Re: [PATCH v2 00/20] objtool: vmlinux.o and CLANG LTO support

From: Nick Desaulniers <ndesaulniers@google.com>
To: Sami Tolvanen <samitolvanen@google.com>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>,
	Sedat Dilek <sedat.dilek@gmail.com>, X86 ML <x86@kernel.org>,
	LKML <linux-kernel@vger.kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Kees Cook <keescook@chromium.org>,
	Clang-Built-Linux ML <clang-built-linux@googlegroups.com>,
	Miroslav Benes <mbenes@suse.cz>,
	Fangrui Song <maskray@google.com>,
	Peter Collingbourne <pcc@google.com>
Subject: Re: [PATCH v2 00/20] objtool: vmlinux.o and CLANG LTO support
Date: Fri, 22 Jan 2021 17:32:43 -0800	[thread overview]
Message-ID: <CAKwvOdkcoCFKD63xtQzRdFikDk-GUUfUG5EEKiCTA3cueEZQNA@mail.gmail.com> (raw)
In-Reply-To: <CABCJKueaW1BEEBKLQzyp77VwTL+bE4x=kOLV3TWmc1s6-r3PeQ@mail.gmail.com>

On Fri, Jan 22, 2021 at 1:17 PM Sami Tolvanen <samitolvanen@google.com> wrote:
>
> Hi Josh,
>
> On Fri, Jan 22, 2021 at 7:42 AM Josh Poimboeuf <jpoimboe@redhat.com> wrote:
> >
> > On Thu, Jan 21, 2021 at 11:38:54PM +0100, Sedat Dilek wrote:
> > > On Thu, Jan 21, 2021 at 10:29 PM Josh Poimboeuf <jpoimboe@redhat.com> wrote:
> > > >
> > > > v2:
> > > > - fix commit description for why xen hypervisor page contents don't
> > > >   matter [Juergen]
> > > > - annotate indirect jumps as safe instead of converting them to
> > > >   retpolines [Andrew, Juergen]
> > > > - drop patch 1 - fake jumps no longer exist
> > > > - add acks
> > > >
> > > > Based on tip/objtool/core.
> > > >
> > > >
> > > > Add support for proper vmlinux.o validation, which will be needed for
> > > > Sami's upcoming x86 LTO set.  (And vmlinux validation is the future for
> > > > objtool anyway, for other reasons.)
> > > >
> > > > This isn't 100% done -- most notably, crypto still needs to be supported
> > > > -- but I think this gets us most of the way there.
> > > >
> > > > This can also be found at
> > > >
> > > >   git://git.kernel.org/pub/scm/linux/kernel/git/jpoimboe/linux.git objtool-vmlinux
> > > >
> > >
> > > Should this be s/objtool-vmlinux/objtool-vmlinux-v2 ?
> >
> > Indeed:
> >
> >   git://git.kernel.org/pub/scm/linux/kernel/git/jpoimboe/linux.git objtool-vmlinux-v2
>
> I tested v2 on top of the LTO tree and with allyesconfig + relevant
> crypto configs disabled, and I only see the warnings I reported
> earlier.
>
> I also tested this on top of the CFI tree and with LTO+CFI enabled, I
> can reproduce the segfault Sedat reported in the previous thread. This
> happens because find_unsuffixed_func is called with a NULL sym in
> read_relocs:
>
> Program received signal SIGSEGV, Segmentation fault.
> find_unsuffixed_func (elf=elf@entry=0x7ffff55a5010, sym=0x0,
> suffix=0x416cbf ".cfi_jt", func=func@entry=0x7fffffffd5f0) at
> elf.c:274
> 274             loc = strstr(sym->name, suffix);
> (gdb) bt
> #0  find_unsuffixed_func (elf=elf@entry=0x7ffff55a5010, sym=0x0,
> suffix=0x416cbf ".cfi_jt", func=func@entry=0x7fffffffd5f0) at
> elf.c:274
> #1  0x000000000040d8f8 in read_relocs (elf=0x7ffff55a5010) at elf.c:637
> ...
>
> In this specific case, find_func_by_offset returns NULL for
> .text..L.cfi.jumptable.43 at addend 0x8, because Clang doesn't emit
> jump table symbols for static functions:
>
> 0000000000000000 <__typeid__ZTSFjmiE_global_addr>:
>    0:   e9 00 00 00 00          jmpq   5 <__typeid__ZTSFjmiE_global_addr+0x5>
>                         1: R_X86_64_PLT32       io_serial_in-0x4
>    5:   cc                      int3
>    6:   cc                      int3
>    7:   cc                      int3
>    8:   e9 00 00 00 00          jmpq   d <__typeid__ZTSFjmiE_global_addr+0xd>
>                         9: R_X86_64_PLT32       mem32_serial_in-0x4
>    d:   cc                      int3
>    e:   cc                      int3
>    f:   cc                      int3
>
> Nick, do you remember if there were plans to change this?

Do you have a link to any previous discussion to help jog my mind; I
don't really remember this one.

Is it that `__typeid__ZTSFjmiE_global_addr` is the synthesized jump
table, and yet there is no `__typeid__ZTSFjmiE_global_addr` entry in
the symbol table?
-- 
Thanks,
~Nick Desaulniers