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Kernel Syscalls
Contents
Note on these
Args go in their normal registers, like arg1 in R0, as usual. Syscall # goes in IP (that's intra-procedural, not instruction pointer!), a.k.a R12.
As in all ARM (i.e. also on Android) the kernel entry is accomplished by the SVC command (SWI in some debuggers and ARM dialects). On the kernel end, a low level CPU exception handler (fleh_swi) is installed as part of the ExceptionVectorsBase, and - upon issuing a SWI/SVC - control is transferred to that address. This handler can check the syscall number to distinguish between POSIX calls (non negative) and Mach traps (negative).
Unix
Usage
MOV IP, #x // number from following list into Intraprocedural, a.k.a. r12 SVC 0x80 // Formerly, SWI (software interrupt)
For example:
(gdb) disass chown 0x30d2ad54 <chown>: mov r12, #16 ; 0x10, being # of chown 0x30d2ad58 <chown+4>: svc 0x00000080
Most of these are the same as you would find in the XNU open source kernel, with ARM idiosyncrasies aside (and #372, ledger)
sysent
The system call table in XNU is known as "sysent", and is no longer a public symbol, for obvious reasons (e.g. syscall hooking). It is fairly straightforward, however, to find the sysent and its calls. While i0nic proposes a heuristic of finding the sysent based on the (still) exported kdebug symbol, this is unreliable, as the latter might change in the future (either be moved or no longer exported). A better way is to home in on the pattern of the struct sysent entries itself, i.e - as defined in bsd/sys/sysent.h:
struct sysent { /* system call table */
int16_t sy_narg; /* number of args */
int8_t sy_resv; /* reserved */
int8_t sy_flags; /* flags */
sy_call_t *sy_call; /* implementing function */
sy_munge_t *sy_arg_munge32; /* system call arguments munger for 32-bit process */
sy_munge_t *sy_arg_munge64; /* system call arguments munger for 64-bit process */
int32_t sy_return_type; /* system call return types */
uint16_t sy_arg_bytes; /* Total size of arguments in bytes for
* 32-bit system calls
*/
};
Because system calls arguments are set in stone, it is straightforward to write code to find the signature of the first few syscalls (syscall, exit, fork..), and from there calculate the other sysent entries. A program to do so reliable on iOS has, in fact, been written, and produces the following output for iOS 5.1:
List of system calls from iOS 5.1
note: Even though a syscall is present in the table, it does not in any way imply it is functional. Auditing, for example, is not enabled in iOS (no CONFIG_AUDIT in the XNU build). Most of these syscalls are the same as those of OS X, with the exception of ledger (which actually makes a comeback in OS X Mountain Lion), and 434+ (CONFIG_EMBEDDED).
$ ./fsysent ~/Documents/projects/iOS.6.0b1.iPod4.kernel mach_trap_table offset in file (for patching purposes): 3064992 (0x2ec4a0) Sysent offset in file (for patching purposes): 3076288 (0x2ef0c0) This appears to be XNU 2107.1.78 syscall 801e9d5c T exit 801d32dc T fork 801d61d4 T read 801e9d7c T write 801ea150 T open 800b12f0 T close 801cb904 T wait4 801d3f10 T 8 old creat 801e9d5c T link 800b1804 T unlink 800b1f0c T 11 old execv 801e9d5c T chdir 800b0be0 T fchdir 800b0a70 T mknod 800b13d8 T chmod 800b2a5c T chown 800b2bb8 T 17 old break 801e9d5c T getfsstat 800b080c T 19 old lseek 801e9d5c T getpid 801daa60 T 21 old mount 801e9d5c T 22 old umount 801e9d5c T setuid 801dad14 T getuid 801daae4 T geteuid 801daaf4 T ptrace 801e6924 T recvmsg 802090f4 T sendmsg 80208c3c T recvfrom 80208d20 T accept 802085f4 T getpeername 802093c0 T getsockname 80209310 T access 800b23c8 T chflags 800b2844 T fchflags 800b290c T sync 800b02a0 T kill 801de620 T 38 old stat 801e9d5c T getppid 801daa68 T 40 old lstat 801e9d5c T dup 801c9a94 T pipe 801ec3dc T getegid 801dab6c T profil 801e9d5c T 45 old ktrace 801e9d5c T sigaction 801dd73c T getgid 801dab5c T sigprocmask 801ddc80 T getlogin 801db93c T setlogin 801db9b4 T acct 801c447c T sigpending 801dde24 T sigaltstack 801de564 T ioctl 801ea514 T reboot 801e6888 T revoke 800b4320 T symlink 800b1a74 T readlink 800b2748 T execve 801d2cb0 T umask 800b42f8 T chroot 800b0cb0 T 62 old fstat 801e9d5c T 63 used internally , reserved 801e9d5c T 64 old getpagesize 801e9d5c T msync 801d6d24 T vfork 801d586c T 67 old vread 801e9d5c T 68 old vwrite 801e9d5c T 69 old sbrk 801e9d5c T 70 old sstk 801e9d5c T 71 old mmap 801e9d5c T 72 old vadvise 801e9d5c T munmap 801d6dd0 T mprotect 801d6e04 T madvise 801d6ebc T 76 old vhangup 801e9d5c T 77 old vlimit 801e9d5c T mincore 801d6f28 T getgroups 801dab7c T setgroups 801db880 T getpgrp 801daa70 T setpgid 801dac1c T setitimer 801e6370 T 84 old wait 801e9d5c T swapon 8021a638 T getitimer 801e6228 T 87 old gethostname 801e9d5c T 88 old sethostname 801e9d5c T getdtablesize 801c966c T dup2 801c9ec0 T 91 old getdopt 801e9d5c T fcntl 801ca2d8 T select 801ea7c0 T 94 old setdopt 801e9d5c T fsync 800b3154 T setpriority 801dbce8 T socket 8020809c T connect 80208614 T 99 old accept 801e9d5c T getpriority 801dbbdc T 101 old send 801e9d5c T 102 old recv 801e9d5c T 103 old sigreturn 801e9d5c T bind 80208168 T setsockopt 80209228 T listen 802082d4 T 107 old vtimes 801e9d5c T 108 old sigvec 801e9d5c T 109 old sigblock 801e9d5c T 110 old sigsetmask 801e9d5c T sigsuspend 801dde4c T 112 old sigstack 801e9d5c T 113 old recvmsg 801e9d5c T 114 old sendmsg 801e9d5c T 115 old vtrace 801e9d5c T gettimeofday 801e6038 T getrusage 801dca80 T getsockopt 8020928c T 119 old resuba 801e9d5c T readv 801ea008 T writev 801ea3a8 T settimeofday 801e6094 T fchown 800b2cc8 T fchmod 800b2b8c T 125 old recvfrom 801e9d5c T setreuid 801db060 T setregid 801db3f4 T rename 800b3344 T 129 old truncate 801e9d5c T 130 old ftruncate 801e9d5c T flock 801cce8c T mkfifo 800b16b4 T sendto 80208960 T shutdown 802091f8 T socketpair 80208804 T mkdir 800b3c38 T rmdir 800b3c78 T utimes 800b2d7c T futimes 800b2f50 T adjtime 801e6198 T 141 old getpeername 801e9d5c T gethostuuid 801ebe9c T 143 old sethostid 801e9d5c T 144 old getrlimit 801e9d5c T 145 old setrlimit 801e9d5c T 146 old killpg 801e9d5c T setsid 801dabd8 T 148 old setquota 801e9d5c T 149 old qquota 801e9d5c T 150 old getsockname 801e9d5c T getpgid 801daa78 T setprivexec 801daa48 T pread 801e9f6c T pwrite 801ea2c8 T nfssvc 801e9d5c T 156 old getdirentries 801e9d5c T statfs 800b0340 T fstatfs 800b05f8 T unmount 800afe08 T 160 old async_daemon 801e9d5c T getfh 801e9d5c T 162 old getdomainname 801e9d5c T 163 old setdomainname 801e9d5c T 164 801e9d5c T quotactl 800b033c T 166 old exportfs 801e9d5c T mount 800aefe8 T 168 old ustat 801e9d5c T csops 801d9824 T 170 old table 801d9d10 T 171 old wait3 801e9d5c T 172 old rpause 801e9d5c T waitid 801d4308 T 174 old getdents 801e9d5c T 175 old gc_control 801e9d5c T add_profil 801e9d5c T 177 801e9d5c T 178 801e9d5c T 179 801e9d5c T kdebug_trace 801c1d58 T setgid 801db1f8 T setegid 801db304 T seteuid 801daf64 T sigreturn 8021cfa8 T chud 8021bcb8 T 186 801e9d5c T fdatasync 800b31cc T stat 800b24a4 T fstat 801cbb98 T lstat 800b25f0 T pathconf 800b26e4 T fpathconf 801cbbf4 T 193 801e9d5c T getrlimit 801dc8c8 T setrlimit 801dc190 T getdirentries 800b3eb0 T mmap 801d6814 T 198 __syscall 801e9d5c T lseek 800b1f84 T truncate 800b2fd0 T ftruncate 800b3090 T __sysctl 801e0ccc T mlock 801d7074 T munlock 801d70cc T undelete 800b1c0c T ATsocket 801e9d5c T ATgetmsg 801e9d5c T ATputmsg 801e9d5c T ATPsndreq 801e9d5c T ATPsndrsp 801e9d5c T ATPgetreq 801e9d5c T ATPgetrsp 801e9d5c T 213 Reserved for AppleTalk 801e9d5c T 214 801e9d5c T 215 801e9d5c T mkcomplex 800b1224 T statv 801e9d5c T lstatv 801e9d5c T fstatv 801e9d5c T getattrlist 8009afe0 T setattrlist 8009b058 T getdirentriesattr 800b4408 T exchangedata 800b45c4 T 224 old checkuseraccess / fsgetpath ( which moved to 427 ) 801e9d5c T searchfs 800b4804 T delete 800b1f48 T copyfile 800b31e8 T fgetattrlist 80098408 T fsetattrlist 8009b760 T poll 801eaf24 T watchevent 801eb84c T waitevent 801eb9f0 T modwatch 801ebb60 T getxattr 800b5478 T fgetxattr 800b55b4 T setxattr 800b56b4 T fsetxattr 800b57c0 T removexattr 800b58bc T fremovexattr 800b5984 T listxattr 800b5a44 T flistxattr 800b5b28 T fsctl 800b4cfc T initgroups 801db6fc T posix_spawn 801d1d74 T ffsctl 800b539c T 246 801e9d5c T nfsclnt 801e9d5c T fhopen 801e9d5c T 249 801e9d5c T minherit 801d6e84 T semsys 801e9d5c T msgsys 801e9d5c T shmsys 801e9d5c T semctl 801e9d5c T semget 801e9d5c T semop 801e9d5c T 257 801e9d5c T msgctl 801e9d5c T msgget 801e9d5c T msgsnd 801e9d5c T msgrcv 801e9d5c T shmat 801e9d5c T shmctl 801e9d5c T shmdt 801e9d5c T shmget 801e9d5c T shm_open 8020d2c0 T shm_unlink 8020dda0 T sem_open 8020c718 T sem_close 8020ceb0 T sem_unlink 8020cc78 T sem_wait 8020cf08 T sem_trywait 8020cfd0 T sem_post 8020d074 T sem_getvalue 8020d118 T sem_init 8020d110 T sem_destroy 8020d114 T open_extended 800b1144 T umask_extended 800b42a8 T stat_extended 800b244c T lstat_extended 800b2598 T fstat_extended 801cb97c T chmod_extended 800b294c T fchmod_extended 800b2a90 T access_extended 800b20bc T settid 801db580 T gettid 801dab04 T setsgroups 801db890 T getsgroups 801dabd0 T setwgroups 801db894 T getwgroups 801dabd4 T mkfifo_extended 800b1610 T mkdir_extended 800b3a4c T identitysvc 801e9d5c T shared_region_check_np 8021ab68 T shared_region_map_np 801e9d5c T vm_pressure_monitor 8021b2cc T psynch_rw_longrdlock 8021415c T psynch_rw_yieldwrlock 80214408 T psynch_rw_downgrade 80214410 T psynch_rw_upgrade 8021440c T psynch_mutexwait 80211374 T psynch_mutexdrop 80212338 T psynch_cvbroad 8021238c T psynch_cvsignal 80212970 T psynch_cvwait 80212df8 T psynch_rw_rdlock 80213530 T psynch_rw_wrlock 80214160 T psynch_rw_unlock 80214414 T psynch_rw_unlock2 8021470c T getsid 801daaa8 T settid_with_pid 801db620 T 312 old __pthread_cond_timedwait 80213430 T aio_fsync 801c4e60 T aio_return 801c5038 T aio_suspend 801c52c0 T aio_cancel 801c49d8 T aio_error 801c4db4 T aio_read 801c5018 T aio_write 801c54d4 T lio_listio 801c54f4 T 321 old __pthread_cond_wait 801e9d5c T iopolicysys 801dcc74 T 323 80218edc T mlockall 801d7108 T munlockall 801d710c T 326 801e9d5c T issetugid 801dad04 T __pthread_kill 801de298 T __pthread_sigmask 801de2f8 T __sigwait 801de3a8 T __disable_threadsignal 801ddf74 T __pthread_markcancel 801ddf90 T __pthread_canceled 801ddfd8 T __semwait_signal 801de178 T 335 old utrace 801e9d5c T proc_info 80216dc0 T sendfile 801e9d5c T stat64 800b24f0 T fstat64 801cbbd4 T lstat64 800b263c T stat64_extended 800b2540 T lstat64_extended 800b268c T fstat64_extended 801cbbb8 T getdirentries64 800b4268 T statfs64 800b0660 T fstatfs64 800b07a8 T getfsstat64 800b09b8 T __pthread_chdir 800b0ca8 T __pthread_fchdir 800b0bd8 T audit 801c0a18 T auditon 801c0a1c T 352 801e9d5c T getauid 801c0a20 T setauid 801c0a24 T getaudit 801e9d5c T setaudit 801e9d5c T getaudit_addr 801c0a28 T setaudit_addr 801c0a2c T auditctl 801c0a30 T bsdthread_create 80215260 T bsdthread_terminate 802154d8 T kqueue 801cddec T kevent 801cde6c T lchown 800b2cb0 T stack_snapshot 801c41a0 T bsdthread_register 8021553c T workq_open 80216190 T workq_kernreturn 802165f8 T kevent64 801ce104 T __old_semwait_signal 801de04c T __old_semwait_signal_nocancel 801de080 T thread_selfid 80216afc T ledger 801ebf04 T 374 801e9d5c T 375 801e9d5c T 376 801e9d5c T 377 801e9d5c T 378 801e9d5c T 379 801e9d5c T __mac_execve 801d2cd0 T __mac_syscall 8027b874 T __mac_get_file 8027b51c T __mac_set_file 8027b764 T __mac_get_link 8027b640 T __mac_set_link 8027b864 T __mac_get_proc 8027b010 T __mac_set_proc 8027b0d0 T __mac_get_fd 8027b3c8 T __mac_set_fd 8027b650 T __mac_get_pid 8027af44 T __mac_get_lcid 8027b184 T __mac_get_lctx 8027b248 T __mac_set_lctx 8027b304 T setlcid 801dba7c T getlcid 801dbb64 T read_nocancel 801e9d9c T write_nocancel 801ea170 T open_nocancel 800b1368 T close_nocancel 801cb920 T wait4_nocancel 801d3f30 T recvmsg_nocancel 80209114 T sendmsg_nocancel 80208c5c T recvfrom_nocancel 80208d40 T accept_nocancel 80208314 T msync_nocancel 801d6d3c T fcntl_nocancel 801ca2f8 T select_nocancel 801ea7dc T fsync_nocancel 800b31c4 T connect_nocancel 8020862c T select_nocancel 801ea7dc T fsync_nocancel 800b31c4 T connect_nocancel 8020862c T sigsuspend_nocancel 801ddf08 T readv_nocancel 801ea028 T writev_nocancel 801ea3c8 T sendto_nocancel 80208980 T pread_nocancel 801e9f8c T pwrite_nocancel 801ea2e8 T waitid_nocancel 801d4324 T poll_nocancel 801eaf44 T msgsnd_nocancel 801e9d5c T msgrcv_nocancel 801e9d5c T sem_wait_nocancel 8020cf24 T aio_suspend_nocancel 801c52e0 T __sigwait_nocancel 801de3e0 T __semwait_signal_nocancel 801de1ac T __mac_mount 800af00c T __mac_get_mount 8027ba6c T __mac_getfsstat 800b0830 T fsgetpath 800b5c0c T audit_session_self 801c0a0c T audit_session_join 801c0a10 T fileport_makeport 801ccf70 T fileport_makefd 801cd0f4 T audit_session_port 801c0a14 T pid_suspend 8021a950 T pid_resume 8021a9c0 T pid_hibernate 8021aa2c T pid_shutdown_sockets 8021aa84 T 437 old shared_region_slide_np 801e9d5c T shared_region_map_and_slide_np 8021b118 T
Mach
XNU also supports the Mach personality, which is distinct from that of the UNIX syscalls discussed above. Mach syscalls (on 32-bit systems like iOS) are encoded as negative numbers, which is clever, since POSIX system calls are all non-negative. For example, consider mach_msg_trap:
_mach_msg_trap:
0001a8b4 e1a0c00d mov ip, sp
0001a8b8 e92d0170 push {r4, r5, r6, r8}
0001a8bc e89c0070 ldm ip, {r4, r5, r6}
0001a8c0 e3e0c01e mvn ip, #30 @ 0x1e ; Move NEGATIVE -30 into IP (R12)
0001a8c4 ef000080 svc 0x00000080 ; issue a supervisor call
0001a8c8 e8bd0170 pop {r4, r5, r6, r8}
0001a8cc e12fff1e bx lr
..
_semaphore_signal_all_trap:
0001a8f8 e3e0c021 mvn ip, #33 @ 0x21 ; NEGATIVE -33 into IP (R12)
0001a8fc ef000080 svc 0x00000080
0001a900 e12fff1e bx lr
Mach system calls are commonly known as "traps", and are maintained in a Mach Trap table. iOS's fleh_swi handler (the kernel entry point on the other side of the "SWI" or "SVC" command) checks the system call number - if it is negative, it is flipped (2's complement), and interpreted as Mach trap instead.
mach_trap_table
In iOS 5.x, the mach_trap_table is not far from the page_size export, and right next to the trap names. kern_invalid is the equivalent of ENOSYS. All the traps are ARM Thumb. The fsysent binary can be used to find the Mach trap table, as well.
$ ./fsysent -m ~/Documents/projects/iOS.5.1.iPod4.kernel This is an ARM binary. Applying iOS kernel signatures mach_trap_table offset in file (for patching purposes): 2855556 (0x2b9284) Kern invalid detected at 0x80025f50 (+1). Ignoring those. ..This appears to be XNU 1878.11.8 // -- New in iOS 5 (and expect these in Mountain Lion) 10 _kernelrpc_mach_vm_allocate_trap 800132ac T 11 _kernelrpc_vm_allocate_trap 80013318 T 12 _kernelrpc_mach_vm_deallocate_trap 800133b4 T 13 _kernelrpc_vm_deallocate_trap 80013374 T 14 _kernelrpc_mach_vm_protect_trap 8001343c T 15 _kernelrpc_vm_protect_trap 800133f8 T 16 _kernelrpc_mach_port_allocate_trap 80013494 T 17 _kernelrpc_mach_port_destroy_trap 800134e4 T 18 _kernelrpc_mach_port_deallocate_trap 80013520 T 19 _kernelrpc_mach_port_mod_refs_trap 8001355c T 20 _kernelrpc_mach_port_move_member_trap 8001359c T 21 _kernelrpc_mach_port_insert_right_trap 800135e0 T 22 _kernelrpc_mach_port_insert_member_trap 8001363c T 23 _kernelrpc_mach_port_extract_member_trap 80013680 T // ----------------------------------------- 26 mach_reply_port 800198ac T 27 thread_self_trap 80019890 T 28 task_self_trap 80019870 T 29 host_self_trap 80017db8 T 31 mach_msg_trap 80013c1c T 32 mach_msg_overwrite_trap 80013ae4 T 33 semaphore_signal_trap 800252d4 T 34 semaphore_signal_all_trap 80025354 T 35 semaphore_signal_thread_trap 80025260 T 36 semaphore_wait_trap 800255e8 T 37 semaphore_wait_signal_trap 8002578c T 38 semaphore_timedwait_trap 800256c8 T 39 semaphore_timedwait_signal_trap 8002586c T 44 task_name_for_pid 801e0734 T 45 task_for_pid 801e0598 T 46 pid_for_task 801e054c T 48 macx_swapon 801e127c T 49 macx_swapoff 801e14cc T 51 macx_triggers 801e1260 T 52 macx_backing_store_suspend 801e11f0 T 53 macx_backing_store_recovery 801e1198 T 58 pfz_exit 80025944 T 59 swtch_pri 800259f4 T 60 swtch 80025948 T 61 thread_switch 80025bb8 T 62 clock_sleep_trap 800160f0 T 89 mach_timebase_info_trap 80015318 T 90 mach_wait_until_trap 80015934 T 91 mk_timer_create_trap 8001d238 T 92 mk_timer_destroy_trap 8001d428 T 93 mk_timer_arm_trap 8001d46c T 94 mk_timer_cancel_trap 8001d4f0 T 100 iokit_user_client_trap (probably) 80234aa0 T
