Radare2 allows manual search for assembly code looking like a syscall operation. For example on ARM platform usually they are represented by the svc instruction, on the others can be a different instructions, e.g. syscall on x86 PC.

[0x0001ece0]> /c svc
0x000187c2   # 2: svc 0x76
0x000189ea   # 2: svc 0xa9
0x00018a0e   # 2: svc 0x82

Syscalls detection is driven by asm.os, asm.bits, and asm.arch. Be sure to setup those configuration options accordingly. You can use asl command to check if syscalls' support is set up properly and as you expect. The command lists syscalls supported for your platform.

[0x0001ece0]> asl
sd_softdevice_enable = 0x80.16
sd_softdevice_disable = 0x80.17
sd_softdevice_is_enabled = 0x80.18

If you setup ESIL stack with aei or aeim, you can use /as command to search the addresses where particular syscalls were found and list them.

[0x0001ece0]> aei
[0x0001ece0]> /as
0x000187c2 sd_ble_gap_disconnect
0x000189ea sd_ble_gatts_sys_attr_set
0x00018a0e sd_ble_gap_sec_info_reply

To reduce searching time it is possible to restrict the searching range for only executable segments or sections with /as

Using the ESIL emulation radare2 can print syscall arguments in the disassembly output. To enable the linear (but very rough) emulation use asm.emu configuration variable:

[0x0001ece0]> e asm.emu=true
[0x0001ece0]> s 0x000187c2
[0x000187c2]> pdf~svc
   0x000187c2   svc 0x76  ; 118 = sd_ble_gap_disconnect

In case of executing aae (or aaaa which calls aae) command radare2 will push found syscalls to a special syscall. flagspace, which can be useful for automation purpose:

[0x000187c2]> fs
0    0 * imports
1    0 * symbols
2 1523 * functions
3  420 * strings
4  183 * syscalls
[0x000187c2]> f~syscall
0x000187c2 1 syscall.sd_ble_gap_disconnect.0
0x000189ea 1 syscall.sd_ble_gatts_sys_attr_set
0x00018a0e 1 syscall.sd_ble_gap_sec_info_reply

It also can be interactively navigated through within HUD mode (V_)

0> syscall.sd_ble_gap_disconnect
 - 0x000187b2  syscall.sd_ble_gap_disconnect
   0x000187c2  syscall.sd_ble_gap_disconnect.0
   0x00018a16  syscall.sd_ble_gap_disconnect.1
   0x00018b32  syscall.sd_ble_gap_disconnect.2
   0x0002ac36  syscall.sd_ble_gap_disconnect.3

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