Mark,
I think the TOC (.got + .plt) has to be contiguous in memory. The on-disk
layout is not that important.
Can you check whats the difference of the in-memory TOC between lld and ld.bfd?
Thanks, Roman
On Fri, Jan 13, 2017 at 02:07:00PM -0800, Mark Millard wrote:
> Just an FYI:
>
> elfdump -a (from -r311950) does not dump .plt or .got.plt or .toc :
>
> # elfdump -a a.out | egrep "(got|toc|plt|:$)" | more
> elf header:
> program header:
> section header:
> sh_name: .rela.plt
> sh_name: .plt
> sh_name: .got
> sh_name: .got.plt
> sh_name: .toc
> interp:
> symbol table (.dynsym):
> relocation with addend (.rela.plt):
> dynamic:
> global offset table:
> symbol table (.symtab):
>
> (The "global offset table" was empty but its title was listed.)
>
> ===
> Mark Millard
> markmi at dsl-only.net
>
> On 2017-Jan-12, at 5:58 PM, Mark Millard <markmi at dsl-only.net> wrote:
>
> On 2017-Jan-12, at 11:22 AM, Roman Divacky <rdivacky at vlakno.cz> wrote:
>
> > Can you check if the TOC is correct? LLD assumes this:
> >
> > static uint64_t PPC64TocOffset = 0x8000;
> >
> > uint64_t getPPC64TocBase() {
> > // The TOC consists of sections .got, .toc, .tocbss, .plt in that order. The
> > // TOC starts where the first of these sections starts. We always create a
> > // .got when we see a relocation that uses it, so for us the start is always
> > // the .got.
> > uint64_t TocVA = In<ELF64BE>::Got->getVA();
> >
> > // Per the ppc64-elf-linux ABI, The TOC base is TOC value plus 0x8000
> > // thus permitting a full 64 Kbytes segment. Note that the glibc startup
> > // code (crt1.o) assumes that you can get from the TOC base to the
> > // start of the .toc section with only a single (signed) 16-bit relocation.
> > return TocVA + PPC64TocOffset;
> > }
>
> [I warn that I'm outside familiar territory here.]
>
> If I understand the 1st comment right the following does not look
> like a match for -fuse-dl=lld (readelf -a output):
>
> Section Headers:
> [Nr] Name Type Address Offset
> Size EntSize Flags Link Info Align
> [ 0] NULL 0000000000000000 00000000
> 0000000000000000 0000000000000000 0 0 0
> . . .
> [10] .rela.plt RELA 0000000010000420 00000420
> 0000000000000048 0000000000000018 A 5 0 8
> . . .
> [15] .plt PROGBITS 0000000010010560 00010560
> 0000000000000060 0000000000000000 AX 0 0 16
> . . .
> [20] .got PROGBITS 0000000010020138 00020138
> 0000000000000000 0000000000000000 WA 0 0 8
> . . .
> [22] .got.plt PROGBITS 0000000010030020 00030020
> 0000000000000030 0000000000000000 WA 0 0 8
> . . .
> [23] .toc PROGBITS 0000000010030050 00030050
> 0000000000000050 0000000000000000 WA 0 0 8
>
> Possibly contributing reasons:
>
> A) .got is not "first" of the 4 sections (by Address or by [Nr]).
> (.got is listed as zero size as well)
> B) There is no reference to .got.plt in the comment.
> C) .got and .toc have .got.plt and other things between
> -- and .got and .got.plt have stuff between.
> D) There is no .tocbss at all (guess: optional so possibly okay).
> E) .plt is before .got by address and by [Nr]
> (it is als not next to .got or .got.plt or .toc).
> F) There is no reference to .got.plt in the comment.
> G) In general there are other things between the sections
> making them spread over a wider address range.
>
> [I guess that .rela.plt does not matter but I showed it
> in case I'm wrong.]
>
> Another potential issue is .plt being PROGBITS instead of
> NOBITS (see below). Related is AX flags above vs. WA
> flags below being a potential issue.
>
>
> By contrast for -fuse-dl-bfd I see:
>
> Section Headers:
> [Nr] Name Type Address Offset
> Size EntSize Flags Link Info Align
> [ 0] NULL 0000000000000000 00000000
> 0000000000000000 0000000000000000 0 0 0
> . . .
> [ 8] .rela.plt RELA 0000000010000370 00000370
> 0000000000000048 0000000000000018 A 4 22 8
> . . .
> [21] .got PROGBITS 0000000010010c48 00000c48
> 0000000000000058 0000000000000008 WA 0 0 8
> [22] .plt NOBITS 0000000010010ca0 00000ca0
> 0000000000000060 0000000000000018 WA 0 0 8
>
> So no .toc or .tocbase sections.
>
> But .got and .plt are next to each other with .got first
> (by address and by [Nr]). This would fit the comments if
> .toc and .tocbss are optional --and apparently they are.
>
> So my guess is that -fuse-dl-bfd looks to be as expected,
> unlike -fuse-dl=lld .
>
>
> > Perhaps thats not true on FreeBSD? Especially the hardcoded constant seems
> > suspicious.
> > When it comes to the actual PLT entry, there's this comment in the code:
> >
> > // FIXME: What we should do, in theory, is get the offset of the function
> > // descriptor in the .opd section, and use that as the offset from %r2 (the
> > // TOC-base pointer). Instead, we have the GOT-entry offset, and that will
> > // be a pointer to the function descriptor in the .opd section. Using
> > // this scheme is simpler, but requires an extra indirection per PLT
> > dispatch.
> >
> > So I think that while it's different it might not be wrong. What might be
> > wrong
> > is the TOC entry (either it's content or it's position).
> >
> > I suspect there might be some Linux vs FreeBSD difference that prevents
> > this from working.
> >
> > Roman
>
> ===
> Mark Millard
> markmi at dsl-only.net
>
> On Thu, Jan 12, 2017 at 12:37:53AM -0800, Mark Millard wrote:
> > 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...
> >>>
> >>> Is the readelf -a reasonable now?
> >>
> >> 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
> >
> > 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
> > => 0x00000000500279b0 <+128>: li r0,1
> > 0x00000000500279b4 <+132>: sc
> > 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
> >
> > 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=bfd 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
> >
> > 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)
> > 0x0000000010010004 in ?? ()
> > (gdb) display/i $pc
> > 1: x/i $pc
> > => 0x10010004: mfcr r12
> > (gdb) stepi
> > 0x0000000010010008 in ?? ()
> > 1: x/i $pc
> > => 0x10010008: std r31,-8(r1)
> > (gdb)
> > 0x000000001001000c in ?? ()
> > 1: x/i $pc
> > => 0x1001000c: std r0,16(r1)
> >
> > . . .
> >
> > (gdb)
> > 0x00000000100100a0 in ?? ()
> > 1: x/i $pc
> > => 0x100100a0: beq 0x100100ac
> > (gdb)
> > 0x00000000100100ac in ?? ()
> > 1: x/i $pc
> > => 0x100100ac: cmpldi r8,0
> > (gdb)
> > 0x00000000100100b0 in ?? ()
> > 1: x/i $pc
> > => 0x100100b0: beq 0x100100c0
> > (gdb)
> > 0x00000000100100c0 in ?? ()
> > 1: x/i $pc
> > => 0x100100c0: addis r3,r2,0
> > (gdb)
> > 0x00000000100100c4 in ?? ()
> > 1: x/i $pc
> > => 0x100100c4: ld r3,32552(r3)
> > (gdb)
> > 0x00000000100100c8 in ?? ()
> > 1: x/i $pc
> > => 0x100100c8: cmpldi r3,0
> > (gdb)
> > 0x00000000100100cc in ?? ()
> > 1: x/i $pc
> > => 0x100100cc: beq 0x100100e0
> > (gdb)
> > 0x00000000100100d0 in ?? ()
> > 1: x/i $pc
> > => 0x100100d0: mr r3,r7
> > (gdb)
> > 0x00000000100100d4 in ?? ()
> > 1: x/i $pc
> > => 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) <<<<<===== 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)
> > 0x0000000010010560 in ?? ()
> > 1: x/i $pc
> > => 0x10010560: std r2,40(r1)
> > (gdb)
> > 0x0000000010010564 in ?? ()
> > 1: x/i $pc
> > => 0x10010564: addis r11,r2,0
> > (gdb)
> > 0x0000000010010568 in ?? ()
> > 1: x/i $pc
> > => 0x10010568: ld r12,32512(r11)
> > (gdb)
> > 0x000000001001056c in ?? ()
> > 1: x/i $pc
> > => 0x1001056c: ld r11,0(r12)
> > (gdb)
> >
> > Program received signal SIGSEGV, Segmentation fault.
> > 0x000000001001056c in ?? ()
> > 1: x/i $pc
> > => 0x1001056c: ld r11,0(r12)
> >
> > The source code (from lib/csu/powerpc64/crt1.c ) is:
> >
> > void
> > _start(int argc, char **argv, char **env,
> > const struct Struct_Obj_Entry *obj __unused, void (*cleanup)(void),
> > struct ps_strings *ps_strings)
> > {
> >
> > handle_argv(argc, argv, env);
> >
> > if (ps_strings != (struct ps_strings *)0)
> > __ps_strings = ps_strings;
> >
> > if (&_DYNAMIC != NULL)
> > atexit(cleanup);
> > else
> > _init_tls();
> >
> > #ifdef GCRT
> > atexit(_mcleanup);
> > monstartup(&eprol, &etext);
> > #endif
> >
> > handle_static_init(argc, argv, env);
> > exit(main(argc, argv, env));
> > }
> >
> > The 3 plt code blocks are for:
> >
> > atexit
> > _init_tls
> > exit
> >
> > from what I can tell, possibly not in that order.
> >
> > Overall: The plt handling seems to be broken.
> >
> >
> >> You can also build rtld with additional debugging by adding -DDEBUG to
> >> CFLAGS. In libexec/rtld-elf/Makefile there's an example command line
> >> for building it locally, but I've just added CFLAGS+=-DDEBUG to the
> >> Makefile in my test tree and built it along with the rest of my full
> >> cross build.
> >
> > # svnlite diff /usr/src/libexec/rtld-elf/Makefile
> > Index: /usr/src/libexec/rtld-elf/Makefile
> > ===================================================================
> > --- /usr/src/libexec/rtld-elf/Makefile (revision 311950)
> > +++ /usr/src/libexec/rtld-elf/Makefile (working copy)
> > @@ -17,6 +17,7 @@
> > malloc.c xmalloc.c debug.c libmap.c
> > MAN= rtld.1
> > CSTD?= gnu99
> > +CFLAGS+=-DDEBUG
> > CFLAGS+= -Wall -DFREEBSD_ELF -DIN_RTLD -ffreestanding
> > CFLAGS+= -I${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/ .
> >
> >
> > ===
> > Mark Millard
> > markmi at dsl-only.net
>
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