[RFC PATCH V2] Keep 3 high personality bytes across exec

[Eric B Munson <ebmunson@us.ibm.com>]

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Currently when a 32 bit process is exec'd on a powerpc 64 bit host the value
in the top three bytes of the personality is clobbered.  This patch adds a
check in the SET_PERSONALITY macro that will carry all the values in the top
three bytes across the exec.

These three bytes currently carry flags to disable address randomisation,
limit the address space, force zeroing of an mmapped page, etc.  Should an
application set any of these bits they will be maintained and honoured on
homogeneous environment but discarded and ignored on a heterogeneous
environment.  So if an application requires all mmapped pages to be initialised
to zero and a wrapper is used to setup the personality and exec the target,
these flags will remain set on an all 32 or all 64 bit envrionment, but they
will be lost in the exec on a mixed 32/64 bit environment.  Losing these bits
means that the same application would behave differently in different
environments.  Tested on a POWER5+ machine with 64bit kernel and a mixed
64/32 bit user space.

Signed-off-by: Eric B Munson <ebmunson@us.ibm.com>

---
V2

Changes from V1:
Updated changelog with a better description of why this change is useful

Based on 2.6.26-rc6

 include/asm-powerpc/elf.h   |    3 ++-
 include/linux/personality.h |    6 ++++++
 2 files changed, 8 insertions(+), 1 deletions(-)

diff --git a/include/asm-powerpc/elf.h b/include/asm-powerpc/elf.h
index 9080d85..2f11a0e 100644
--- a/include/asm-powerpc/elf.h
+++ b/include/asm-powerpc/elf.h
@@ -257,7 +257,8 @@ do {								\
 	else							\
 		clear_thread_flag(TIF_ABI_PENDING);		\
 	if (personality(current->personality) != PER_LINUX32)	\
-		set_personality(PER_LINUX);			\
+		set_personality(PER_LINUX |			\
+			(current->personality & PER_INHERIT));	\
 } while (0)
 /*
  * An executable for which elf_read_implies_exec() returns TRUE will
diff --git a/include/linux/personality.h b/include/linux/personality.h
index a84e9ff..362eb90 100644
--- a/include/linux/personality.h
+++ b/include/linux/personality.h
@@ -36,6 +36,12 @@ enum {
 	ADDR_LIMIT_3GB = 	0x8000000,
 };
 
+/* Mask for the above personality values */
+#define PER_INHERIT (ADDR_NO_RANDOMIZE|FDPIC_FUNCPTRS|MMAP_PAGE_ZERO| \
+			ADDR_COMPAT_LAYOUT|READ_IMPLIES_EXEC|ADDR_LIMIT_32BIT| \
+			SHORT_INODE|WHOLE_SECONDS|STICKY_TIMEOUTS| \
+			ADDR_LIMIT_3GB)
+
 /*
  * Security-relevant compatibility flags that must be
  * cleared upon setuid or setgid exec:


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