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

[Sebastian Andrzej Siewior <sebastian@breakpoint.cc>]

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@breakpoint.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