Keep 3 high personality bytes across exec

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.

---
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(-)

index 9080d85..2f11a0e 100644
@@ -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

index a84e9ff..362eb90 100644
@@ -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:

Yeah, ~PER_MASK will work fine. I used PER_INHERIT first because I was not sure if there were values that should not be carried forward. I will have an updated patch out shortly.

Eric

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.

---
V3
Based on 2.6.26-rc8

Changes from V2:
Use ~PER_MASK instead of PER_INHERIT
Remove PER_INHERIT
Rebase to 2.6.26-rc8

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

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

index 9080d85..5eee73e 100644
@@ -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_MASK)));	\
 } while (0)
 /*
  * An executable for which elf_read_implies_exec() returns TRUE will

1.5.6.1