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

Sami Tolvanen
2020-11-09T23:12:00Z

On Fri, Oct 23, 2020 at 10:36 AM Sami Tolvanen <samitolvanen@google.com> wrote:
>
> 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:

OK, I spent some time looking at these warnings and the configs needed
to reproduce them without building allyesconfig:

CONFIG_XEN

__switch_to_asm()+0x0: undefined stack state
xen_hypercall_set_trap_table()+0x0: <=== (sym)

CONFIG_XEN_PV

.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

Backtrace doesn’t print out anything useful for the “redundant CLD”
error, but it occurs when validate_branch is looking at
xen_sysenter_target.

do_suspend_lowlevel()+0x116: sibling call from callable instruction
with modified stack frame
do_suspend_lowlevel()+0x9a: (branch)
do_suspend_lowlevel()+0x0: <=== (sym)

.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)

It looks like the stack state mismatch warnings can be fixed by adding
unwind hints also to entry_SYSCALL_64_after_hwframe,
entry_SYSENTER_compat_after_hwframe, and
entry_SYSCALL_compat_after_hwframe. Does that sound correct?

CONFIG_AMD_MEM_ENCRYPT

.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)

This happens because sev_es_play_dead calls start_cpu0. It always has,
but objtool hasn’t been able to follow the call when processing only
sev-es.o. Any thoughts on the preferred way to fix this one?

CONFIG_CRYPTO_CRC32C_INTEL

__x86_retpoline_rdi()+0x10: return with modified stack frame
__x86_retpoline_rdi()+0x0: (branch)
.altinstr_replacement+0x147: (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+0x265: (branch)
__x86_indirect_thunk_rdi()+0x0: (alt)
__x86_indirect_thunk_rdi()+0x0: <=== (sym)

This is different from the warnings in the rest of the arch/x86/crypto
code. Do we need some kind of a hint before the JMP_NOSPEC in crc_pcl?

CONFIG_FUNCTION_TRACER

__x86_retpoline_rdi()+0x0: stack state mismatch: cfa1=7+32 cfa2=-1+0
.altinstr_replacement+0x111: (branch)
.text+0x28a5: (alt)
.text+0x2880: <=== (hint)

This unwind hint is in return_to_handler. Removing it obviously stops
the warning and doesn’t seem to result in any other complaints from
objtool. Is this hint correct?

The remaining warnings are all “unsupported stack pointer realignment”
issues in the crypto code and can be reproduced with the following
configs:

CONFIG_CRYPTO_AES_NI_INTEL
CONFIG_CRYPTO_CAMELLIA_AESNI_AVX2_X86_64
CONFIG_CRYPTO_SHA1_SSSE3
CONFIG_CRYPTO_SHA256_SSSE3
CONFIG_CRYPTO_SHA512_SSSE3

Josh, have you had a chance to look at the crypto patches you mentioned earlier?

Sami