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

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

On Sun, Jan 30, 2022 at 11:49 PM Michael Hudson-Doyle
<michael.hudson@canonical.com> wrote:
>
>
>
> 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/
>

A patch is posted at

https://sourceware.org/pipermail/libc-alpha/2022-January/135905.html

-- 
H.J.