1.. SPDX-License-Identifier: GPL-2.0
2
3Undefined Behavior Sanitizer - UBSAN
4====================================
5
6UBSAN is a runtime undefined behaviour checker.
7
8UBSAN uses compile-time instrumentation to catch undefined behavior (UB).
9Compiler inserts code that perform certain kinds of checks before operations
10that may cause UB. If check fails (i.e. UB detected) __ubsan_handle_*
11function called to print error message.
12
13GCC has that feature since 4.9.x [1_] (see ``-fsanitize=undefined`` option and
14its suboptions). GCC 5.x has more checkers implemented [2_].
15
16Report example
17--------------
18
19::
20
21	 ================================================================================
22	 UBSAN: Undefined behaviour in ../include/linux/bitops.h:110:33
23	 shift exponent 32 is to large for 32-bit type 'unsigned int'
24	 CPU: 0 PID: 0 Comm: swapper Not tainted 4.4.0-rc1+ #26
25	  0000000000000000 ffffffff82403cc8 ffffffff815e6cd6 0000000000000001
26	  ffffffff82403cf8 ffffffff82403ce0 ffffffff8163a5ed 0000000000000020
27	  ffffffff82403d78 ffffffff8163ac2b ffffffff815f0001 0000000000000002
28	 Call Trace:
29	  [<ffffffff815e6cd6>] dump_stack+0x45/0x5f
30	  [<ffffffff8163a5ed>] ubsan_epilogue+0xd/0x40
31	  [<ffffffff8163ac2b>] __ubsan_handle_shift_out_of_bounds+0xeb/0x130
32	  [<ffffffff815f0001>] ? radix_tree_gang_lookup_slot+0x51/0x150
33	  [<ffffffff8173c586>] _mix_pool_bytes+0x1e6/0x480
34	  [<ffffffff83105653>] ? dmi_walk_early+0x48/0x5c
35	  [<ffffffff8173c881>] add_device_randomness+0x61/0x130
36	  [<ffffffff83105b35>] ? dmi_save_one_device+0xaa/0xaa
37	  [<ffffffff83105653>] dmi_walk_early+0x48/0x5c
38	  [<ffffffff831066ae>] dmi_scan_machine+0x278/0x4b4
39	  [<ffffffff8111d58a>] ? vprintk_default+0x1a/0x20
40	  [<ffffffff830ad120>] ? early_idt_handler_array+0x120/0x120
41	  [<ffffffff830b2240>] setup_arch+0x405/0xc2c
42	  [<ffffffff830ad120>] ? early_idt_handler_array+0x120/0x120
43	  [<ffffffff830ae053>] start_kernel+0x83/0x49a
44	  [<ffffffff830ad120>] ? early_idt_handler_array+0x120/0x120
45	  [<ffffffff830ad386>] x86_64_start_reservations+0x2a/0x2c
46	  [<ffffffff830ad4f3>] x86_64_start_kernel+0x16b/0x17a
47	 ================================================================================
48
49Usage
50-----
51
52To enable UBSAN, configure the kernel with::
53
54  CONFIG_UBSAN=y
55
56To exclude files from being instrumented use::
57
58  UBSAN_SANITIZE_main.o := n
59
60and to exclude all targets in one directory use::
61
62  UBSAN_SANITIZE := n
63
64When disabled for all targets, specific files can be enabled using::
65
66  UBSAN_SANITIZE_main.o := y
67
68Detection of unaligned accesses controlled through the separate option -
69CONFIG_UBSAN_ALIGNMENT. It's off by default on architectures that support
70unaligned accesses (CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS=y). One could
71still enable it in config, just note that it will produce a lot of UBSAN
72reports.
73
74References
75----------
76
77.. _1: https://gcc.gnu.org/onlinedocs/gcc-4.9.0/gcc/Debugging-Options.html
78.. _2: https://gcc.gnu.org/onlinedocs/gcc/Debugging-Options.html
79.. _3: https://clang.llvm.org/docs/UndefinedBehaviorSanitizer.html
80