linux-kernel - [PATCH] arm64: do not enforce strict 16 byte alignment to stack pointer

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Message-Id: <1462985814-16146-1-git-send-email-colin.king@canonical.com>
Date:	Wed, 11 May 2016 17:56:54 +0100
From:	Colin King <colin.king@...onical.com>
To:	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will.deacon@....com>,
	AKASHI Takahiro <takahiro.akashi@...aro.org>,
	Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Janet Liu <janet.liu@...eadtrum.com>,
	Jiri Slaby <jslaby@...e.cz>,
	Jisheng Zhang <jszhang@...vell.com>,
	James Morse <james.morse@....com>,
	Mark Rutland <mark.rutland@....com>,
	linux-arm-kernel@...ts.infradead.org
Cc:	linux-kernel@...r.kernel.org
Subject: [PATCH] arm64: do not enforce strict 16 byte alignment to stack pointer

From: Colin Ian King <colin.king@...onical.com>

copy_thread should not be enforcing 16 byte aligment and returning
-EINVAL. Other architectures trap misaligned stack access with SIGBUS
so arm64 should follow this convention, so remove the strict enforcement
check.

For example, currently clone(2) fails with -EINVAL when passing
a misaligned stack and this gives little clue to what is wrong. Instead,
it is arguable that a SIGBUS on the fist access to a misaligned stack
allows one to figure out that it is a misaligned stack issue rather
than trying to figure out why an unconventional (and undocumented)
-EINVAL is being returned.

Signed-off-by: Colin Ian King <colin.king@...onical.com>
---
 arch/arm64/kernel/process.c | 3 ---
 1 file changed, 3 deletions(-)

diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c
index 5655f756..8414971 100644
--- a/arch/arm64/kernel/process.c
+++ b/arch/arm64/kernel/process.c
@@ -258,9 +258,6 @@ int copy_thread(unsigned long clone_flags, unsigned long stack_start,
 		if (stack_start) {
 			if (is_compat_thread(task_thread_info(p)))
 				childregs->compat_sp = stack_start;
-			/* 16-byte aligned stack mandatory on AArch64 */
-			else if (stack_start & 15)
-				return -EINVAL;
 			else
 				childregs->sp = stack_start;
 		}
-- 
2.8.1