This part adds support for sys_indirect on x86 and x86-64.
 b/arch/x86/ia32/ia32entry.S          |    1 +

 b/arch/x86/kernel/syscall_table_32.S |    1 +

 b/include/asm-x86/indirect.h         |    5 +++++

 b/include/asm-x86/indirect_32.h      |   27 +++++++++++++++++++++++++++

 b/include/asm-x86/indirect_64.h      |   30 ++++++++++++++++++++++++++++++

 b/include/asm-x86/unistd_32.h        |    3 ++-

 b/include/asm-x86/unistd_64.h        |    2 ++

 7 files changed, 68 insertions(+), 1 deletion(-)
--- a/arch/x86/ia32/ia32entry.S

+++ b/arch/x86/ia32/ia32entry.S

@@ -726,4 +726,5 @@ ia32_sys_call_table:

 	.quad compat_sys_timerfd

 	.quad sys_eventfd

 	.quad sys32_fallocate

+	.quad sys_indirect		/* 325  */

 ia32_syscall_end:

diff --git a/arch/x86/kernel/syscall_table_32.S b/arch/x86/kernel/syscall_table_32.S

index 8344c70..92095b2 100644

--- a/arch/x86/kernel/syscall_table_32.S

+++ b/arch/x86/kernel/syscall_table_32.S

@@ -324,3 +324,4 @@ ENTRY(sys_call_table)

 	.long sys_timerfd

 	.long sys_eventfd

 	.long sys_fallocate

+	.long sys_indirect		/* 325 */

diff --git a/include/asm-x86/unistd_32.h b/include/asm-x86/unistd_32.h

index 9b15545..8ee0b20 100644

--- a/include/asm-x86/unistd_32.h

+++ b/include/asm-x86/unistd_32.h

@@ -330,10 +330,11 @@

 #define __NR_timerfd		322

 #define __NR_eventfd		323

 #define __NR_fallocate		324

+#define __NR_indirect		325
 #ifdef __KERNEL__
-#define NR_syscalls 325

+#define NR_syscalls 326
 #define __ARCH_WANT_IPC_PARSE_VERSION

 #define __ARCH_WANT_OLD_READDIR

diff --git a/include/asm-x86/unistd_64.h b/include/asm-x86/unistd_64.h

index 5ff4d3e..66eab33 100644

--- a/include/asm-x86/unistd_64.h

+++ b/include/asm-x86/unistd_64.h

@@ -635,6 +635,8 @@ __SYSCALL(__NR_timerfd, sys_timerfd)

 __SYSCALL(__NR_eventfd, sys_eventfd)

 #define __NR_fallocate				285

 __SYSCALL(__NR_fallocate, sys_fallocate)

+#define __NR_indirect				286

+__SYSCALL(__NR_indirect, sys_indirect)
 #ifndef __NO_STUBS

 #define __ARCH_WANT_OLD_READDIR

diff --git a/i...
[ message continues ]

[ Ingo, Thomas - see the whole series on linux-kernel ]
The thing is, not all system calls can do this.
Some system calls are magic, and don't just take the arguments in

registers: they also care about the actual stack pointer and the whole

pt_regs struct when returning to user mode. 
So this does need more infrastructure: some way of marking which system

calls cannot be executed indirectly.
The magic system calls are things like:
 - sys_iopl() - this one changes the eflags value restored on iret

 - execve/clone/vfork() - need direct access to pt_regs

 - vm86() - does magic with the stack, cares about pt_regs

 - sigreturn - magic pt_regs accesses again
and there may be others I have forgotten about.
Calling these system calls from C code will just corrupt the kernel stack,

and is a big big no-no.
		Linus

-