linux-kernel - [PATCH] x86_64: Record stack pointer before task execution begins

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Message-Id: <1330273075-2949-1-git-send-email-siddhesh.poyarekar@gmail.com>
Date:	Sun, 26 Feb 2012 21:47:55 +0530
From:	Siddhesh Poyarekar <siddhesh.poyarekar@...il.com>
To:	x86@...nel.org
Cc:	KOSAKI Motohiro <kosaki.motohiro@...il.com>,
	linux-kernel@...r.kernel.org, Jamie Lokier <jamie@...reable.org>,
	Mike Frysinger <vapier@...too.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Siddhesh Poyarekar <siddhesh.poyarekar@...il.com>
Subject: [PATCH] x86_64: Record stack pointer before task execution begins

task->thread.usersp is unusable immediately after a binary is exec()'d
until it undergoes a context switch cycle. The start_thread() function
called during execve() saves the stack pointer into pt_regs and into
old_rsp, but fails to record it into task->thread.usersp.

Because of this, KSTK_ESP(task) returns an incorrect value for a
64-bit program until the task is switched out and back in since
switch_to swaps %rsp values in and out into task->thread.usersp.

Signed-off-by: Siddhesh Poyarekar <siddhesh.poyarekar@...il.com>
---
 arch/x86/kernel/process_64.c |    1 +
 1 files changed, 1 insertions(+), 0 deletions(-)

diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c
index 9b9fe4a..702a3b9 100644
--- a/arch/x86/kernel/process_64.c
+++ b/arch/x86/kernel/process_64.c
@@ -341,6 +341,7 @@ start_thread_common(struct pt_regs *regs, unsigned long new_ip,
 	loadsegment(es, _ds);
 	loadsegment(ds, _ds);
 	load_gs_index(0);
+	current->thread.usersp	= new_sp;
 	regs->ip		= new_ip;
 	regs->sp		= new_sp;
 	percpu_write(old_rsp, new_sp);
-- 
1.7.7.4

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