linux-kernel - [PATCH 8/9] x86/fpu: correctly check for kthreads

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Date:   Wed, 14 Aug 2019 11:41:30 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     linux-kernel@...r.kernel.org
Cc:     ak@...ux.intel.com, akpm@...ux-foundation.org,
        bigeasy@...utronix.de, bp@...e.de, catalin.marinas@....com,
        davem@...emloft.net, hch@....de, kan.liang@...el.com,
        mark.rutland@....com, mingo@...nel.org, peterz@...radead.org,
        riel@...riel.com, will@...nel.org
Subject: [PATCH 8/9] x86/fpu: correctly check for kthreads

Per commit:

  0cecca9d03c964ab ("x86/fpu: Eager switch PKRU state")

... switch_fpu_state() is trying to distinguish user threads from
kthreads, such that kthreads consistently use init_pkru_value. It does
do by looking at current->mm.

In general, a non-NULL current->mm doesn't imply that current is a
kthread, as kthreads can install an mm via use_mm(), and so it's
preferable to use is_kthread() to determine whether a thread is a
kthread.

For consistency, let's use is_kthread() here.

Signed-off-by: Mark Rutland <mark.rutland@....com>
Cc: Borislav Petkov <bp@...e.de>
Cc: Christoph Hellwig <hch@....de>
Cc: Ingo Molnar <mingo@...nel.org>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Rik van Riel <riel@...riel.com>
Cc: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
---
 arch/x86/include/asm/fpu/internal.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/fpu/internal.h b/arch/x86/include/asm/fpu/internal.h
index 4c95c365058a..ed3d85fa7c67 100644
--- a/arch/x86/include/asm/fpu/internal.h
+++ b/arch/x86/include/asm/fpu/internal.h
@@ -607,7 +607,7 @@ static inline void switch_fpu_finish(struct fpu *new_fpu)
 	 * PKRU state is switched eagerly because it needs to be valid before we
 	 * return to userland e.g. for a copy_to_user() operation.
 	 */
-	if (current->mm) {
+	if (!is_kthread(current)) {
 		pk = get_xsave_addr(&new_fpu->state.xsave, XFEATURE_PKRU);
 		if (pk)
 			pkru_val = pk->pkru;
-- 
2.11.0