linux-kernel - Re: [PATCH 2/2] x86/retpoline: Fix return buffer filling

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Date:   Tue, 13 Feb 2018 12:21:18 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Andi Kleen <andi@...stfloor.org>
Cc:     tglx@...utronix.de, x86@...nel.org, linux-kernel@...r.kernel.org,
        dwmw2@...radead.org, jpoimboe@...hat.com,
        Andi Kleen <ak@...ux.intel.com>
Subject: Re: [PATCH 2/2] x86/retpoline: Fix return buffer filling

On Mon, Feb 12, 2018 at 04:04:22PM -0800, Andi Kleen wrote:
> From: Andi Kleen <ak@...ux.intel.com>
> 
> An earlier patch moved the RSB filling out of line, ending
> it with a return. This results in the return buffer filling
> only giving 15 instead of 16 usable returns because
> the return from fill_rsb already uses one up.

Or, we can get rid of the RET:

---
diff --git a/arch/x86/lib/retpoline.S b/arch/x86/lib/retpoline.S
index 480edc3a5e03..359130ceaa64 100644
--- a/arch/x86/lib/retpoline.S
+++ b/arch/x86/lib/retpoline.S
@@ -91,7 +91,8 @@ GENERATE_THUNK(r15)
 
 ENTRY(__fill_rsb)
 	STUFF_RSB RSB_FILL_LOOPS, %_ASM_SP
-	ret
+	pop %_ASM_BX
+	jmp *%_ASM_BX
 END(__fill_rsb)
 EXPORT_SYMBOL_GPL(__fill_rsb)
 
@@ -99,6 +100,7 @@ EXPORT_SYMBOL_GPL(__fill_rsb)
 
 ENTRY(__clear_rsb)
 	STUFF_RSB RSB_CLEAR_LOOPS, %_ASM_SP
-	ret
+	pop %_ASM_BX
+	jmp *%_ASM_BX
 END(__clear_rsb)
 EXPORT_SYMBOL_GPL(__clear_rsb)

-- 
Regards/Gruss,
    Boris.

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