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SubjectRe: [linus:master] BUILD REGRESSION a2e5790d841658485d642196dbb0927303d6c22f
On Wed, Feb 07, 2018 at 11:43:37AM -0800, Linus Torvalds wrote:
> On Wed, Feb 7, 2018 at 11:28 AM, Borislav Petkov <bp@alien8.de> wrote:
> > On Wed, Feb 07, 2018 at 08:14:51PM +0100, Peter Zijlstra wrote:
> >> Then someone went and wrecked it.
> >
> > Yeah, note says UD0 didn't eat a ModRM byte on old CPUs. But then that
> > changed too. Fun stuff changing insn encoding underway.
> >
> > So if we opt for adding a ModRM byte, could a 0x90 NOP work so that it
> > doesn't shit itself on those old CPUs?
>
> We could just also decide that the only thing that the modrm bytes of
> UD0 actually *affect* is how the CPU might act for a page-crossing
> instruction.
>
> Because I think that's the only semantic difference: if it's a
> page-crosser, the instruction could take a page fault before raising
> the #UD.
>
> Is there any other decode issue we might want to look out for?

_The_ problem is that new binutils cannot sanely decode any function
that has a WARN in (this very much includes perf annotate):

old:

00000000000016a0 <copy_overflow>:
16a0: 48 89 f2 mov %rsi,%rdx
16a3: 89 fe mov %edi,%esi
16a5: 48 c7 c7 00 00 00 00 mov $0x0,%rdi
16a8: R_X86_64_32S .rodata.str1.8+0x288
16ac: e8 00 00 00 00 callq 16b1 <copy_overflow+0x11>
16ad: R_X86_64_PC32 __warn_printk-0x4
16b1: 0f ff (bad)
16b3: c3 retq
16b4: 66 90 xchg %ax,%ax
16b6: 66 2e 0f 1f 84 00 00 nopw %cs:0x0(%rax,%rax,1)
16bd: 00 00 00

new:

00000000000016a0 <copy_overflow>:
16a0: 48 89 f2 mov %rsi,%rdx
16a3: 89 fe mov %edi,%esi
16a5: 48 c7 c7 00 00 00 00 mov $0x0,%rdi
16a8: R_X86_64_32S .rodata.str1.8+0x288
16ac: e8 00 00 00 00 callq 16b1 <copy_overflow+0x11>
16ad: R_X86_64_PC32 __warn_printk-0x4
16b1: 0f ff c3 ud0 %ebx,%eax
16b4: 66 90 xchg %ax,%ax
16b6: 66 2e 0f 1f 84 00 00 nopw %cs:0x0(%rax,%rax,1)
16bd: 00 00 00


I went through the register opcodes and matched it against the ModR/M
encoding, and the best option I've found so far is using 0xd6 as the
next byte.

That yields:

old:

0000000000001690 <copy_overflow>:
1690: 48 89 f2 mov %rsi,%rdx
1693: 89 fe mov %edi,%esi
1695: 48 c7 c7 00 00 00 00 mov $0x0,%rdi
1698: R_X86_64_32S .rodata.str1.8+0x270
169c: e8 00 00 00 00 callq 16a1 <copy_overflow+0x11>
169d: R_X86_64_PC32 __warn_printk-0x4
16a1: 0f ff (bad)
16a3: d6 (bad)
16a4: c3 retq
16a5: 90 nop
16a6: 66 2e 0f 1f 84 00 00 nopw %cs:0x0(%rax,%rax,1)

new:

0000000000001690 <copy_overflow>:
1690: 48 89 f2 mov %rsi,%rdx
1693: 89 fe mov %edi,%esi
1695: 48 c7 c7 00 00 00 00 mov $0x0,%rdi
1698: R_X86_64_32S .rodata.str1.8+0x270
169c: e8 00 00 00 00 callq 16a1 <copy_overflow+0x11>
169d: R_X86_64_PC32 __warn_printk-0x4
16a1: 0f ff d6 ud0 %esi,%edx
16a4: c3 retq
16a5: 90 nop
16a6: 66 2e 0f 1f 84 00 00 nopw %cs:0x0(%rax,%rax,1)
16ad: 00 00 00


And only grows a defconfig image by 91 bytes, purely for the purpose of
being able to disassemble it :/


text data bss dec hex filename
17307211 4890808 1052880 23250899 162c7d3 defconfig-build/vmlinux
17307302 4890808 1052880 23250990 162c82e defconfig-build/vmlinux


---
arch/x86/include/asm/bug.h | 9 +++++++--
1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/arch/x86/include/asm/bug.h b/arch/x86/include/asm/bug.h
index 34d99af43994..f0d5b4a1512d 100644
--- a/arch/x86/include/asm/bug.h
+++ b/arch/x86/include/asm/bug.h
@@ -12,16 +12,21 @@
* (binutils knows about "ud1" but {en,de}codes it as 2 bytes, whereas
* our kernel decoder thinks it takes a ModRM byte, which seems consistent
* with various things like the Intel SDM instruction encoding rules)
+ *
+ * And now someone went and taught binutils about UD0 as taking a ModR/M too
+ * and it generates crap when disassembling the kernel. Stick a pointless 0xD6
+ * ModR/M on, which the old binutils decodes as (bad) and the new binutils sees
+ * as a valid single byte ModR/M.
*/

-#define ASM_UD0 ".byte 0x0f, 0xff"
+#define ASM_UD0 ".byte 0x0f, 0xff, 0xd6"
#define ASM_UD1 ".byte 0x0f, 0xb9" /* + ModRM */
#define ASM_UD2 ".byte 0x0f, 0x0b"

#define INSN_UD0 0xff0f
#define INSN_UD2 0x0b0f

-#define LEN_UD0 2
+#define LEN_UD0 3

#ifdef CONFIG_GENERIC_BUG

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