Re: [RFC][ PATCH -tip v2 0/7] kprobes: Kprobes jump optimization support

[Masami Hiramatsu <mhiramat@redhat.com>]

Andi Kleen wrote:
> Masami Hiramatsu <mhiramat@redhat.com> writes:
> 
>> Hi Andi,
>>
>> Andi Kleen wrote:
>>> Masami Hiramatsu <mhiramat@redhat.com> writes:
>>>> The gcc's crossjumping unifies equivalent code by inserting indirect
>>>> jumps which jump into other function body. It is hard to know to where
>>>> these jumps jump, so I decided to disable it when setting
>>>> CONFIG_OPTPROBES=y.
>>> That sounds quite bad. Tail call optimization is an important optimization
>>> that especially on kernel style code (lots of indirect pointers
>>> and sometimes deep call chains) is very useful.  It would be quite
>>> sad if production kernels would lose that optimization.
>> I think the crossjumping is not the tail call optimization,
>> http://gcc.gnu.org/onlinedocs/gcc-3.4.6/gccint/Passes.html
> 
> Statement didn't make sense then. The RTL crossjump pass you're referring
> AFAIK does not jump into other functions, it only optimizes jumps 
> inside a function (unless you're talking about inlines)

If so, that's a good news for me. Then just drop the disable crossjumping
patch is enough.

>>> Also tail calls in C should always jump directly to another function,
>>> so they shouldn't be particularly complex to manage.
>> Tail call jumps directly into the head of another function,
>> not the middle. Thus it is safe.
> 
> cross jumping does neither.
> 
>>>> I also decided not to optimize probes when it is in functions which
>>>> will cause exceptions, because the exception in the kernel will jump
>>>> to a fixup code and the fixup code jumps back to the middle of the
>>>> same function body.
>>> Note that not only exceptions do that, there are a few other cases
>>> where jumps in and out of out of line sections happen. You might
>>> need a more general mechanism to detect this.
>> As far as I can see (under arch/x86), Almost all fixup entries are
>> defined with ex_table entries, and others jump to the head of
>> symbols(or functions). The jumps which jump into the middle of
>> some functions are what I need to find, and, as far as I know,
>> those fixup jumps are used with exception tables. Of course,
>> I might miss some fixup codes, in that case, please let me know:-)
> 
> One case for example are out of line sections generated by gcc itself
> with the right options.

Hmm, would you know what is the actual name of that section and option?

I think another possible solution is decoding those sections and
black-listing the target functions when making vmlinux or loading
modules.

Thank you,
-- 
Masami Hiramatsu

Software Engineer
Hitachi Computer Products (America), Inc.
Software Solutions Division

e-mail: mhiramat@redhat.com