linux-kernel - Re: [PATCH v6 22/25] x86/asm: annotate indirect jumps

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Date:   Wed, 21 Oct 2020 17:22:59 -0700
From:   Sami Tolvanen <samitolvanen@...gle.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Josh Poimboeuf <jpoimboe@...hat.com>, Jann Horn <jannh@...gle.com>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Will Deacon <will@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>,
        linux-arch <linux-arch@...r.kernel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        linux-kbuild <linux-kbuild@...r.kernel.org>,
        kernel list <linux-kernel@...r.kernel.org>,
        linux-pci@...r.kernel.org
Subject: Re: [PATCH v6 22/25] x86/asm: annotate indirect jumps

On Wed, Oct 21, 2020 at 1:56 AM Peter Zijlstra <peterz@...radead.org> wrote:
>
> On Tue, Oct 20, 2020 at 12:24:37PM -0700, Sami Tolvanen wrote:
> > > > Building allyesconfig with this series and LTO enabled, I still see
> > > > the following objtool warnings for vmlinux.o, grouped by source file:
> > > >
> > > > arch/x86/entry/entry_64.S:
> > > > __switch_to_asm()+0x0: undefined stack state
> > > > .entry.text+0xffd: sibling call from callable instruction with
> > > > modified stack frame
> > > > .entry.text+0x48: stack state mismatch: cfa1=7-8 cfa2=-1+0
> > >
> > > Not sure what this one's about, there's no OBJECT_FILES_NON_STANDARD?
> >
> > Correct, because with LTO, we won't have an ELF binary to process
> > until we compile everything into vmlinux.o, and at that point we can
> > no longer skip individual object files.
>
> I think what Josh was trying to say is; this file is subject to objtool
> on a normal build and does not generate warnings. So why would it
> generate warnings when subject to objtool as result of a vmlinux run
> (due to LTO or otherwise).
>
> In fact, when I build a x86_64-defconfig and then run:
>
>   $ objtool check -barf defconfig-build/vmlinux.o

Note that I'm passing also --vmlinux and --duplicate to objtool when
processing vmlinux.o, and this series has a patch to split noinstr
validation from --vmlinux, so that's skipped.

> I do not see these in particular, although I do see a lot of:
>
>   "sibling call from callable instruction with modified stack frame"
>   "falls through to next function"
>
> that did not show up in the individual objtool runs during the build.

I'm able to reproduce these warnings with gcc 9.3 + allyesconfig, with
KASAN and GCOV_KERNEL disabled, as they are not enabled in LTO builds
either. This is without the LTO series applied, so we also have the
retpoline warnings:

$ ./tools/objtool/objtool check -arfld vmlinux.o 2>&1 | grep -vE
'(sibling|instr)'
vmlinux.o: warning: objtool: wakeup_long64()+0x61: indirect jump found
in RETPOLINE build
vmlinux.o: warning: objtool: .text+0x826308a: indirect jump found in
RETPOLINE build
vmlinux.o: warning: objtool: .text+0x82630c5: indirect jump found in
RETPOLINE build
vmlinux.o: warning: objtool: .head.text+0x748: indirect jump found in
RETPOLINE build
vmlinux.o: warning: objtool:
set_bringup_idt_handler.constprop.0()+0x0: undefined stack state
vmlinux.o: warning: objtool: .entry.text+0x1634: redundant CLD
vmlinux.o: warning: objtool: camellia_cbc_dec_32way()+0xb3: stack
state mismatch: cfa1=7+520 cfa2=7+8
vmlinux.o: warning: objtool: camellia_ctr_32way()+0x1a: stack state
mismatch: cfa1=7+520 cfa2=7+8
vmlinux.o: warning: objtool: aesni_gcm_init_avx_gen2()+0x12:
unsupported stack pointer realignment
vmlinux.o: warning: objtool: aesni_gcm_enc_update_avx_gen2()+0x12:
unsupported stack pointer realignment
vmlinux.o: warning: objtool: aesni_gcm_dec_update_avx_gen2()+0x12:
unsupported stack pointer realignment
vmlinux.o: warning: objtool: aesni_gcm_finalize_avx_gen2()+0x12:
unsupported stack pointer realignment
vmlinux.o: warning: objtool: aesni_gcm_init_avx_gen4()+0x12:
unsupported stack pointer realignment
vmlinux.o: warning: objtool: aesni_gcm_enc_update_avx_gen4()+0x12:
unsupported stack pointer realignment
vmlinux.o: warning: objtool: aesni_gcm_dec_update_avx_gen4()+0x12:
unsupported stack pointer realignment
vmlinux.o: warning: objtool: aesni_gcm_finalize_avx_gen4()+0x12:
unsupported stack pointer realignment
vmlinux.o: warning: objtool: sha1_transform_avx2()+0xc: unsupported
stack pointer realignment
vmlinux.o: warning: objtool: sha1_ni_transform()+0x7: unsupported
stack pointer realignment
vmlinux.o: warning: objtool: sha256_transform_rorx()+0x13: unsupported
stack pointer realignment
vmlinux.o: warning: objtool: sha512_transform_ssse3()+0x14:
unsupported stack pointer realignment
vmlinux.o: warning: objtool: sha512_transform_avx()+0x14: unsupported
stack pointer realignment
vmlinux.o: warning: objtool: sha512_transform_rorx()+0x7: unsupported
stack pointer realignment
vmlinux.o: warning: objtool: __x86_retpoline_rdi()+0x10: return with
modified stack frame
vmlinux.o: warning: objtool: __x86_retpoline_rdi()+0x0: stack state
mismatch: cfa1=7+32 cfa2=7+8
vmlinux.o: warning: objtool: __x86_retpoline_rdi()+0x0: stack state
mismatch: cfa1=7+32 cfa2=-1+0
vmlinux.o: warning: objtool: reset_early_page_tables()+0x0: stack
state mismatch: cfa1=7+8 cfa2=-1+0
vmlinux.o: warning: objtool: .entry.text+0x48: stack state mismatch:
cfa1=7-8 cfa2=-1+0
vmlinux.o: warning: objtool: .entry.text+0x15fd: stack state mismatch:
cfa1=7-8 cfa2=-1+0
vmlinux.o: warning: objtool: .entry.text+0x168c: stack state mismatch:
cfa1=7-8 cfa2=-1+0

There are a couple of differences, like the first "undefined stack
state" warning pointing to set_bringup_idt_handler.constprop.0()
instead of __switch_to_asm(). I tried running this with --backtrace,
but objtool segfaults at the first .entry.text warning:

$ ./tools/objtool/objtool check -barfld vmlinux.o
...
vmlinux.o: warning: objtool:
set_bringup_idt_handler.constprop.0()+0x0: undefined stack state
vmlinux.o: warning: objtool:   xen_hypercall_set_trap_table()+0x0: <=== (sym)
...
vmlinux.o: warning: objtool: .entry.text+0xffd: sibling call from
callable instruction with modified stack frame
vmlinux.o: warning: objtool:   .entry.text+0xfcb: (branch)
Segmentation fault

Going back to the allyesconfig+LTO vmlinux.o, the "undefined stack
state" warning looks quite similar:

$ ./tools/objtool/objtool check -barlfd vmlinux.o
vmlinux.o: warning: objtool: __switch_to_asm()+0x0: undefined stack state
vmlinux.o: warning: objtool:   xen_hypercall_set_trap_table()+0x0: <=== (sym)
vmlinux.o: warning: objtool: .entry.text+0xffd: sibling call from
callable instruction with modified stack frame
vmlinux.o: warning: objtool:   .entry.text+0xfcb: (branch)
Segmentation fault

Sami