Re: [PATCH v6 2/4] x86/stackvalidate: Compile-time stack validation

[Andy Lutomirski]

On Tue, Jul 7, 2015 at 4:29 PM, Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote:
> On Tue, Jul 07, 2015 at 03:57:14PM -0700, Andy Lutomirski wrote:
>> On Tue, Jul 7, 2015 at 7:54 AM, Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote:
>> >
>> > It currently only supports x86_64.  I tried to make the code generic so
>> > that support for other architectures can hopefully be plugged in
>> > relatively easily.
>> >
>> > Currently with my Fedora config it's reporting over 1400 warnings, but
>> > most of them are duplicates.  The warnings affect 37 .c files and 18 .S
>> > files.  The C file warnings are generally due to inline assembly, which
>> > doesn't seem to play nice with frame pointers.
>>
>> This issue might be worth bringing up on the gcc and binutils lists.
>> If we need better toolchain support, let's ask for it.
>
> I think we found a good solution for this.  See my update at:
>
>   https://lkml.kernel.org/r/20150707223519.GA31294@xxxxxxxxxxxxxxxxx

Does that force frame pointer generation?  If so, then once we have a
real kernel unwinder, we might want a non-frame-pointer-forcing
version for better code generation.  (That can wait, of course.)


>> > +
>> > +   This is a context switch instruction like sysenter or sysret.  Such
>> > +   instructions aren't allowed in a callable function, and are most
>> > +   likely part of kernel entry code.
>> > +
>> > +   If the instruction isn't actually in a callable function, change
>> > +   ENDPROC to END.
>> > +
>> > +
>> > +6. stackvalidate: asm_file.o: func()+0x26: jump to outside file from callable function
>> > +   or
>> > +   stackvalidate: asm_file.o: func()+0xd9: jump to dynamic address from callable function
>> > +
>> > +   These are constraints imposed by stackvalidate so that it can
>> > +   properly analyze all jump targets.  Dynamic jump targets and jumps to
>> > +   code in other object files aren't allowed.
>>
>> Does this not trigger due to optimized sibling calls to different files?
>
> This is a great point.  With CONFIG_FRAME_POINTER it's not a problem,
> because it adds -fno-optimize-sibling-calls.
>
> Without it, I think stackvalidate would spit out a ton of "jump to
> outside file" warnings.
>
> I haven't yet looked at the details of how exactly sibling calls work.
> I'd assume they're disabled because they break frame pointers somehow.
> Any idea if they'd also break DWARF CFI stack traces?

They'll certainly prevent unwinding from finding the pre-optimization
caller, but the rest of unwinding should work.  I don't know why we
turn it off, though.

You might want special-case jump-out-of-translation-unit to be okay if
the stack frame is in its initial state.  That is:

func:
     jmp elsewhere

could be considered okay, as could:

func:
     push %rax
     pop %rax
     jmp elsewhere

and similar.

>
> I probably need to do some digging there.  If sibling calls don't break
> CFI stack traces and we end up needing them, stackvalidate might need to
> analyze the entire kernel image at once instead of its current per-.o
> checking.
>
> Anyway, thanks a bunch for all your insightful feedback Andy!
>

I'm just pretending to be insightful :)

--Andy
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