linux-kernel - Re: [PATCH 0/7] arm: Add livepatch support

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Date:   Wed, 7 Dec 2016 16:19:48 +0100
From:   Petr Mladek <pmladek@...e.com>
To:     Abel Vesa <abelvesa@...ux.com>
Cc:     linux@...linux.org.uk, jpoimboe@...hat.com, jeyu@...hat.com,
        jikos@...nel.org, mbenes@...e.cz, rostedt@...dmis.org,
        mingo@...hat.com, gregkh@...uxfoundation.org,
        geert+renesas@...der.be, davem@...emloft.net,
        akpm@...ux-foundation.org, emil.l.velikov@...il.com,
        mchehab@...nel.org, linux@...ck-us.net, ard.biesheuvel@...aro.org,
        jens.wiklander@...aro.org, jean-philippe.brucker@....com,
        viro@...iv.linux.org.uk, stefano.stabellini@...citrix.com,
        chris.brandt@...esas.com, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, live-patching@...r.kernel.org
Subject: Re: [PATCH 0/7] arm: Add livepatch support

On Tue 2016-12-06 17:06:00, Abel Vesa wrote:
> This is just an idea I've been trying out for a while now. 
> 
> Just in case somebody wants to play with it, this applies to linux-arm/for-next.
> 
> Also please note that this was only tested in qemu, but I will do some testing 
> on some real hardware in the following days.
> 
> FWICT, on this arch the compiler always generates a function prologue somewhere
> between these lines:
> 
> e1a0c00d        mov     ip, sp
> e92ddff0        push    {r4-r9, sl, fp, ip, lr, pc}
> e24cb004        sub     fp, ip, #4
> e24dd064        sub     sp, sp, #100    ; 0x64 <--- local variables
> e52de004        push    {lr}            ; (str lr, [sp, #-4]!)
> ebf9c2c9        bl      80110364 <__gnu_mcount_nc>
> ....
> 
> Every function that follows this pattern (the number of registers pushed and the
> sp subtraction for the local variables being the only acceptable exception) can
> be patched with this mechanism. IIRC, only the inline functions and notrace 
> functions do not follow this pattern.

Please, where do you check that the given function follows this
pattern? I do not have experience with arm at all. But compiler
is able to do crazy optimizations these days.

I think that this was already mentioned somewhere. But please, put
this detailed explanation also to related patch/code so that it
can later be found in the git commits. It will also help to
better understand/review the particular patches.

Best Regards,
Petr