8 <+120>:	ld      r8,136(r1)
   0x00000000500279ac <+124>:	blrl
=3D> 0x00000000500279b0 <+128>:	li      r0,1
   0x00000000500279b4 <+132>:	sc     =20
   0x00000000500279b8 <+136>:	nop
   0x00000000500279bc <+140>:	nop
End of assembler dump.

So setting a breakpoint at 0x00000000500279ac and
trying again:

(gdb) run
Starting program: /root/c_tests/a.out=20

Breakpoint 3, ._rtld_start () at =
/usr/src/libexec/rtld-elf/powerpc64/rtld_start.S:104
104		blrl	/* _start(argc, argv, envp, obj, cleanup, =
ps_strings) */
(gdb) info registers
r0             0x50027980	1342339456
r1             0xffffffffffffdaf0	18446744073709542128
r2             0x10028138	268599608
r3             0x1	1
r4             0xffffffffffffdbb8	18446744073709542328
r5             0xffffffffffffdbc8	18446744073709542344
r6             0x5004c000	1342488576
r7             0x50058b30	1342540592
r8             0x0	0
r9             0x0	0
r10            0x0	0
r11            0x0	0
r12            0x20000000	536870912
r13            0x50057010	1342533648
r14            0x0	0
r15            0x0	0
r16            0x0	0
r17            0x0	0
r18            0x0	0
r19            0x0	0
r20            0x0	0
r21            0x0	0
r22            0x0	0
r23            0x0	0
r24            0x0	0
r25            0x0	0
r26            0x0	0
r27            0x0	0
r28            0x0	0
r29            0x0	0
r30            0x0	0
r31            0x0	0
pc             0x500279ac	0x500279ac <._rtld_start+124>
msr            <unavailable>
cr             0x22000c00	570428416
lr             0x10010000	0x10010000
ctr            0x50043a80	1342454400
xer            0x20000000	536870912
(gdb) stepi
0x0000000010010000 in ?? ()

and that is effectively at ._start .

NOTE: There is no ._start name in the disassembly
listed by objdump.

By contrast for -fuse-ld=3Dbfd building a.out objdump shows:

0000000010000438 <._start> mflr    r0
000000001000043c <._start+0x4> mfcr    r12
0000000010000440 <._start+0x8> std     r31,-8(r1)
0000000010000444 <._start+0xc> std     r0,16(r1)
0000000010000448 <._start+0x10> stw     r12,8(r1)
000000001000044c <._start+0x14> stdu    r1,-176(r1)
. . .


In gdb for ld.lld used:

Reading symbols from a.out...done.
(gdb) br *0x00000000500279ac
Breakpoint 1 at 0x500279ac
(gdb) run
Starting program: /root/c_tests/a.out=20

Breakpoint 1, ._rtld_start () at =
/usr/src/libexec/rtld-elf/powerpc64/rtld_start.S:104
104		blrl	/* _start(argc, argv, envp, obj, cleanup, =
ps_strings) */
(gdb) stepi
0x0000000010010000 in ?? ()
(gdb)=20
0x0000000010010004 in ?? ()
(gdb) display/i $pc
1: x/i $pc
=3D> 0x10010004:	mfcr    r12
(gdb) stepi
0x0000000010010008 in ?? ()
1: x/i $pc
=3D> 0x10010008:	std     r31,-8(r1)
(gdb)=20
0x000000001001000c in ?? ()
1: x/i $pc
=3D> 0x1001000c:	std     r0,16(r1)

. . .

(gdb)=20
0x00000000100100a0 in ?? ()
1: x/i $pc
=3D> 0x100100a0:	beq     0x100100ac
(gdb)=20
0x00000000100100ac in ?? ()
1: x/i $pc
=3D> 0x100100ac:	cmpldi  r8,0
(gdb)=20
0x00000000100100b0 in ?? ()
1: x/i $pc
=3D> 0x100100b0:	beq     0x100100c0
(gdb)=20
0x00000000100100c0 in ?? ()
1: x/i $pc
=3D> 0x100100c0:	addis   r3,r2,0
(gdb)=20
0x00000000100100c4 in ?? ()
1: x/i $pc
=3D> 0x100100c4:	ld      r3,32552(r3)
(gdb)=20
0x00000000100100c8 in ?? ()
1: x/i $pc
=3D> 0x100100c8:	cmpldi  r3,0
(gdb)=20
0x00000000100100cc in ?? ()
1: x/i $pc
=3D> 0x100100cc:	beq     0x100100e0
(gdb)=20
0x00000000100100d0 in ?? ()
1: x/i $pc
=3D> 0x100100d0:	mr      r3,r7
(gdb)=20
0x00000000100100d4 in ?? ()
1: x/i $pc
=3D> 0x100100d4:	bl      0x10010560

Note: Below is from plt :

Disassembly of section .plt:
0000000010010560 <.plt> std     r2,40(r1)
0000000010010564 <.plt+0x4> addis   r11,r2,0
0000000010010568 <.plt+0x8> ld      r12,32512(r11)
000000001001056c <.plt+0xc> ld      r11,0(r12) <<<<<=3D=3D=3D=3D=3D =
Fails here.
0000000010010570 <.plt+0x10> mtctr   r11
0000000010010574 <.plt+0x14> ld      r2,8(r12)
0000000010010578 <.plt+0x18> ld      r11,16(r12)
000000001001057c <.plt+0x1c> bctr

(By setting breakpoints in the 3 such .plt code blocks:
this is the first .plt code block executed and it fails.)

The .plt is different from what ld.bfd generates:
no __glink_PLTresolve or its use and the code does
not appear strictly equivalent to me.

Back to gdb based information:

(gdb) info registers
r0             0x500279b0	1342339504
r1             0xffffffffffffda40	18446744073709541952
r2             0x10028138	268599608
r3             0x50058b30	1342540592
r4             0x0	0
r5             0xffffffffffffdbc8	18446744073709542344
r6             0x5004c000	1342488576
r7             0x50058b30	1342540592
r8             0x0	0
r9             0x0	0
r10            0x0	0
r11            0x0	0
r12            0x22000c00	570428416
r13            0x50057010	1342533648
r14            0x0	0
r15            0x0	0
r16            0x0	0
r17            0x0	0
r18            0x0	0
r19            0x0	0
r20            0x0	0
r21            0x0	0
r22            0x0	0
r23            0x0	0
r24            0x0	0
r25            0x10028138	268599608
r26            0x0	0
r27            0x0	0
r28            0x1	1
r29            0xffffffffffffdbb8	18446744073709542328
r30            0xffffffffffffdbc8	18446744073709542344
r31            0xffffffffffffda40	18446744073709541952
pc             0x10010560	0x10010560
msr            <unavailable>
cr             0x42000c00	1107299328
lr             0x100100d8	0x100100d8
ctr            0x50043a80	1342454400
xer            0x20000000	536870912

(gdb)=20
0x0000000010010560 in ?? ()
1: x/i $pc
=3D> 0x10010560:	std     r2,40(r1)
(gdb)=20
0x0000000010010564 in ?? ()
1: x/i $pc
=3D> 0x10010564:	addis   r11,r2,0
(gdb)=20
0x0000000010010568 in ?? ()
1: x/i $pc
=3D> 0x10010568:	ld      r12,32512(r11)
(gdb)=20
0x000000001001056c in ?? ()
1: x/i $pc
=3D> 0x1001056c:	ld      r11,0(r12)
(gdb)=20

Program received signal SIGSEGV, Segmentation fault.
0x000000001001056c in ?? ()
1: x/i $pc
=3D> 0x1001056c:	ld      r11,0(r12)

The source code (from lib/csu/powerpc64/crt1.c ) is:

void
_start(int argc, char **argv, char **env,
    const struct Struct_Obj_Entry *obj __unused, void (*cleanup)(void),
    struct ps_strings *ps_strings)
{

        handle_argv(argc, argv, env);

        if (ps_strings !=3D (struct ps_strings *)0)
                __ps_strings =3D ps_strings;

        if (&_DYNAMIC !=3D NULL)
                atexit(cleanup);
        else
                _init_tls();

#ifdef GCRT
        atexit(_mcleanup);
        monstartup(&eprol, &etext);
#endif

        handle_static_init(argc, argv, env);
        exit(main(argc, argv, env));
}

The 3 plt code blocks are for:

atexit
_init_tls
exit

from what I can tell, possibly not in that order.

Overall: The plt handling seems to be broken.


> You can also build rtld with additional debugging by adding -DDEBUG to
> CFLAGS. In libexec/rtld-elf/Makefile there's an example command line
> for building it locally, but I've just added CFLAGS+=3D-DDEBUG to the
> Makefile in my test tree and built it along with the rest of my full
> cross build.

# svnlite diff /usr/src/libexec/rtld-elf/Makefile
Index: /usr/src/libexec/rtld-elf/Makefile
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
--- /usr/src/libexec/rtld-elf/Makefile	(revision 311950)
+++ /usr/src/libexec/rtld-elf/Makefile	(working copy)
@@ -17,6 +17,7 @@
 		malloc.c xmalloc.c debug.c libmap.c
 MAN=3D		rtld.1
 CSTD?=3D		gnu99
+CFLAGS+=3D-DDEBUG
 CFLAGS+=3D	-Wall -DFREEBSD_ELF -DIN_RTLD -ffreestanding
 CFLAGS+=3D	-I${SRCTOP}/lib/csu/common
 .if exists(${.CURDIR}/${MACHINE_ARCH})

The above did not seem to make much of a difference for the
code involved, likely because crt1.c is from
lib/csu/powerpc64/ instead of from libexec/rtld-elf/ .


=3D=3D=3D
Mark Millard
markmi at dsl-only.net




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