Re: linux-next: Tree for Aug 27 (objtool)

Masami Hiramatsu
2019-08-29T01:54:06Z

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Hi Josh,

On Wed, 28 Aug 2019 11:34:33 -0500
Josh Poimboeuf <jpoimboe@redhat.com> wrote:

> On Wed, Aug 28, 2019 at 11:13:31AM -0500, Josh Poimboeuf wrote:
> > Turns out this patch does break something:
> >
> > arch/x86/xen/enlighten_pv.o: warning: objtool: xen_cpuid()+0x25: can't find jump dest instruction at .text+0x9c
> >
> > I'll need to figure out a better way to whitelist that
> > XEN_EMULATE_PREFIX fake instruction thing. I'll probably just teach
> > the objtool decoder about it.
>
> Hi Masami,
>
> Is it possible for the kernel x86 decoder to recognize the
> XEN_EMULATE_PREFIX prefix?
>
> asm(XEN_EMULATE_PREFIX "cpuid"
> : "=a" (*ax),
> "=b" (*bx),
> "=c" (*cx),
> "=d" (*dx)
> : "0" (*ax), "2" (*cx));
>
> is disassembled to:
>
> 33: 0f 0b ud2
> 35: 78 65 js 9c <xen_store_tr+0xc>
> 37: 6e outsb %ds:(%rsi),(%dx)
> 38: 0f a2 cpuid
>
> which confuses objtool. Presumably that would confuse other users of
> the decoder as well.

Good catch! It should be problematic, since x86 decoder sanity test is
based on objtool. But I don't want to change the test code itself,
because this problem is highly depending on Xen.

> That's a highly unlikely sequence of instructions, maybe the kernel
> decoder should recognize it as a single instruction.

OK, it is better to be done in decoder (only for CONFIG_XEN_PVHVM)

BTW, could you also share what test case would you using?

And what about attached patch? (just compile checked with/without CONFIG_XEN_PVHVM)

Thank you,

--
Masami Hiramatsu <mhiramat@kernel.org>

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From 9a46833c54fd320afd3836c0e51ade82e4bc6f96 Mon Sep 17 00:00:00 2001
From: Masami Hiramatsu <mhiramat@kernel.org>
Date: Thu, 29 Aug 2019 10:01:55 +0900
Subject: [PATCH] x86: xen: insn: Decode XEN_EMULATE_PREFIX correctly

Add XEN_EMULATE_PREFIX prefix support to x86 insn decoder.
This treats a special sequence of instructions of XEN_EMULATE_PREFIX
as a prefix bytes in x86 insn decoder only if CONFIG_XEN_PVHVM=y.
Note that this prefix is treated as just a dummy code.

Reported-by: Josh Poimboeuf <jpoimboe@redhat.com>
Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
---
arch/x86/include/asm/xen/interface.h | 8 +++++--
arch/x86/lib/insn.c | 35 ++++++++++++++++++++++++++++
2 files changed, 41 insertions(+), 2 deletions(-)

diff --git a/arch/x86/include/asm/xen/interface.h b/arch/x86/include/asm/xen/interface.h
index 62ca03ef5c65..fbee520b1f07 100644
--- a/arch/x86/include/asm/xen/interface.h
+++ b/arch/x86/include/asm/xen/interface.h
@@ -27,6 +27,8 @@
#ifndef _ASM_X86_XEN_INTERFACE_H
#define _ASM_X86_XEN_INTERFACE_H

+#include <linux/stringify.h>
+
/*
* XEN_GUEST_HANDLE represents a guest pointer, when passed as a field
* in a struct in memory.
@@ -379,11 +381,13 @@ struct xen_pmu_arch {
* Prefix forces emulation of some non-trapping instructions.
* Currently only CPUID.
*/
+#define __XEN_EMULATE_PREFIX 0x0f,0x0b,0x78,0x65,0x6e
+#define __XEN_EMULATE_PREFIX_STR __stringify(__XEN_EMULATE_PREFIX)
#ifdef __ASSEMBLY__
-#define XEN_EMULATE_PREFIX .byte 0x0f,0x0b,0x78,0x65,0x6e ;
+#define XEN_EMULATE_PREFIX .byte __XEN_EMULATE_PREFIX ;
#define XEN_CPUID XEN_EMULATE_PREFIX cpuid
#else
-#define XEN_EMULATE_PREFIX ".byte 0x0f,0x0b,0x78,0x65,0x6e ; "
+#define XEN_EMULATE_PREFIX ".byte " __XEN_EMULATE_PREFIX_STR " ; "
#define XEN_CPUID XEN_EMULATE_PREFIX "cpuid"
#endif

diff --git a/arch/x86/lib/insn.c b/arch/x86/lib/insn.c
index 0b5862ba6a75..2401a6fc9509 100644
--- a/arch/x86/lib/insn.c
+++ b/arch/x86/lib/insn.c
@@ -7,6 +7,9 @@

#ifdef __KERNEL__
#include <linux/string.h>
+#include <linux/kernel.h>
+/* For special Xen prefix */
+#include <asm/xen/interface.h>
#else
#include <string.h>
#endif
@@ -58,6 +61,34 @@ void insn_init(struct insn *insn, const void *kaddr, int buf_len, int x86_64)
insn->addr_bytes = 4;
}

+#ifdef CONFIG_XEN_PVHVM
+static const insn_byte_t xen_prefix[] = { XEN_EMULATE_PREFIX };
+
+static int insn_xen_prefix(struct insn *insn, insn_byte_t b)
+{
+ struct insn_field *prefixes = &insn->prefixes;
+ int i = 0;
+
+ while (i < ARRAY_SIZE(xen_prefix) && b == xen_prefix[i])
+ b = peek_nbyte_next(insn_byte_t, insn, ++i);
+
+ if (unlikely(i == ARRAY_SIZE(xen_prefix))) {
+ memcpy(prefixes->bytes, xen_prefix, 3);
+ prefixes->bytes[3] = xen_prefix[ARRAY_SIZE(xen_prefix) - 1];
+ prefixes->nbytes = ARRAY_SIZE(xen_prefix);
+ insn->next_byte += prefixes->nbytes;
+ prefixes->got = 1;
+
+ return 1;
+ }
+
+err_out:
+ return 0;
+}
+#else
+#define insn_xen_prefix(insn,b) (0)
+#endif
+
/**
* insn_get_prefixes - scan x86 instruction prefix bytes
* @insn: &struct insn containing instruction
@@ -79,6 +110,10 @@ void insn_get_prefixes(struct insn *insn)
nb = 0;
lb = 0;
b = peek_next(insn_byte_t, insn);
+
+ if (insn_xen_prefix(insn, b))
+ return;
+
attr = inat_get_opcode_attribute(b);
while (inat_is_legacy_prefix(attr)) {
/* Skip if same prefix */
--
2.20.1