1The Undefined Behavior Sanitizer - UBSAN
2========================================
3
4UBSAN is a runtime undefined behaviour checker.
5
6UBSAN uses compile-time instrumentation to catch undefined behavior (UB).
7Compiler inserts code that perform certain kinds of checks before operations
8that may cause UB. If check fails (i.e. UB detected) __ubsan_handle_*
9function called to print error message.
10
11GCC has that feature since 4.9.x [1_] (see ``-fsanitize=undefined`` option and
12its suboptions). GCC 5.x has more checkers implemented [2_].
13
14Report example
15--------------
16
17::
18
19	 ================================================================================
20	 UBSAN: Undefined behaviour in ../include/linux/bitops.h:110:33
21	 shift exponent 32 is to large for 32-bit type 'unsigned int'
22	 CPU: 0 PID: 0 Comm: swapper Not tainted 4.4.0-rc1+ #26
23	  0000000000000000 ffffffff82403cc8 ffffffff815e6cd6 0000000000000001
24	  ffffffff82403cf8 ffffffff82403ce0 ffffffff8163a5ed 0000000000000020
25	  ffffffff82403d78 ffffffff8163ac2b ffffffff815f0001 0000000000000002
26	 Call Trace:
27	  [<ffffffff815e6cd6>] dump_stack+0x45/0x5f
28	  [<ffffffff8163a5ed>] ubsan_epilogue+0xd/0x40
29	  [<ffffffff8163ac2b>] __ubsan_handle_shift_out_of_bounds+0xeb/0x130
30	  [<ffffffff815f0001>] ? radix_tree_gang_lookup_slot+0x51/0x150
31	  [<ffffffff8173c586>] _mix_pool_bytes+0x1e6/0x480
32	  [<ffffffff83105653>] ? dmi_walk_early+0x48/0x5c
33	  [<ffffffff8173c881>] add_device_randomness+0x61/0x130
34	  [<ffffffff83105b35>] ? dmi_save_one_device+0xaa/0xaa
35	  [<ffffffff83105653>] dmi_walk_early+0x48/0x5c
36	  [<ffffffff831066ae>] dmi_scan_machine+0x278/0x4b4
37	  [<ffffffff8111d58a>] ? vprintk_default+0x1a/0x20
38	  [<ffffffff830ad120>] ? early_idt_handler_array+0x120/0x120
39	  [<ffffffff830b2240>] setup_arch+0x405/0xc2c
40	  [<ffffffff830ad120>] ? early_idt_handler_array+0x120/0x120
41	  [<ffffffff830ae053>] start_kernel+0x83/0x49a
42	  [<ffffffff830ad120>] ? early_idt_handler_array+0x120/0x120
43	  [<ffffffff830ad386>] x86_64_start_reservations+0x2a/0x2c
44	  [<ffffffff830ad4f3>] x86_64_start_kernel+0x16b/0x17a
45	 ================================================================================
46
47Usage
48-----
49
50To enable UBSAN configure kernel with::
51
52	CONFIG_UBSAN=y
53
54and to check the entire kernel::
55
56        CONFIG_UBSAN_SANITIZE_ALL=y
57
58To enable instrumentation for specific files or directories, add a line
59similar to the following to the respective kernel Makefile:
60
61- For a single file (e.g. main.o)::
62
63    UBSAN_SANITIZE_main.o := y
64
65- For all files in one directory::
66
67    UBSAN_SANITIZE := y
68
69To exclude files from being instrumented even if
70``CONFIG_UBSAN_SANITIZE_ALL=y``, use::
71
72  UBSAN_SANITIZE_main.o := n
73
74and::
75
76  UBSAN_SANITIZE := n
77
78Detection of unaligned accesses controlled through the separate option -
79CONFIG_UBSAN_ALIGNMENT. It's off by default on architectures that support
80unaligned accesses (CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS=y). One could
81still enable it in config, just note that it will produce a lot of UBSAN
82reports.
83
84References
85----------
86
87.. _1: https://gcc.gnu.org/onlinedocs/gcc-4.9.0/gcc/Debugging-Options.html
88.. _2: https://gcc.gnu.org/onlinedocs/gcc/Debugging-Options.html
89