Re: [PATCH] tst-p_alignmod3.so: Disable GNU_RELRO segment

[Michael Hudson-Doyle <michael.hudson@canonical.com>]

On Mon, 31 Jan 2022 at 17:49, H.J. Lu <hjl.tools@gmail.com> wrote:

> On Sun, Jan 30, 2022 at 6:36 PM Michael Hudson-Doyle
> <michael.hudson@canonical.com> wrote:
> >
> >
> >
> > On Thu, 27 Jan 2022 at 10:41, H.J. Lu via Libc-alpha <
> libc-alpha@sourceware.org> wrote:
> >>
> >> tst-p_alignmod3.so has invalid p_align on LOAD segments which can't work
> >> with GNU_RELRO.  Pass -z norelro to linker to disable GNU_RELRO segment
> >> to trigger
> >
> >
> > This helps for me on most Ubuntu architectures (s390x, arm64, ppc64el,
> probably armhf although that build hasn't finished yet) but I still see a
> failure on amd64 which still seems to hit the "cannot change memory
> protections" case (full log here:
> https://launchpad.net/~mwhudson/+archive/ubuntu/devirt/+build/23110381)
> and i386 (full log here:
> https://launchpad.net/~mwhudson/+archive/ubuntu/devirt/+build/23110384)
> where the loader seems to be segfaulting:
> >
> > (gdb) r
> > Starting program:
> /build/glibc-xBQSrs/glibc-2.34.9000-596-g4556b6edae/build-tree/i386-libc/elf/ld-linux.so.2
> --library-path
> ../build-tree/i386-libc:../build-tree/i386-libc/math:../build-tree/i386-libc/elf:../build-tree/i386-libc/dlfcn:../build-tree/i386-libc/nss:../build-tree/i386-libc/nis:../build-tree/i386-libc/rt:../build-tree/i386-libc/resolv:../build-tree/i386-libc/mathvec:../build-tree/i386-libc/support:../build-tree/i386-libc/nptl
> ../build-tree/i386-libc/elf/tst-p_align3
> >
> > Program received signal SIGSEGV, Segmentation fault.
> > 0xf7fe9f08 in mprotect () at ../sysdeps/unix/syscall-template.S:120
> > 120 T_PSEUDO (SYSCALL_SYMBOL, SYSCALL_NAME, SYSCALL_NARGS)
> > (gdb) bt
> > #0  0xf7fe9f08 in mprotect () at ../sysdeps/unix/syscall-template.S:120
> > #1  0xf7fcd299 in _dl_map_segments (loader=<optimized out>,
> has_holes=true, maplength=2044, nloadcmds=<optimized out>,
> loadcmds=0xffffc650, type=3,
> >     header=<optimized out>, fd=3, l=0xf7fb7a70) at
> ./dl-map-segments.h:116
> > #2  _dl_map_object_from_fd (name=name@entry=0xf7fb9b03
> "/build/glibc-xBQSrs/glibc-2.34.9000-596-g4556b6edae/build-tree/i386-libc/elf/tst-p_alignmod3.so",
> >     origname=origname@entry=0x0, fd=<optimized out>, fbp=<optimized
> out>, realname=<optimized out>, loader=<optimized out>, l_type=<optimized
> out>, mode=<optimized out>,
> >     stack_endp=<optimized out>, nsid=<optimized out>) at dl-load.c:1258
> > #3  0xf7fcebcd in _dl_map_object (loader=0xf7ffda70, name=0xf7fb9b03
> "/build/glibc-xBQSrs/glibc-2.34.9000-596-g4556b6edae/build-tree/i386-libc/elf/tst-p_alignmod3.so",
> >     type=1, trace_mode=0, mode=0, nsid=<optimized out>) at dl-load.c:2327
> > #4  0xf7fc8378 in openaux (a=0xffffcc98) at dl-deps.c:64
> > #5  0xf7fdf8c4 in _dl_catch_exception (exception=0xffffcc8c,
> operate=0xf7fc8340 <openaux>, args=0xffffcc98) at dl-error-skeleton.c:208
> > #6  0xf7fc87f0 in _dl_map_object_deps (map=<optimized out>,
> preloads=<optimized out>, npreloads=<optimized out>, trace_mode=<optimized
> out>, open_mode=<optimized out>)
> >     at dl-deps.c:248
> > #7  0xf7fe5bcf in dl_main (phdr=<optimized out>, phnum=<optimized out>,
> user_entry=<optimized out>, auxv=<optimized out>) at rtld.c:1969
> > #8  0xf7fe1c66 in _dl_sysdep_start (start_argptr=0xffffd440,
> dl_main=0xf7fe3ba0 <dl_main>) at ../elf/dl-sysdep.c:256
> > #9  0xf7fe393f in _dl_start_final (arg=0xffffd440) at rtld.c:506
> > #10 _dl_start (arg=<optimized out>) at rtld.c:595
> > #11 0xf7fe273b in _start () from
> /build/glibc-xBQSrs/glibc-2.34.9000-596-g4556b6edae/build-tree/i386-libc/elf/ld-linux.so.2
> >
> > I think but am not sure that "loadcmds[nloadcmds - 1].mapstart" is 0 and
> "c->mapend" is 4096 so mprotect is getting called with an insane len in
> this code:
> >
> >           if (__glibc_unlikely
> >               (__mprotect ((caddr_t) (l->l_addr + c->mapend),
> >                            loadcmds[nloadcmds - 1].mapstart - c->mapend,
> >                            PROT_NONE) < 0))
> >             return DL_MAP_SEGMENTS_ERROR_MPROTECT;
> >         }
> >
> > but it's all pretty new to me. It's also possible that this is something
> about how Ubuntu's binutils is configured, I suppose.
>
> tst-p_alignmod3.so is invalid and is very sensitive to how it is built.
> But the loader shouldn't crash in any case.  Please provide i386 and
> x86-64 tst-p_alignmod3.so so that I can fix it.
>

I've uploaded them to https://people.canonical.com/~mwh/p_align3/

Cheers,
mwh


> > Cheers,
> > mwh
> >
> >>
> >> .../elf/tst-p_alignmod3.so: ELF load command address/offset not
> page-aligned
> >>
> >> instead of
> >>
> >> .../elf/tst-p_alignmod3.so: cannot change memory protections
> >> ---
> >>  elf/Makefile | 2 +-
> >>  1 file changed, 1 insertion(+), 1 deletion(-)
> >>
> >> diff --git a/elf/Makefile b/elf/Makefile
> >> index daafb5cf12..6229add1fc 100644
> >> --- a/elf/Makefile
> >> +++ b/elf/Makefile
> >> @@ -2619,7 +2619,7 @@ $(objpfx)tst-p_alignmod2.so:
> $(objpfx)tst-p_alignmod-base.so
> >>         cp $(objpfx)tst-p_alignmod-base.so $@
> >>         $(PYTHON) $(..)scripts/tst-elf-edit.py -a 1 $@
> >>
> >> -LDFLAGS-tst-p_alignmod3.so +=
> -Wl,-z,max-page-size=0x100,-z,common-page-size=0x100
> >> +LDFLAGS-tst-p_alignmod3.so +=
> -Wl,-z,max-page-size=0x100,-z,common-page-size=0x100,-z,norelro
> >>
> >>  $(objpfx)tst-p_align3: $(objpfx)tst-p_alignmod3.so
> >>  $(objpfx)tst-p_align3.out: tst-p_align3.sh $(objpfx)tst-p_align3
> >> --
> >> 2.34.1
> >>
>
>
> --
> H.J.
>