[PATCHv3 0/4] sys_indirect system call

Ulrich Drepper
2007-11-17T05:33:17Z

wing patches provide an alternative implementation of the
sys_indirect system call which has been discussed a few times.
This no system call allows us to extend existing system call
interfaces with adding more system calls.

Davide's previous implementation is IMO far more complex than
warranted. This code here is trivial, as you can see. I've
discussed this approach with Linus last week and for a brief moment
we actually agreed on something.

We pass an additional block of data to the kernel, it is copied into
the task_struct, and then it is up to the function implementing the system
call to interpret the data. Each system call, which is meant to be
extended this way, has to be white-listed in sys_indirect. The
alternative is to filter out those system calls which absolutely cannot
be handled using sys_indirect (like clone, execve) since they require
the stack layout of an ordinary system call. This is more dangerous
since it is too easy to miss a call.

The code for x86 and x86-64 gets by without a single line of assembly
code. This is likely to be true for most/all the other archs as well.
There is architecture-dependent code, though. For x86 and x86-64 I've
also fixed up UML (although only x86-64 is tested, that's my setup).

The last patch shows the first application of the functionality. It also
shows a complication: we need the test for valid sub-syscalls in the
main implementation and in the compatibility code. And more: the actual
sources and generated binary are very different (the numbers differ).
Duplicating the information is a big problem, though. I've used some macro
tricks to avoid this. All the information about the flags and the system
calls using them is concentrated in one header. This should maintenance
bearable.

This patch to use sys_indirect is just the beginngin. More will follow,
but I want to see how these patches are received before I spend more time
on it. This code is enough to test the implementation with the following
test program. Adjust it for architectures other than x86 and x86-64.

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
#include <fcntl.h>
#include <stdint.h>
#include <stdio.h>
#include <unistd.h>
#include <netinet/in.h>
#include <sys/socket.h>
#include <sys/syscall.h>

typedef uint32_t __u32;
typedef uint64_t __u64;

union indirect_params {
struct {
int flags;
} file_flags;
};

#ifdef __x86_64__
# define __NR_indirect 286
struct indirect_registers {
__u64 rax;
__u64 rdi;
__u64 rsi;
__u64 rdx;
__u64 r10;
__u64 r8;
__u64 r9;
};
#elif defined __i386__
# define __NR_indirect 325
struct indirect_registers {
__u32 eax;
__u32 ebx;
__u32 ecx;
__u32 edx;
__u32 esi;
__u32 edi;
__u32 ebp;
};
#else
# error "need to define __NR_indirect and struct indirect_params"
#endif

#define FILL_IN(var, values...) \
var = (struct indirect_registers) { values }

int
main (void)
{
int fd = socket (AF_INET, SOCK_DGRAM, IPPROTO_IP);
int s1 = fcntl (fd, F_GETFD);
printf ("old: FD_CLOEXEC %s set\n", s1 == 0 ? "not" : "is");
close (fd);

union indirect_params i;
i.file_flags.flags = O_CLOEXEC;

struct indirect_registers r;
#ifdef __NR_socketcall
# define SOCKOP_socket 1
long args[3] = { AF_INET, SOCK_DGRAM, IPPROTO_IP };
FILL_IN (r, __NR_socketcall, SOCKOP_socket, (long) args);
#else
FILL_IN (r, __NR_socket, AF_INET, SOCK_DGRAM, IPPROTO_IP);
#endif

fd = syscall (__NR_indirect, &r, &i, sizeof (i));
int s2 = fcntl (fd, F_GETFD);
printf ("new: FD_CLOEXEC %s set\n", s2 == 0 ? "not" : "is");
close (fd);

return s1 != 0 || s2 == 0;
}
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Signed-off-by: Ulrich Drepper <drepper@redhat.com>

arch/x86/ia32/Makefile | 1
arch/x86/ia32/ia32entry.S | 2 +
arch/x86/ia32/sys_ia32.c | 37 +++++++++++++++++++++++++++++++++-
arch/x86/kernel/syscall_table_32.S | 1
include/asm-um/indirect.h | 6 +++++
include/asm-x86/ia32_unistd.h | 1
include/asm-x86/indirect.h | 5 ++++
include/asm-x86/indirect_32.h | 23 +++++++++++++++++++++
include/asm-x86/indirect_64.h | 34 +++++++++++++++++++++++++++++++
include/asm-x86/unistd_32.h | 3 +-
include/asm-x86/unistd_64.h | 2 +
include/linux/indirect.h | 39 ++++++++++++++++++++++++++++++++++++
include/linux/sched.h | 4 +++
include/linux/syscalls.h | 6 ++++-
kernel/Makefile | 4 ++-
kernel/indirect.c | 40 +++++++++++++++++++++++++++++++++++++
net/socket.c | 29 +++++++++++++++-----------
17 files changed, 221 insertions(+), 16 deletions(-)