Re: kasan: false use-after-scope warnings with KCOV

[Mark Rutland]

On Tue, Nov 28, 2017 at 06:52:32PM +0100, Dmitry Vyukov wrote:
> On Tue, Nov 28, 2017 at 4:24 PM, Mark Rutland <mark.rutland@xxxxxxx> wrote:

> >> ... it looks suspiciously like something is setting up non-zero shadow
> >> bytes, but not zeroing them upon return.
> >
> > It looks like this is the case.
> >
> > The hack below detects leftover poison on an exception return *before*
> > the false-positive warning (example splat at the end of the email). With
> > scripts/Makefile.kasan hacked to not pass
> > -fsanitize-address-use-after-scope, I see no leftover poison.
> >
> > Unfortunately, there's not enough information left to say where exactly
> > that happened.

> ASAN stack instrumentation actually contains information about frames.
> I just never got around to using it in KASAN. But user-space ASAN
> prints the following on stack bugs:
> 
> Address 0x7ffdb1c75140 is located in stack of thread T0 at offset 64 in frame
>     #0 0x527fff  in main test.c:5
> 
>   This frame has 2 object(s):
>     [32, 40) 'p'
>     [64, 68) 'x' <== Memory access at offset 64 is inside this variable
> 
> Function prologue contains code similar to this:
> 
>   528062:       48 ba f0 7f 52 00 00    movabs $0x527ff0,%rdx
>   52806c:       48 be 9c e5 53 00 00    movabs $0x53e59c,%rsi
>   528076:       48 89 c7                mov    %rax,%rdi
>   528079:       48 83 c7 20             add    $0x20,%rdi
>   52807d:       49 89 c0                mov    %rax,%r8
>   528080:       49 83 c0 40             add    $0x40,%r8
>   528084:       48 c7 00 b3 8a b5 41    movq   $0x41b58ab3,(%rax)
>   52808b:       48 89 70 08             mov    %rsi,0x8(%rax)
>   52808f:       48 89 50 10             mov    %rdx,0x10(%rax)
> 
> Here 0x41b58ab3 is marker of frame start, and after it 0x527ff0 and
> 0x53e59c should be pointers to globals that contain function name and
> other aux information. Note that's on stack itself, not in shadow.
> If you can find any of 0x41b58ab3 in the corrupted part of stack, you
> can figure out what function has left garbage.

Thanks for the info! I'll try to give this a go, but I'm probably not
going to have the chance to investigate much this week.

I'm afraid I'm not that good at reading x86 assembly. IIUC there are
records on the stack something like:

struct record {
	u64 magic; /* 0x41b58ab3 */
	char *func_name;
	struct aux *data;
};

... is that correct?

Is there any documentation on this that I can refer to?

Thanks,
Mark.