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