Re: Need help in understanding x86 syscall

Zachary Amsden
2005-08-11T19:58:27Z

[-- Attachment #1: Type: text/plain, Size: 3564 bytes --]

Steven Rostedt wrote:

>OK, I get the same on my machine.
>
>
>
>>On a machine that does not support sysenter, this will give you:
>>
>>int $0x80
>>ret
>>
>>The int $0x80 system calls are still fully supported by a sysenter
>>capable kernel, since it must run older binaries and potentially support
>>syscalls during early boot up before it is known that sysenter is supported.
>>
>>
>
>Now is the latest glibc using this. Since I put in a ud2 op in my
>sysenter_entry code, which is not triggered, as well as an objdump of
>
>
>libc.so shows a bunch of int 0x80 calls.
>
>

The NPTL version of glibc (the TLS library) uses this.

zach-dev2:~ $ ldd /bin/ls
linux-gate.so.1 => (0xffffe000)
librt.so.1 => /lib/tls/librt.so.1 (0x4002e000)
libacl.so.1 => /lib/libacl.so.1 (0x40038000)
libselinux.so.1 => /lib/libselinux.so.1 (0x4003e000)
--> libc.so.6 => /lib/tls/libc.so.6 (0x4004c000)
libpthread.so.0 => /lib/tls/libpthread.so.0 (0x40162000)
/lib/ld-linux.so.2 => /lib/ld-linux.so.2 (0x40000000)
libattr.so.1 => /lib/libattr.so.1 (0x40174000)

You'll find getpid much faster with TLS libraries (it's cached, no
longer a system call):

With TLS:
zach-dev2:Micro-bench $ time ./getpid

real 0m0.080s
user 0m0.080s
sys 0m0.000s

Without TLS:
zach-dev:Micro-bench $ time ./getpid

real 0m5.041s
user 0m2.520s
sys 0m2.520s

If you're feeling really masochistic, I've added a demonstration of how
you can call sysenter from userspace without glibc. The code verifies
that there is no way to exploit the kernel to achieve reading arbitrary
memory through a non-flat data segment. It deliberately segfaults at
the end. Let me point out this is a very wrong way to do things - you
should always use the vsyscall page, and in fact, this code actually
depends on the vsyscall page even if it is not apparent. I fake the
same frame structure that the vsyscall page would have pushed to
simulate a vsyscall entry, but the kernel will always return to the
vsyscall page, which then returns back to us. Fun stuff. If you leave
the kernel hack for ud2 in your kernel, I would expect it to blow up in
amazing fashion when running the code below.

zach-dev2:~ $ gcc sysenter.S sysenter.c -o sys
sysenter.c: In function `main':
sysenter.c:34: warning: passing arg 2 of `signal' from incompatible
pointer type
sysenter.c:49: warning: passing arg 3 of `sysenter_call_2' makes pointer
from in
teger without a cast
sysenter.c:22: warning: return type of `main' is not `int'
zach-dev2:~ $ ./sys
interrupted %ebp = 0xbaadf00d
phew
Segmentation fault (core dumped)
zach-dev2:~ $ gdb sys core
GNU gdb 6.2.1
Copyright 2004 Free Software Foundation, Inc.
GDB is free software, covered by the GNU General Public License, and you are
welcome to change it and/or distribute copies of it under certain
conditions.
Type "show copying" to see the conditions.
There is absolutely no warranty for GDB. Type "show warranty" for details.
This GDB was configured as "i586-suse-linux"...Using host libthread_db
library "
/lib/tls/libthread_db.so.1".

Core was generated by `./sys'.
Program terminated with signal 11, Segmentation fault.

warning: current_sos: Can't read pathname for load map: Input/output error

Reading symbols from /lib/tls/libc.so.6...done.
Loaded symbols for /lib/tls/libc.so.6
Reading symbols from /lib/ld-linux.so.2...done.
Loaded symbols for /lib/ld-linux.so.2
#0 0xffffe410 in ?? ()
(gdb) print $eax
$1 = -14
(gdb)

#define EFAULT 14 /* Bad address */

[-- Attachment #2: getpid.c --]
[-- Type: text/plain, Size: 195 bytes --]

int main(int argc, char *argv[]) {
int j;
for (j = 0; j < 1000000; j++) {
getpid(); getpid(); getpid(); getpid(); getpid();
getpid(); getpid(); getpid(); getpid(); getpid();
}
}

[-- Attachment #3: sysenter.S --]
[-- Type: text/plain, Size: 804 bytes --]

#include <sys/syscall.h>

.text
.global sysenter_call
.global sysenter_call_2

/* void sysenter_call(pid_t pid, int signo, short ds, void *addr) */

sysenter_call:
push %ebx
push %edi
push %ebp
push %ds
movl %esp, %edi
movl 20(%esp), %ebx /* pid */
movl 24(%esp), %ecx /* signo */
movl 28(%esp), %ds /* exploit DS */
movl 32(%esp), %ebp
movl %ebp, %esp
push $sysenter_return
push %ecx
push %edx
subl $16, %ebp
push $0xbaadf00d
movl $SYS_kill, %eax
sysenter

/* vsyscall page will ret to us here */
sysenter_return:
mov %edi, %esp
pop %ds
pop %ebp
pop %edi
pop %ebx
ret

sysenter_call_2:
push %ebx
push %ebp
movl 12(%esp), %ebx /* pid */
movl 16(%esp), %ecx /* signo */
movl 20(%esp), %ebp
movl $SYS_kill, %eax
sysenter

.data
test: .long 0

[-- Attachment #4: sysenter.c --]
[-- Type: text/plain, Size: 1384 bytes --]

#include <stdio.h>
#include <signal.h>
#include <asm/ldt.h>
#include <asm/segment.h>
#include <sys/types.h>
#include <unistd.h>
#include <sys/mman.h>
#define __KERNEL__
#include <asm/page.h>

extern void sysenter_call(pid_t pid, int signo, short ds, void *addr);
extern void sysenter_call_2(pid_t pid, int signo, void *addr);
void catch_sig(int signo, struct sigcontext ctx)
{
__asm__ __volatile__("mov %0, %%ds" : : "r" (__USER_DS));
printf("interrupted %%ebp = 0x%x\n", ctx.ebp);
if (ctx.ebp == 0xbaadf00d)
printf("phew\n");
}

void main(void)
{
struct user_desc desc;
short ds;
unsigned long addr;
unsigned *stack;
unsigned long offset;

stack = (unsigned *)mmap(0, 4096, PROT_EXEC|PROT_READ|PROT_WRITE,
MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
stack = &stack[1024];
addr = 0xf0000;
offset = __PAGE_OFFSET-(unsigned)stack+addr+16;
signal(SIGUSR1, catch_sig);
desc.entry_number = 0;
desc.base_addr = offset;
desc.limit = 0xffffff;
desc.seg_32bit = 1;
desc.contents = MODIFY_LDT_CONTENTS_DATA;
desc.read_exec_only = 0;
desc.limit_in_pages = 1;
desc.seg_not_present = 0;
desc.useable = 1;
if (modify_ldt(1, &desc, sizeof(desc)) != 0) {
perror("modify_ldt");
}
ds = 0x7; /* TI | RPL 3 */
sysenter_call(getpid(), SIGUSR1, ds, stack);
sysenter_call_2(getpid(), SIGSTOP, __PAGE_OFFSET+4096);
printf("not reached - core should show %%eax == -EFAULT\n");
}