Kernel Syscalls

Note on these

Args go in their normal registers, like arg1 in R0/X0, as usual. Syscall # goes in IP (that's intra-procedural, not instruction pointer!), a.k.a R12/X16.

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 on arm32 and x16 on arm64
SVC 0x80   // Formerly, SWI (software interrupt)

For example, arm32:

(gdb) disass chown
0x30d2ad54 <chown>:	mov	r12, #16	       ; 0x10, being # of chown
0x30d2ad58 <chown+4>:	svc	0x00000080

And arm64:

libsystem_kernel.dylib`chown:
    0x1866c6084 <+0>:  mov    x16, #0x10
    0x1866c6088 <+4>:  svc    #0x80

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 exported kdebug symbol, this is unreliable, as the symbol is 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 6.0b1:

List of system calls from iOS 6.0 GM

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).

A good reference on these can be found at Wiley's OS X and iOS Internals online appendix. The joker tool (shown below) can be downloaded from the same site.

$ joker -u ~/Documents/projects/iOS.6.0.iPod4.kernel 
This is an ARM binary. Applying iOS kernel signatures
Entry point is 0x80085084....This appears to be XNU 2107.2.33
Syscall names are @2a70f0
Sysent offset in file/memory (for patching purposes): 0x2ef0c0/0x802f00c0

Suppressing enosys (0x800b3429)  T = Thumb
1. exit                  801d4a74 T
2. fork                  801d7980 T
3. read                  801eb584 T
4. write                 801eb958 T
5. open                  800b13a4 T
6. close                 801ccab4 T
7. wait4                 801d56bc T
9. link                  800b18e8 T
10. unlink               800b1ff0 T
12. chdir                800b0c60 T
13. fchdir               800b0af0 T
14. mknod                800b14bc T
15. chmod                800b2b40 T
16. chown                800b2c9c T
18. getfsstat            800b088c T
20. getpid               801dc20c T
23. setuid               801dc4c0 T
24. getuid               801dc290 T
25. geteuid              801dc2a0 T
26. ptrace               801e812c T
27. recvmsg              8020a8fc T
28. sendmsg              8020a444 T
29. recvfrom             8020a528 T
30. accept               80209dfc T
31. getpeername          8020abc8 T
32. getsockname          8020ab18 T
33. access               800b24ac T
34. chflags              800b2928 T
35. fchflags             800b29f0 T
36. sync                 800b0320 T
37. kill                 801dfdcc T
39. getppid              801dc214 T
41. dup                  801cab04 T
42. pipe                 801edbe4 T
43. getegid              801dc318 T
46. sigaction            801deee8 T
47. getgid               801dc308 T
48. sigprocmask          801df42c T
49. getlogin             801dd0e8 T
50. setlogin             801dd160 T
51. acct                 801c54ec T
52. sigpending           801df5d0 T
53. sigaltstack          801dfd10 T
54. ioctl                801ebd1c T
55. reboot               801e8090 T
56. revoke               800b43f8 T
57. symlink              800b1b58 T
58. readlink             800b282c T
59. execve               801d4448 T
60. umask                800b43d0 T
61. chroot               800b0d30 T
65. msync                801d84d0 T
66. vfork                801d7018 T
73. munmap               801d857c T
74. mprotect             801d85b0 T
75. madvise              801d8668 T
78. mincore              801d86d4 T
79. getgroups            801dc328 T
80. setgroups            801dd02c T
81. getpgrp              801dc21c T
82. setpgid              801dc3c8 T
83. setitimer            801e7b78 T
85. swapon               8021be68 T
86. getitimer            801e7a30 T
89. getdtablesize        801ca6dc T
90. dup2                 801caf54 T
92. fcntl                801cb420 T
93. select               801ebfc8 T
95. fsync                800b3238 T
96. setpriority          801dd494 T
97. socket               802098a4 T
98. connect              80209e1c T
100. getpriority          801dd388 T
104. bind                 80209970 T
105. setsockopt           8020aa30 T
106. listen               80209adc T
 111. sigsuspend           801df5f8 T
116. gettimeofday         801e7840 T
117. getrusage            801de22c T
118. getsockopt           8020aa94 T
120. readv                801eb810 T
121. writev               801ebbb0 T
122. settimeofday         801e789c T
123. fchown               800b2dac T
124. fchmod               800b2c70 T
126. setreuid             801dc80c T
127. setregid             801dcba0 T
128. rename               800b3428 T
131. flock                801ce20c T
132. mkfifo               800b1798 T
133. sendto               8020a168 T
134. shutdown             8020aa00 T
135. socketpair           8020a00c T
136. mkdir                800b3d1c T
137. rmdir                800b3d5c T
138. utimes               800b2e60 T
139. futimes              800b3034 T
140. adjtime              801e79a0 T
142. gethostuuid          801ed6a4 T
147. setsid               801dc384 T
151. getpgid              801dc224 T
152. setprivexec          801dc1f4 T
153. pread                801eb774 T
154. pwrite               801ebad0 T
157. statfs               800b03c0 T
158. fstatfs              800b0678 T
159. unmount              800afe88 T
165. quotactl             800b03bc T
167. mount                800af068 T
169. csops                801dafd0 T
170. 170  old table       801db4bc T
173. waitid               801d5ab4 T
180. kdebug_trace         801c2db4 T
181. setgid               801dc9a4 T
182. setegid              801dcab0 T
183. seteuid              801dc710 T
184. sigreturn            8021e7e4 T
185. chud                 8021d4f4 T
187. fdatasync            800b32b0 T
188. stat                 800b2588 T
189. fstat                801ccfec T
190. lstat                800b26d4 T
191. pathconf             800b27c8 T
192. fpathconf            801cd048 T
194. getrlimit            801de074 T
195. setrlimit            801dd93c T
196. getdirentries        800b3f94 T
197. mmap                 801d7fc0 T
199. lseek                800b2068 T
200. truncate             800b30b4 T
201. ftruncate            800b3174 T
202. __sysctl             801e2478 T
203. mlock                801d8820 T
204. munlock              801d8878 T
205. undelete             800b1cf0 T
216. mkcomplex            800b12c4 T
220. getattrlist          8009b060 T
221. setattrlist          8009b0d8 T
222. getdirentriesattr    800b44e0 T
223. exchangedata         800b469c T
225. searchfs             800b48dc T
226. delete               800b202c T
227. copyfile             800b32cc T
228. fgetattrlist         80098488 T
229. fsetattrlist         8009b7e0 T
230. poll                 801ec72c T
231. watchevent           801ed054 T
232. waitevent            801ed1f8 T
233. modwatch             801ed368 T
234. getxattr             800b5550 T
235. fgetxattr            800b568c T
236. setxattr             800b578c T
237. fsetxattr            800b5898 T
238. removexattr          800b5994 T
239. fremovexattr         800b5a5c T
240. listxattr            800b5b1c T
241. flistxattr           800b5c00 T
242. fsctl                800b4dd4 T
243. initgroups           801dcea8 T
244. posix_spawn          801d351c T
245. ffsctl               800b5474 T
250. minherit             801d8630 T
266. shm_open             8020eb24 T
267. shm_unlink           8020f604 T
268. sem_open             8020df80 T
269. sem_close            8020e718 T
270. sem_unlink           8020e4e0 T
271. sem_wait             8020e76c T
272. sem_trywait          8020e834 T
273. sem_post             8020e8d8 T
274. sem_getvalue         8020e97c T
275. sem_init             8020e974 T
276. sem_destroy          8020e978 T
277. open_extended        800b11d8 T
278. umask_extended       800b4380 T
279. stat_extended        800b2530 T
280. lstat_extended       800b267c T
281. fstat_extended       801ccdd0 T
282. chmod_extended       800b2a30 T
283. fchmod_extended      800b2b74 T
284. access_extended      800b21a0 T
285. settid               801dcd2c T
286. gettid               801dc2b0 T
287. setsgroups           801dd03c T
288. getsgroups           801dc37c T
289. setwgroups           801dd040 T
290. getwgroups           801dc380 T
291. mkfifo_extended      800b16f4 T
292. mkdir_extended       800b3b30 T
294. shared_region_check_np 8021c3a4 T
296. vm_pressure_monitor  8021cb08 T
297. psynch_rw_longrdlock 802159ac T
298. psynch_rw_yieldwrlock 80215c60 T
299. psynch_rw_downgrade  80215c68 T
300. psynch_rw_upgrade    80215c64 T
301. psynch_mutexwait     80212bd8 T
302. psynch_mutexdrop     80213b9c T
303. psynch_cvbroad       80213bf0 T
304. psynch_cvsignal      802141c0 T
305. psynch_cvwait        80214648 T
306. psynch_rw_rdlock     80214d7c T
307. psynch_rw_wrlock     802159b0 T
308. psynch_rw_unlock     80215c6c T
309. psynch_rw_unlock2    80215f64 T
310. getsid               801dc254 T
311. settid_with_pid      801dcdcc T
312. psynch_cvclrprepost  80214c7c T
313. aio_fsync            801c5ed0 T
314. aio_return           801c60a8 T
315. aio_suspend          801c6330 T
316. aio_cancel           801c5a48 T
317. aio_error            801c5e24 T
318. aio_read             801c6088 T
319. aio_write            801c6544 T
320. lio_listio           801c6564 T
322. iopolicysys          801de420 T
323. process_policy       8021a72c T
324. mlockall             801d88b4 T
325. munlockall           801d88b8 T
327. issetugid            801dc4b0 T
328. __pthread_kill       801dfa44 T
329. __pthread_sigmask    801dfaa4 T
330. __sigwait            801dfb54 T
331. __disable_threadsignal 801df720 T
332. __pthread_markcancel 801df73c T
333. __pthread_canceled   801df784 T
334. __semwait_signal     801df924 T
336. proc_info            80218618 T
338. stat64               800b25d4 T
339. fstat64              801cd028 T
340. lstat64              800b2720 T
341. stat64_extended      800b2624 T
342. lstat64_extended     800b2770 T
343. fstat64_extended     801cd00c T
344. getdirentries64      800b4340 T
345. statfs64             800b06e0 T
346. fstatfs64            800b0828 T
347. getfsstat64          800b0a38 T
348. __pthread_chdir      800b0d28 T
349. __pthread_fchdir     800b0c58 T
350. audit                801c1a74 T
351. auditon              801c1a78 T
353. getauid              801c1a7c T
354. setauid              801c1a80 T
357. getaudit_addr        801c1a84 T
358. setaudit_addr        801c1a88 T
359. auditctl             801c1a8c T
360. bsdthread_create     80216ab8 T
361. bsdthread_terminate  80216d30 T
362. kqueue               801cf594 T
363. kevent               801cf614 T
364. lchown               800b2d94 T
365. stack_snapshot       801c520c T
366. bsdthread_register   80216d94 T
367. workq_open           802179e8 T
368. workq_kernreturn     80217e50 T
369. kevent64             801cf8ac T
370. __old_semwait_signal 801df7f8 T
371. __old_semwait_signal_nocancel 801df82c T
372. thread_selfid        80218354 T
373. ledger               801ed70c T
380. __mac_execve         801d4468 T
381. __mac_syscall        8027d0a8 T
382. __mac_get_file       8027cd50 T
383. __mac_set_file       8027cf98 T
384. __mac_get_link       8027ce74 T
385. __mac_set_link       8027d098 T
386. __mac_get_proc       8027c844 T
387. __mac_set_proc       8027c904 T
388. __mac_get_fd         8027cbfc T
389. __mac_set_fd         8027ce84 T
390. __mac_get_pid        8027c778 T
391. __mac_get_lcid       8027c9b8 T
392. __mac_get_lctx       8027ca7c T
393. __mac_set_lctx       8027cb38 T
394. setlcid              801dd228 T
395. getlcid              801dd310 T
396. read_nocancel        801eb5a4 T
397. write_nocancel       801eb978 T
398. open_nocancel        800b1434 T
399. close_nocancel       801ccad0 T
400. wait4_nocancel       801d56dc T
401. recvmsg_nocancel     8020a91c T
402. sendmsg_nocancel     8020a464 T
403. recvfrom_nocancel    8020a548 T
404. accept_nocancel      80209b1c T
405. msync_nocancel       801d84e8 T
406. fcntl_nocancel       801cb440 T
407. select_nocancel      801ebfe4 T
408. fsync_nocancel       800b32a8 T
409. connect_nocancel     80209e34 T
410. sigsuspend_nocancel  801df6b4 T
411. readv_nocancel       801eb830 T
412. writev_nocancel      801ebbd0 T
413. sendto_nocancel      8020a188 T
414. pread_nocancel       801eb794 T
415. pwrite_nocancel      801ebaf0 T
416. waitid_nocancel      801d5ad0 T
417. poll_nocancel        801ec74c T
420. sem_wait_nocancel    8020e788 T
421. aio_suspend_nocancel 801c6350 T
422. __sigwait_nocancel   801dfb8c T
423. __semwait_signal_nocancel 801df958 T
424. __mac_mount          800af08c T
425. __mac_get_mount      8027d2a0 T
426. __mac_getfsstat      800b08b0 T
427. fsgetpath            800b5ce4 T
428. audit_session_self   801c1a68 T
429. audit_session_join   801c1a6c T
430. fileport_makeport    801ce2f0 T
431. fileport_makefd      801ce494 T
432. audit_session_port   801c1a70 T
433. pid_suspend          8021c180 T
434. pid_resume           8021c1f0 T
435. pid_hibernate        8021c268 T
436. pid_shutdown_sockets 8021c2c0 T
438. shared_region_map_and_slide_np 8021c954 T
439. kas_info             8021cb50 T   ; Provides ASLR information to user space 
                                       ; (intentionally crippled in iOS, works in ML)
440. memorystatus_control 801e62a0 T   ;; Controls JetSam - supersedes old sysctl interface
441. guarded_open_np      801cead0 T  
442. guarded_close_np     801cebdc 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 joker binary can be used to find the Mach trap table, as well. The following shows iOS 6.0.b1's table:

$ ./joker -ls mach kernel.iPod4.iOS6.0b1
This is an ARM binary. Applying iOS kernel signatures
mach_trap_table offset in file (for patching purposes): 3064992 (0x2ec4a0)
Kern invalid should be 0x80027ec1. Ignoring those
..This appears to be XNU 2107.1.78
 10 _kernelrpc_mach_vm_allocate_trap         80014460 T
 12 _kernelrpc_mach_vm_deallocate_trap       800144cc T
 14 _kernelrpc_mach_vm_protect_trap          80014510 T
 16 _kernelrpc_mach_port_allocate_trap       80014564 T
 17 _kernelrpc_mach_port_destroy_trap        800145b4 T
 18 _kernelrpc_mach_port_deallocate_trap     800145f0 T
 19 _kernelrpc_mach_port_mod_refs_trap       8001462c T
 20 _kernelrpc_mach_port_move_member_trap    8001466c T
 21 _kernelrpc_mach_port_insert_right_trap   800146b0 T
 22 _kernelrpc_mach_port_insert_member_trap  80014710 T
 23 _kernelrpc_mach_port_extract_member_trap 80014754 T
 26 mach_reply_port                          8001b5b4 T
 27 thread_self_trap                         8001b598 T
 28 task_self_trap                           8001b578 T
 29 host_self_trap                           80019910 T
 31 mach_msg_trap                            80014ec0 T
 32 mach_msg_overwrite_trap                  80014d20 T
 33 semaphore_signal_trap                    80027188 T
 34 semaphore_signal_all_trap                8002720c T
 35 semaphore_signal_thread_trap             80027114 T
 36 semaphore_wait_trap                      800274b0 T
 37 semaphore_wait_signal_trap               80027658 T
 38 semaphore_timedwait_trap                 80027598 T
 39 semaphore_timedwait_signal_trap          8002773c T
 44 task_name_for_pid                        8021a838 T
 45 task_for_pid                             8021a688 T
 46 pid_for_task                             8021a63c T
 48 macx_swapon                              8021b414 T
 49 macx_swapoff                             8021b668 T
 51 macx_triggers                            8021b3f4 T
 52 macx_backing_store_suspend               8021b370 T
 53 macx_backing_store_recovery              8021b318 T
 58 pfz_exit                                 80027818 T
 59 swtch_pri                                800278e4 T
 60 swtch                                    8002781c T
 61 thread_switch                            80027ad4 T
 62 clock_sleep_trap                         80017520 T
 89 mach_timebase_info_trap                  80016658 T
 90 mach_wait_until_trap                     80016d20 T
 91 mk_timer_create_trap                     8001f2f4 T
 92 mk_timer_destroy_trap                    8001f500 T
 93 mk_timer_arm_trap                        8001f544 T
 94 mk_timer_cancel_trap                     8001f5c8 T
100 iokit_user_client_trap                   8026c11c T

References

• Wiley's OS X and iOS Internals online appendix