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From: Peter Zijlstra <peterz@infradead.org>
To: Julien Thierry <jthierry@redhat.com>
Cc: Alexandre Chartre <alexandre.chartre@oracle.com>,
	x86@kernel.org, linux-kernel@vger.kernel.org,
	jpoimboe@redhat.com, tglx@linutronix.de
Subject: Re: [PATCH 4/7] objtool: Add support for return trampoline call
Date: Thu, 2 Apr 2020 17:40:22 +0200	[thread overview]
Message-ID: <20200402154022.GG20730@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <a250f29d-969a-b704-6dd6-c6cc7b84f526@redhat.com>

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.


  reply	other threads:[~2020-04-02 15:40 UTC|newest]

Thread overview: 51+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-04-02  8:22 [PATCH 0/7] objtool changes to remove most ANNOTATE_NOSPEC_ALTERNATIVE Alexandre Chartre
2020-04-02  8:22 ` [PATCH 1/7] objtool: is_fentry_call() crashes if call has no destination Alexandre Chartre
2020-04-02  8:22 ` [PATCH 2/7] objtool: Allow branches within the same alternative Alexandre Chartre
2020-04-02 12:03   ` Julien Thierry
2020-04-02 12:38     ` Alexandre Chartre
2020-04-02  8:22 ` [PATCH 3/7] objtool: Add support for intra-function calls Alexandre Chartre
2020-04-02 12:53   ` Julien Thierry
2020-04-02 13:24     ` Alexandre Chartre
2020-04-02 13:38       ` Julien Thierry
2020-04-02 14:56         ` Alexandre Chartre
2020-04-02 15:04       ` Peter Zijlstra
2020-04-02 15:54         ` Josh Poimboeuf
2020-04-03  7:06           ` Alexandre Chartre
2020-04-02 15:49     ` Josh Poimboeuf
2020-04-02 17:27       ` Josh Poimboeuf
2020-04-03  8:01       ` Julien Thierry
2020-04-03 12:41         ` Peter Zijlstra
2020-04-03 12:49           ` Julien Thierry
2020-04-03 14:37             ` Peter Zijlstra
2020-04-03 14:44         ` Josh Poimboeuf
2020-04-02  8:22 ` [PATCH 4/7] objtool: Add support for return trampoline call Alexandre Chartre
2020-04-02 13:26   ` Julien Thierry
2020-04-02 14:46     ` Alexandre Chartre
2020-04-02 15:31       ` Julien Thierry
2020-04-02 15:40         ` Peter Zijlstra [this message]
2020-04-03  8:11           ` Julien Thierry
2020-04-03 15:17             ` Josh Poimboeuf
2020-04-03 15:22               ` Josh Poimboeuf
2020-04-03 15:32                 ` Josh Poimboeuf
2020-04-03 15:46               ` Peter Zijlstra
2020-04-03 15:55                 ` Josh Poimboeuf
2020-04-04 13:32                 ` Peter Zijlstra
2020-04-04 14:22                   ` Josh Poimboeuf
2020-04-04 15:51                     ` Peter Zijlstra
2020-04-06  8:19                       ` Alexandre Chartre
2020-04-06  9:31                         ` Peter Zijlstra
2020-04-06 11:03                           ` Alexandre Chartre
2020-04-06 14:16                       ` Josh Poimboeuf
2020-04-02 15:27   ` Peter Zijlstra
2020-04-03  7:19     ` Alexandre Chartre
2020-04-06 14:34     ` Alexandre Chartre
2020-04-06 14:55       ` Alexandre Chartre
2020-04-02  8:22 ` [PATCH 5/7] x86/speculation: Annotate intra-function calls Alexandre Chartre
2020-04-03 16:05   ` Josh Poimboeuf
2020-04-03 16:16     ` Josh Poimboeuf
2020-04-03 17:14       ` Alexandre Chartre
2020-04-03 17:18         ` Peter Zijlstra
2020-04-03 17:24           ` Josh Poimboeuf
2020-04-03 18:20             ` Peter Zijlstra
2020-04-02  8:22 ` [PATCH 6/7] x86/speculation: Annotate retpoline return instructions Alexandre Chartre
2020-04-02  8:22 ` [PATCH 7/7] x86/speculation: Remove most ANNOTATE_NOSPEC_ALTERNATIVE Alexandre Chartre

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