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1 Undefined Behavior Sanitizer - UBSAN
2
3 Overview
4 --------
5
6 UBSAN is a runtime undefined behaviour checker.
7
8 UBSAN uses compile-time instrumentation to catch undefined behavior (UB).
9 Compiler inserts code that perform certain kinds of checks before operations
10 that may cause UB. If check fails (i.e. UB detected) __ubsan_handle_*
11 function called to print error message.
12
13 GCC has that feature since 4.9.x [1] (see -fsanitize=undefined option and
14 its suboptions). GCC 5.x has more checkers implemented [2].
15
16 Report example
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
47 Usage
48 -----
49
50 To enable UBSAN configure kernel with:
51
52         CONFIG_UBSAN=y
53
54 and to check the entire kernel:
55
56         CONFIG_UBSAN_SANITIZE_ALL=y
57
58 To enable instrumentation for specific files or directories, add a line
59 similar to the following to the respective kernel Makefile:
60
61         For a single file (e.g. main.o):
62                 UBSAN_SANITIZE_main.o := y
63
64         For all files in one directory:
65                 UBSAN_SANITIZE := y
66
67 To exclude files from being instrumented even if
68 CONFIG_UBSAN_SANITIZE_ALL=y, use:
69
70                 UBSAN_SANITIZE_main.o := n
71         and:
72                 UBSAN_SANITIZE := n
73
74 Detection of unaligned accesses controlled through the separate option -
75 CONFIG_UBSAN_ALIGNMENT. It's off by default on architectures that support
76 unaligned accesses (CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS=y). One could
77 still enable it in config, just note that it will produce a lot of UBSAN
78 reports.
79
80 References
81 ----------
82
83 [1] - https://gcc.gnu.org/onlinedocs/gcc-4.9.0/gcc/Debugging-Options.html
84 [2] - https://gcc.gnu.org/onlinedocs/gcc/Debugging-Options.html