Re: [musl] Static linking is broken after creation of DT_TEXTREL segment

["Андрей Аладьев" <aladjev.andrew@gmail.com>]

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Thank you, I will definitely report this issue to gmp or provide more
information under existing one. Please see here:

if (dso == &ldso) {
  /* Only ldso's REL table needs addend saving/reuse. */
  if (rel == apply_addends_to)
    reuse_addends = 1;
  skip_relative = 1;
}

if (skip_relative && IS_RELATIVE(rel[1], dso->syms)) continue;

Musl already has "skip_relative" flag. This flag can be improved like:

# ifdef SKIP_RELLOCATION_IN_READONLY_MODE
if (readonly_mode) {
  skip_relative = 1;
}
# endif

if (IS_RELATIVE(rel[1], dso->syms)) {
  if (skip_relative) {
    continue;
  } else if (readonly_mode) {
    error("Error it is not possible to make relocations in readonly mode");
    a_crash();
  }
}

It will make musl more stronger and user friendlier.


ср, 29 янв. 2020 г. в 23:53, Rich Felker <dalias@libc.org>:

> On Wed, Jan 29, 2020 at 09:41:46PM +0300, Андрей Аладьев wrote:
> > Hello. Please use the following docker image
> > "puchuu/test_x86_64-gentoo-linux-musl". I will write here complete steps
> so
> > everyone can reproduce this issue.
> >
> > docker run -it puchuu/test_x86_64-gentoo-linux-musl bash
> > env-update && source /etc/profile
> > echo "dev-libs/gmp static-libs" > /etc/portage/package.use/gmp
> > MAKEOPTS='-j16' emerge -v dev-libs/gmp
> > cd /tmp && wget "
> >
> https://raw.githubusercontent.com/andrew-aladev/lzws/master/cmake/checks/GMP/main.c
> > "
> >
> > gcc main.c -static -lgmp -o main && ./main
> > We can see that "-static -lgmp" works perfect.
> >
> > gcc main.c /usr/lib/libgmp.a -o main && ./main
> >
> /usr/lib/gcc/x86_64-gentoo-linux-musl/9.2.0/../../../../x86_64-gentoo-linux-musl/bin/ld:
> > /usr/lib/libgmp.a(bdiv_q_1.o): warning: relocation against
> > `__gmp_binvert_limb_table' in read-only section `.text'
> >
> /usr/lib/gcc/x86_64-gentoo-linux-musl/9.2.0/../../../../x86_64-gentoo-linux-musl/bin/ld:
> > warning: creating a DT_TEXTREL in object
> > Segmentation fault (core dumped)
> > We can see that direct usage of "/usr/lib/libgmp.a" provided DT_TEXTREL
> > segment.
> >
> > MAKEOPTS='-j16' emerge -v gdb dev-vcs/git
> > CFLAGS='-O0 -g -ggdb -ggdb3' CXXFLAGS='-O0 -g -ggdb -ggdb3'
> > FEATURES='nostrip' MAKEOPTS='-j16' emerge -v musl gmp
> > git clone git://git.musl-libc.org/musl --depth=1 --single-branch -b
> > "v1.1.24"
> > gcc -O0 -g -ggdb -ggdb3 main.c /usr/lib/libgmp.a -o main && gdb -ex=run
> -d
> > musl ./main
> >
> > Program received signal SIGSEGV, Segmentation fault.
> > 0x00007fe2f8c4f231 in do_relocs (dso=0x7fe2f8c8eb40 <app>,
> > rel=0x55af0a0cd568, rel_size=456, stride=3) at ldso/dynlink.c:423
> > 423                             *reloc_addr = (size_t)base + addend;
> > (gdb) where
> > #0  0x00007fe2f8c4f231 in do_relocs (dso=0x7fe2f8c8eb40 <app>,
> > rel=0x55af0a0cd568, rel_size=456, stride=3) at ldso/dynlink.c:423
> > #1  0x00007fe2f8c51e60 in reloc_all (p=0x7fe2f8c8eb40 <app>) at
> > ldso/dynlink.c:1328
> > #2  0x00007fe2f8c53a03 in __dls3 (sp=0x7ffc82457260) at
> ldso/dynlink.c:1906
> > #3  0x00007fe2f8c52de6 in __dls2b (sp=0x7ffc82457260) at
> ldso/dynlink.c:1672
> > #4  0x00007fe2f8c52d4e in __dls2 (base=0x7fe2f8bba000
> > "\177ELF\002\001\001", sp=0x7ffc82457260) at ldso/dynlink.c:1650
> > #5  0x00007fe2f8c4e5a0 in _dlstart_c (sp=0x7ffc82457260,
> > dynv=0x7fe2f8c8be20) at ldso/dlstart.c:147
> > #6  0x00007fe2f8c4e246 in _dlstart () from /lib/ld-musl-x86_64.so.1
> > #7  0x0000000000000001 in ?? ()
> > #8  0x00007ffc82458635 in ?? ()
> > #9  0x0000000000000000 in ?? ()
> >
> > (gdb) info locals
> > base = 0x561041462000 "\177ELF\002\001\001"
> > syms = 0x5610414622c8
> > strings = 0x561041462478 ""
> > sym = 0x0
> > name = 0x5610414624f8 "free"
> > ctx = 0x7f9eea640b40 <app>
> > type = 8
> > sym_index = 0
> > def = {sym = 0x0, dso = 0x7f9eea640b40 <app>}
> > reloc_addr = 0x56104147cf79 <__gmpn_bdiv_q_1+25>
> > sym_val = 0
> > tls_val = 0
> > addend = 131264
> > skip_relative = 0
> > reuse_addends = 0
> > save_slot = 0
> >
> > (gdb) p laddr(dso, rel[0])
> > $27 = (void *) 0x56104147cf79 <__gmpn_bdiv_q_1+25>
> >
> > (gdb) p dso->loadmap
> > $28 = (struct fdpic_loadmap *) 0x0
> >
> > (gdb) p (dso->base + rel[0])
> > $29 = (unsigned char *) 0x56104147cf79 <__gmpn_bdiv_q_1+25> "\300"
> >
> > We can see that "laddr" provided pointer to "dso->base + rel[0]", than
> > switch tried to override it's value and segfault appeared. Pointer is
> > wrong, most likely readonly.
> >
> > You can replace "gcc" with "clang" and everything will be the same.
> Updated
> > binutils to most recent version 2.33.1 and rebuilt toolchain - nothing
> > changed. Pointer is still invalid and SEGV_ACCERR appears.
> >
> > So I think that bug is inside musl itself. Glibc container is the same
> > situation works fine. I see no way to create a workaround for this issue.
>
> musl only has limited support for TEXTRELs as a legacy feature, and
> only on some archs. It does not support them in PIE executables or
> other "new settings".
>
> I think what you're hitting is a somewhat-known issue in libgmp where
> it uses PIC-incompatible code for the .a library but not for the .so.
> There is no good reason for it to do this; the PIC-compatible code is
> just as efficient and should always be used.
>
> I'm not sure if there's a published patch for this issue. One solution
> is using --disable-asm or whatever the option is called to turn off
> the PIC-incompatible asm. But if you want the performance it would be
> preferable to fix it in some other way.
>
> An easy workaround if you don't need PIE is just linking with -no-pie.
>
> Rich
>

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