github.com/google/syzkaller@v0.0.0-20240517125934-c0f1611a36d6/docs/linux/coverage.md (about) 1 # Coverage 2 3 Syzkaller uses [kcov](https://www.kernel.org/doc/html/latest/dev-tools/kcov.html) to collect coverage from the kernel. kcov exports the address of each executed basic block, and syzkaller runtime uses tools from `binutils` (`objdump`, `nm`, `addr2line` and `readelf`) to map these addresses to lines and functions in the source code. 4 5 ## Binutils 6 7 Note that if you are fuzzing in cross-arch environment you need to provide correct `binutils` cross-tools to syzkaller before starting `syz-manager`: 8 9 ``` bash 10 mkdir -p ~/bin/mips64le 11 ln -s `which mips64el-linux-gnuabi64-addr2line` ~/bin/mips64le/addr2line 12 ln -s `which mips64el-linux-gnuabi64-nm` ~/bin/mips64le/nm 13 ln -s `which mips64el-linux-gnuabi64-objdump` ~/bin/mips64le/objdump 14 ln -s `which mips64el-linux-gnuabi64-readelf` ~/bin/mips64le/readelf 15 export PATH=~/bin/mips64le:$PATH 16 ``` 17 18 The target-triple prefix is determined based on the `target` config option. 19 20 ### readelf 21 22 `readelf` is used to detect virtual memory offset. 23 24 ``` 25 readelf -SW kernel_image 26 ``` 27 28 The meaning of the flags is as follows: 29 30 * `-S' - list section headers in the kernel image file 31 * `-W' - output each section header entry in a single line 32 33 Example output of the command: 34 35 ``` 36 There are 59 section headers, starting at offset 0x3825258: 37 38 Section Headers: 39 [Nr] Name Type Address Off Size ES Flg Lk Inf Al 40 [ 0] NULL 0000000000000000 000000 000000 00 0 0 0 41 [ 1] .text PROGBITS ffffffff81000000 200000 e010f7 00 AX 0 0 4096 42 [ 2] .rela.text RELA 0000000000000000 23ff488 684720 18 I 56 1 8 43 [ 3] .rodata PROGBITS ffffffff82000000 1200000 2df790 00 WA 0 0 4096 44 [ 4] .rela.rodata RELA 0000000000000000 2a83ba8 0d8e28 18 I 56 3 8 45 [ 5] .pci_fixup PROGBITS ffffffff822df790 14df790 003180 00 A 0 0 16 46 [ 6] .rela.pci_fixup RELA 0000000000000000 2b5c9d0 004a40 18 I 56 5 8 47 [ 7] .tracedata PROGBITS ffffffff822e2910 14e2910 000078 00 A 0 0 1 48 [ 8] .rela.tracedata RELA 0000000000000000 2b61410 000120 18 I 56 7 8 49 [ 9] __ksymtab PROGBITS ffffffff822e2988 14e2988 011b68 00 A 0 0 4 50 [10] ... 51 ``` 52 53 Executor truncates PC values into `uint32` before sending them to `syz-manager` and `syz-manager` uses section header information to recover the offset. Only the section headers of type `PROGBITS` are considered. The `Address` field represents the virtual address of a section in memory (for the sections that are loaded). It is required that all `PROGBITS` sections have same upper 32 bits in the `Address` field. These 32 bits are used as recovery offset. 54 55 56 ## Reporting coverage data 57 58 `MakeReportGenerator` factory creates an object database for the report. It requires target data, as well as information on the location of the source files and build directory. The first step in building this database is 59 extracting the function data from the target binary. 60 ### nm 61 62 `nm` is used to parse address and size of each function in the kernel image 63 64 ``` 65 nm -Ptx kernel_image 66 ``` 67 68 The meaning of the flags is as follows: 69 70 * `-P` - use the portable output format (Standard Output) 71 * `-tx` - write the numeric values in the hex format 72 73 Output is of the following form: 74 75 ``` 76 tracepoint_module_nb d ffffffff84509580 0000000000000018 77 ... 78 udp_lib_hash t ffffffff831a4660 0000000000000007 79 ``` 80 81 The first column is a symbol name and the second column is its type (e.g. text section, data section, debugging symbol, undefined, zero-init section, etc.). The third column is the symbol value in hex format while the forth column contains its size. The size is always rounded to up to 16 in syzkaller. For the report, we are only interested in the code sections so the `nm` output is filtered for the symbols with type `t` or `T`. 82 The final result is a map with symbol names as keys, values being starting and ending address of a symbol. This information is used to map coverage data to symbols (functions). This step is needed to find out whether certain functions are called at all. 83 84 ## Object Dump and Symbolize 85 86 In order to provide the necessary information for tracking the coverage information with syzkaller, the compiler is instrumented to insert the `__sanitizer_cov_trace_pc` call into every basic block generated during the build process. These instructions are then used as anchor points to backtrack the covered code lines. 87 88 ### objdump 89 90 `objdump` is used to parse PC value of each call to `__sanitizer_cov_trace_pc` in the kernel image. These PC values are representing all code that is built into kernel image. PC values exported by kcov are compared against these to determine coverage. 91 92 The kernel image is disassembled using the following command: 93 94 ``` 95 objdump -d --no-show-raw-insn kernel_image 96 ``` 97 98 The meaning of the flags is as follows: 99 100 * `-d` - disassemble executable code blocks 101 * `-no-show-raw-insn` - prevent printing hex alongside symbolic disassembly 102 103 Excerpt output looks like this: 104 105 ``` 106 ... 107 ffffffff81000f41: callq ffffffff81382a00 <__sanitizer_cov_trace_pc> 108 ffffffff81000f46: lea -0x80(%r13),%rdx 109 ffffffff81000f4a: lea -0x40(%r13),%rsi 110 ffffffff81000f4e: mov $0x1c,%edi 111 ffffffff81000f53: callq ffffffff813ed680 <perf_trace_buf_alloc> 112 ffffffff81000f58: test %rax,%rax 113 ffffffff81000f5b: je ffffffff8100110e <perf_trace_initcall_finish+0x2ae> 114 ffffffff81000f61: mov %rax,-0xd8(%rbp) 115 ffffffff81000f68: callq ffffffff81382a00 <__sanitizer_cov_trace_pc> 116 ffffffff81000f6d: mov -0x40(%r13),%rdx 117 ffffffff81000f71: mov 0x8(%rbp),%rsi 118 ... 119 ``` 120 121 From this output coverage trace calls are identified to determine the start of the executable block addresses: 122 123 ``` 124 ffffffff81000f41: callq ffffffff81382a00 <__sanitizer_cov_trace_pc> 125 ffffffff81000f68: callq ffffffff81382a00 <__sanitizer_cov_trace_pc> 126 ``` 127 128 ### addr2line 129 130 `addr2line` is used for mapping PC values exported by kcov and parsed by `objdump` to source code files and lines. 131 132 ``` 133 addr2line -afi -e kernel_image 134 ``` 135 136 The meaning of the flags is as follows: 137 138 * `-afi` - means show addresses, function names and unwind inlined functions 139 * `-e` - is switch for specifying executable instead of using default 140 141 `addr2line` reads hexadecimal addresses from standard input and prints the filename 142 function and line number for each address on standard output. Example usage: 143 144 ``` 145 >> ffffffff8148ba08 146 << 0xffffffff8148ba08 147 << generic_file_read_iter 148 << /home/user/linux/mm/filemap.c:2363 149 ``` 150 151 where `>>` represents the query and `<<` is the response from the `addr2line`. 152 153 The final goal is to have a hash table of frames where key is a program counter 154 and value is a frame array consisting of a following information: 155 156 * `PC` - 64bit program counter value (same as key) 157 * `Func` - function name to which the frame belongs 158 * `File` - file where function/frame code is located 159 * `Line` - Line in a file to which program counter maps 160 * `Inline` - boolean inlining information 161 162 Multiple frames can be linked to a single program counter value due to inlining. 163 164 ## Creating report 165 166 Once the database of the frames and function address ranges is created the next step is to determine the program coverage. Each program is represented here as a series of program counter values. As the function address ranges are known at this point it is easy to determine which functions were called by simply comparing the program counters against these address intervals. In addition, the coverage information is aggregated over the source files based on the program counters that are keys in the frame hash map. These are marked as `coveredPCs`. The resulting coverage is not line based but the basic block based. The end result is stored in the `file` struct containing the following information: 167 168 * `lines` - lines covered in the file 169 * `totalPCs` - total program counters identified for this file 170 * `coveredPCs` - the program counters that were executed in the program run 171 * `totalInline` - total number of program counters mapped to inlined frames 172 * `coveredInline` - the program counters mapped to inlined frames that were executed in the program run