linux-kernel - [PATCH v2] consider the kind of stack incrementation while checking for alternate signal stack

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Message-ID: <20091025143758.GA6653@Chamillionaire.breakpoint.cc>
Date:	Sun, 25 Oct 2009 15:37:58 +0100
From:	Sebastian Andrzej Siewior <sebastian@...akpoint.cc>
To:	Roland McGrath <roland@...hat.com>
Cc:	Sebastian Andrzej Siewior <sebastian@...akpoint.cc>,
	Ingo Molnar <mingo@...e.hu>, Oleg Nesterov <oleg@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-kernel@...r.kernel.org,
	Haavard Skinnemoen <hskinnemoen@...el.com>
Subject: [PATCH v2] consider the kind of stack incrementation while
 checking for alternate signal stack

On PRE_INC and PRE_DEC architectures the stack is first incremented /
decremented and than the value is saved. Therefore sas_ss_sp == sp is
not on the alternative signal stack while sas_ss_sp + sas_ss_size == sp
is on the alternative signal stack.
This was reported as Debian bug #544905 on AMD64 where gcc-4.3 with -O2
created such code.

Signed-off-by: Sebastian Andrzej Siewior <sebastian@...akpoint.cc>
---
v1..v2: - remove support for POST_INC / POST_DEC archs. Every linux
          architecture is either PRE_INC or POST_INC
        - replaced  !(sp - current->sas_ss_sp)) with
          sp > current->sas_ss_sp
          with catches the equal case. Recommended by Roland.

AMD64 test case at [0]

[0] [0] http://download.breakpoint.cc/tc-sig-stack.c

 include/linux/sched.h |    9 ++++++---
 1 files changed, 6 insertions(+), 3 deletions(-)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 75e6e60..6ea5d12 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -2086,11 +2086,14 @@ static inline int is_si_special(const struct siginfo *info)
 	return info <= SEND_SIG_FORCED;
 }
 
-/* True if we are on the alternate signal stack.  */
-
+/*
+ * True if we are on the alternate signal stack.
+ * The implementation considers PRE_DEC and PRE_INC architectures.
+ */
 static inline int on_sig_stack(unsigned long sp)
 {
-	return (sp - current->sas_ss_sp < current->sas_ss_size);
+	return sp > current->sas_ss_sp &&
+		sp - current->sas_ss_sp <= current->sas_ss_size;
 }
 
 static inline int sas_ss_flags(unsigned long sp)
-- 
1.6.4.GIT

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