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

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Date:   Fri, 23 Oct 2020 10:36:17 -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 05:22:59PM -0700, Sami Tolvanen wrote:
> 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:

Looks like it segfaults when calling BT_FUNC() for an instruction that
doesn't have a section (?). Applying this patch allows objtool to finish
with --backtrace:

diff --git a/tools/objtool/check.c b/tools/objtool/check.c
index c216dd4d662c..618b0c4f2890 100644
--- a/tools/objtool/check.c
+++ b/tools/objtool/check.c
@@ -2604,7 +2604,7 @@ static int validate_branch(struct objtool_file *file, struct symbol *func,
 				ret = validate_branch(file, func,
 						      insn->jump_dest, state);
 				if (ret) {
-					if (backtrace)
+					if (backtrace && insn->sec)
 						BT_FUNC("(branch)", insn);
 					return ret;
 				}

Running objtool -barfld on an allyesconfig+LTO vmlinux.o prints out the
following, ignoring the crypto warnings for now:

__switch_to_asm()+0x0: undefined stack state
  xen_hypercall_set_trap_table()+0x0: <=== (sym)
.entry.text+0xffd: sibling call from callable instruction with modified stack frame
  .entry.text+0xfcb: (branch)
  .entry.text+0xfb5: (alt)
  .entry.text+0xfb0: (alt)
  .entry.text+0xf78: (branch)
  .entry.text+0x9c: (branch)
  xen_syscall_target()+0x15: (branch)
  xen_syscall_target()+0x0: <=== (sym)
.entry.text+0x1754: unsupported instruction in callable function
  .entry.text+0x171d: (branch)
  .entry.text+0x1707: (alt)
  .entry.text+0x1701: (alt)
  xen_syscall32_target()+0x15: (branch)
  xen_syscall32_target()+0x0: <=== (sym)
.entry.text+0x1634: redundant CLD
do_suspend_lowlevel()+0x116: sibling call from callable instruction with modified stack frame
  do_suspend_lowlevel()+0x9a: (branch)
  do_suspend_lowlevel()+0x0: <=== (sym)
... [skipping crypto stack pointer alignment warnings] ...
__x86_retpoline_rdi()+0x10: return with modified stack frame
  __x86_retpoline_rdi()+0x0: (branch)
  .altinstr_replacement+0x13d: (branch)
  .text+0xaf4c7: (alt)
  .text+0xb03b0: (branch)
  .text+0xaf482: (branch)
  crc_pcl()+0x10: (branch)
  crc_pcl()+0x0: <=== (sym)
__x86_retpoline_rdi()+0x0: stack state mismatch: cfa1=7+32 cfa2=7+8
  .altinstr_replacement+0x20b: (branch)
  __x86_indirect_thunk_rdi()+0x0: (alt)
  __x86_indirect_thunk_rdi()+0x0: <=== (sym)
.head.text+0xfb: unsupported instruction in callable function
  .head.text+0x207: (branch)
  sev_es_play_dead()+0xff: (branch)
  sev_es_play_dead()+0xd2: (branch)
  sev_es_play_dead()+0xa8: (alt)
  sev_es_play_dead()+0x144: (branch)
  sev_es_play_dead()+0x10b: (branch)
  sev_es_play_dead()+0x1f: (branch)
  sev_es_play_dead()+0x0: <=== (sym)
__x86_retpoline_rdi()+0x0: stack state mismatch: cfa1=7+32 cfa2=-1+0
  .altinstr_replacement+0x107: (branch)
  .text+0x2885: (alt)
  .text+0x2860: <=== (hint)
.entry.text+0x48: stack state mismatch: cfa1=7-8 cfa2=-1+0
  .altinstr_replacement+0xffffffffffffffff: (branch)
  .entry.text+0x21: (alt)
  .entry.text+0x1c: (alt)
  .entry.text+0x10: <=== (hint)
.entry.text+0x15fd: stack state mismatch: cfa1=7-8 cfa2=-1+0
  .altinstr_replacement+0xffffffffffffffff: (branch)
  .entry.text+0x15dc: (alt)
  .entry.text+0x15d7: (alt)
  .entry.text+0x15d0: <=== (hint)
.entry.text+0x168c: stack state mismatch: cfa1=7-8 cfa2=-1+0
  .altinstr_replacement+0xffffffffffffffff: (branch)
  .entry.text+0x166b: (alt)
  .entry.text+0x1666: (alt)
  .entry.text+0x1660: <=== (hint)

Sami