linux-kernel - Re: [PATCH 4/7] objtool: Add support for return trampoline call

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <bc3a31dc-9d59-5756-aad3-187533f05654@redhat.com>
Date:   Fri, 3 Apr 2020 09:11:55 +0100
From:   Julien Thierry <jthierry@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Alexandre Chartre <alexandre.chartre@...cle.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org, jpoimboe@...hat.com,
        tglx@...utronix.de
Subject: Re: [PATCH 4/7] objtool: Add support for return trampoline call



On 4/2/20 4:40 PM, Peter Zijlstra wrote:
> On Thu, Apr 02, 2020 at 04:31:05PM +0100, Julien Thierry wrote:
>> My understanding is that once you find an intra-function call, either you
>> hit a return, ending the branch, so the return should undo the modification
>> the intra-function call did (whether is it a retpoline return or not).
>> Otherwise, the intra-function call branch will need to reach an end in some
>> way (e.g. hitting a CONTEXT_SWITCH instruction, calling a
>> dead_end_function).
>>
>> Am I missing something?
> 
> The thing is basically doing:
> 
> 	mov  $n, cx
> 1:	call 2f
> 2:	dec  cx
> 	jnz  1b
> 	add  8*n, sp
> 
> So it does N calls to self, then subtracts N words from the stack.
> 
> The reason being that the CPU has a return-stack-buffer for predicting
> returns, and call/ret being naturally paired, that works. The above
> is a software flush of the RSB.
> 

Ah, lovely... Maybe that's where SAVE/RESTORE unwind hints could be nice 
;) .

Otherwise, I don't really have a good suggestion for this...

-- 
Julien Thierry