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SubjectRe: [lkp-robot] [x86/asm] 51bad67ffb: int3:#[##]
On Wed, 16 May 2018, Alexey Dobriyan wrote:

> On Tue, May 15, 2018 at 09:25:53PM +0000, Anvin, H Peter wrote:
> > Why is that a problem?
> > Code: 00 00 00 eb e6 cc cc cc cc cc cc cc cc cc cc cc cc cc fa 8d b6 00 00 00 00 e8 5d e8 8f ff 8b 44 24 34 83 e0 03 83 f8 03 72 28 cc <cc> cc cc cc cc cc cc fa 8d b6 00 00 00 00 e8 3d e8 8f ff 89 e0
> >
> > EIP: ret_from_intr+0xd/0x14 SS:ESP: 0068:8603bfb4
> >
> > INT3 slipped through M586 => X86_ALIGNMENT_16 :-\
>
> I could make the patch x86_64 only, but!
>
> It crashed into the middle of the padding.
>
> 796ef8fc <ret_from_intr>:
> 796ef8fc: 8b 44 24 34 mov eax,DWORD PTR [esp+0x34]
> 796ef900: 83 e0 03 and eax,0x3
> 796ef903: 83 f8 03 cmp eax,0x3
> 796ef906: 72 28 jb 796ef930 <resume_kernel>
> 796ef908: cc int3
> 796ef909: cc <========> int3

EIP points to the second int3 because that's where a simple return would
return to. do_trap() does not handle int3 special.

From a quick check that seems to be the only code sequence which has:

jcc lbl

ENTRY(foo)

and ENTRY() aligns to a 16 byte boundary. So the simple fix for that is
below. We need objtool support to detect such places ...

Thanks,

tglx

8<-------------------

--- a/arch/x86/entry/entry_32.S
+++ b/arch/x86/entry/entry_32.S
@@ -338,6 +338,11 @@ END(ret_from_fork)
cmpl $USER_RPL, %eax
jb resume_kernel # not returning to v8086 or userspace

+ /*
+ * Jump over the alignment padding which is filled with int3 instructions
+ */
+ jmp resume_userspace
+
ENTRY(resume_userspace)
DISABLE_INTERRUPTS(CLBR_ANY)
TRACE_IRQS_OFF
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