[PATCH v2] add -fprolog-pad=N option to c-family

[AKASHI Takahiro]

On Fri, Sep 30, 2016 at 12:01:47PM +0200, Jose E. Marchesi wrote:
> 
>     In case anybody missed it, the Linux kernel side to make use
>     of this has also been finished meanwhile. Of course it can not
>     be accepted without compiler support; and this feature patch
>     is much more versatile than just Linux kernel live patching
>     on a single architecture.
> 
> How is this supposed to be exploited atomically in RISC arches such as
> sparc?  In such architectures you usually need to patch several
> instructions to load an absolute address into a register.

We had some disucssions in the context of arm64:
https://gcc.gnu.org/ml/gcc-patches/2016-04/msg01093.html

But I don't think that we reached a final consensus at that time.

Thanks,
-Takahiro AKASHI

> If a general mechanism is what is intended I would suggest to offer the
> possibility of extending the nops _before_ the function entry point,
> like in:
> 
> (a) nop   ! Load address
>     nop   ! Load address
>     nop   ! Load address
>     nop   ! Load address
>     nop   ! Jump to loaded address.
> entry:
> (b) nop   ! PC-relative jump to (a)
>     save %sp, bleh, %sp
>     ...
> 
> So after the live-patcher patches the loading of the destination address
> and the jump, it can atomically patch (b) to effectively replace the
> implementation of `entry'.
> 
> Wdyt?
>