Linux 5.11: Test 32bit compat with in_compat_syscall

Linux 5.11 removed the TIF_IA32 thread flag with commit:
  x86: Reclaim TIF_IA32 and TIF_X32 (8d71d2bf6efec)

The flag TIF_IA32 was being used by openafs to determine if the task was
handling a syscall request from a 32 bit process.  Building against a
Linux 5.11 kernel results in a build failure as TIF_IA32 is undefined.

The function 'in_compat_syscall' was introduced in Linux 4.6 as
the preferred method to determine if a syscall needed to handle a
compatible call (e.g. 32bit application).

To resolve the build problem, use 'in_compat_syscall' if present (Linux
4.6 and later) to determine if the syscall needs to handle a
compatibility mode call.

Add autoconf check for in_compat_syscall.

Notes about in_compat_syscall:

In Linux 4.6 'in_compat_syscall' was defined for all architectures with
a generic return of 'is_compat_task', but allows architecture specific
overriding implementations (x86 and sparc).

At 4.6 (and later), the function 'is_compat_task' is defined only for
the following architectures to return:

Arch              Returns
=======           ==============================
arm64             test_thread_flag(TIF_32BIT);
mips              test_thread_flag(TIF_32BIT_ADDR)
parisc            test_ti_thread_flag(task_thread_info(t), TIF_32BIT)
powerpc           is_32bit_task()
s390              test_thread_flag(TIF_31BIT)
sparc             test_thread_flag(TIF_32BIT)

If the Linux kernel is not built with compat mode, is_compat_task and
in_compat_syscall is set to always return 0

Linux commit that introduced in_compat_syscall:
  compat: add in_compat_syscall to ask whether we're in a compat syscall
  (5180e3e24fd3e8e7)

Change-Id: I59deebfe5d8cddaf845b15ef69e65a684a961280
Reviewed-on: https://gerrit.openafs.org/14499
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: BuildBot <buildbot@rampaginggeek.com>

diff --git a/src/afs/LINUX/osi_machdep.h b/src/afs/LINUX/osi_machdep.h
index ecc2d93..fe48781 100644 (file)
--- a/src/afs/LINUX/osi_machdep.h
+++ b/src/afs/LINUX/osi_machdep.h
@@ -166,7 +166,9 @@ static inline long copyinstr(char *from, char *to, int count, int *length) {
 static inline int
 afs_in_compat_syscall(void)
 {
-# if defined(AFS_SPARC64_LINUX26_ENV)
+# if defined(HAVE_LINUX_IN_COMPAT_SYSCALL)
+    return in_compat_syscall();
+# elif defined(AFS_SPARC64_LINUX26_ENV)
     return test_thread_flag(TIF_32BIT);
 # elif defined(AFS_SPARC64_LINUX24_ENV)
     return (current->thread.flags & SPARC_FLAG_32BIT) != 0;

diff --git a/src/cf/linux-kernel-func.m4 b/src/cf/linux-kernel-func.m4
index 4651b5b..0ca3e44 100644 (file)
--- a/src/cf/linux-kernel-func.m4
+++ b/src/cf/linux-kernel-func.m4
@@ -160,6 +160,12 @@ AC_CHECK_LINUX_FUNC([lru_cache_add_file],
                     [#include <linux/swap.h>],
                     [lru_cache_add_file(NULL);])
 
+dnl Linux 4.6 introduced in_compat_syscall as replacement for is_compat_task
+dnl for certain platforms.
+AC_CHECK_LINUX_FUNC([in_compat_syscall],
+                    [#include <linux/compat.h>],
+                    [in_compat_syscall();])
+
 dnl lru_cache_add exported in Linux 5.8
 dnl    replaces lru_cache_add_file
 AC_CHECK_LINUX_FUNC([lru_cache_add],