From 20368da6816350d8f63a7848c2ccc79c04808f4b Mon Sep 17 00:00:00 2001 From: Andrey Konovalov Date: Wed, 14 Jun 2017 13:23:51 +0200 Subject: docs: move sys/README.md to docs --- docs/syscall_descriptions.md | 229 +++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 229 insertions(+) create mode 100644 docs/syscall_descriptions.md (limited to 'docs') diff --git a/docs/syscall_descriptions.md b/docs/syscall_descriptions.md new file mode 100644 index 000000000..27239ca41 --- /dev/null +++ b/docs/syscall_descriptions.md @@ -0,0 +1,229 @@ +# Syscall descriptions + +`syzkaller` uses declarative description of syscalls to generate, mutate, minimize, +serialize and deserialize programs (sequences of syscalls). Below you can see +(hopefully self-explanatory) excerpt from the description: + +``` +open(file filename, flags flags[open_flags], mode flags[open_mode]) fd +read(fd fd, buf buffer[out], count len[buf]) len[buf] +close(fd fd) +open_mode = S_IRUSR, S_IWUSR, S_IXUSR, S_IRGRP, S_IWGRP, S_IXGRP, S_IROTH, S_IWOTH, S_IXOTH +``` + +The description is contained in `sys/*.txt` files. See for example [sys/sys.txt](/sys/sys.txt) file. + +## Syntax + +Pseudo-formal grammar of syscall description: +``` + syscallname "(" [arg ["," arg]*] ")" [type] + arg = argname type + argname = identifier + type = typename [ "[" type-options "]" ] + typename = "const" | "intN" | "intptr" | "flags" | "array" | "ptr" | + "buffer" | "string" | "strconst" | "filename" | + "len" | "bytesize" | "vma" | "proc" + type-options = [type-opt ["," type-opt]] +``` +common type-options include: +``` + "opt" - the argument is optional (like mmap fd argument, or accept peer argument) +``` +rest of the type-options are type-specific: +``` + "const": integer constant, type-options: + value, underlying type (one if "intN", "intptr") + "intN"/"intptr": an integer without a particular meaning, type-options: + optional range of values (e.g. "5:10", or "-100:200") + "flags": a set of flags, type-options: + reference to flags description (see below) + "array": a variable/fixed-length array, type-options: + type of elements, optional size (fixed "5", or ranged "5:10", boundaries inclusive) + "ptr": a pointer to an object, type-options: + type of the object; direction (in/out/inout) + "buffer": a pointer to a memory buffer (like read/write buffer argument), type-options: + direction (in/out/inout) + "string": a zero-terminated memory buffer (no pointer indirection implied), type-options: + either a string value in quotes for constant strings (e.g. "foo"), + or a reference to string flags, + optionally followed by a buffer size (string values will be padded with \x00 to that size) + "filename": a file/link/dir name, no pointer indirection implied, in most cases you want `ptr[in, filename]` + "fileoff": offset within a file + "len": length of another field (for array it is number of elements), type-options: + argname of the object + "bytesize": similar to "len", but always denotes the size in bytes, type-options: + argname of the object + "vma": a pointer to a set of pages (used as input for mmap/munmap/mremap/madvise), type-options: + optional number of pages (e.g. vma[7]), or a range of pages (e.g. vma[2-4]) + "proc": per process int (see description below), type-options: + underlying type, value range start, how many values per process + "text16", "text32", "text64": machine code of the specified bitness +``` +flags/len/flags also have trailing underlying type type-option when used in structs/unions/pointers. + +Flags are described as: +``` + flagname = const ["," const]* +``` +or for string flags as: +``` + flagname = "\"" literal "\"" ["," "\"" literal "\""]* +``` + +### Ints + +You can use `int8`, `int16`, `int32`, `int64` and `int64` to denote an integer of the corresponding size. + +By appending `be` suffix (like `int16be`) integers become big-endian. + +It's possible to specify range of values for an integer in the format of `int32[0:100]`. + +To denote a bitfield of size N use `int64:N`. + +It's possible to use these various kinds of ints as base types for `const`, `flags`, `len` and `proc`. + +``` +example_struct { + f0 int8 # random 1-byte integer + f1 const[0x42, int16be] # const 2-byte integer with value 0x4200 (big-endian 0x42) + f2 int32[0:100] # random 4-byte integer with values from 0 to 100 inclusive + f3 int64:20 # random 20-bit bitfield +} +``` + +### Structs + +Structs are described as: +``` + structname "{" "\n" + (fieldname type "\n")+ + "}" +``` +Structs can have trailing attributes "packed" and "align_N", +they are specified in square brackets after the struct. + +### Unions + +Unions are described as: +``` + unionname "[" "\n" + (fieldname type "\n")+ + "]" +``` +Unions can have a trailing "varlen" attribute (specified in square brackets after the union), +which means that union length is not maximum of all option lengths, +but rather length of a particular chosen option. + +### Resources + +Custom resources are described as: +``` + resource identifier "[" underlying_type "]" [ ":" const ("," const)* ] +``` +`underlying_type` is either one of `int8`, `int16`, `int32`, `int64`, `intptr` or another resource. +Resources can then be used as types. For example: +``` +resource fd[int32]: 0xffffffffffffffff, AT_FDCWD, 1000000 +resource sock[fd] +resource sock_unix[sock] + +socket(...) sock +accept(fd sock, ...) sock +listen(fd sock, backlog int32) +``` + +### Length + +You can specify length of a particular field in struct or a named argument by using `len` and `bytesize` types, for example: +``` +write(fd fd, buf buffer[in], count len[buf]) len[buf] + +sock_fprog { + len len[filter, int16] + filter ptr[in, array[sock_filter]] +} +``` + +If `len`'s argument is a pointer (or a `buffer`), then the length of the pointee argument is used. + +To denote the length of a field in N-byte words use `bytesizeN`, possible values for N are 1, 2, 4 and 8. + +To denote the length of the parent struct, you can use `len[parent, int8]`. +To denote the length of the higher level parent when structs are embedded into one another, you can specify the type name of the particular parent: +``` +struct s1 { + f0 len[s2] # length of s2 +} + +struct s2 { + f0 s1 + f1 array[int32] +} + +``` + +### Proc + +The `proc` type can be used to denote per process integers. +The idea is to have a separate range of values for each executor, so they don't interfere. + +The simplest example is a port number. +The `proc[int16be, 20000, 4]` type means that we want to generate an `int16be` integer starting from `20000` and assign no more than `4` integers for each process. +As a result the executor number `n` will get values in the `[20000 + n * 4, 20000 + (n + 1) * 4)` range. + +### Misc + +Description files also contain `include` directives that refer to Linux kernel header files +and `define` directives that define symbolic constant values. See the following section for details. + +## Code generation + +Textual syscall descriptions are translated into code used by `syzkaller`. +This process consists of 2 steps. The first step is extraction of values of symbolic +constants from Linux sources using `syz-extract` utility. +`syz-extract` generates a small C program that includes kernel headers referenced +by `include` directives, defines macros as specified by `define` directives and +prints values of symbolic constants. Results are stored in `.const` files, one per arch. +For example, [sys/tty.txt](/sys/tty.txt) is translated into [sys/tty_amd64.const](/sys/tty_amd64.const). + +The second step is generation of Go code for syzkaller. This step uses syscall descriptions +and the const files generated during the first step. You can see a result in [sys/sys_amd64.go](/sys/sys_amd64.go) +and in [executor/syscalls.h](/executor/syscalls.h). + +## Describing new system calls + +This section describes how to extend syzkaller to allow fuzz testing of a new system call; +this is particularly useful for kernel developers who are proposing new system calls. + +First, add a declarative description of the new system call to the appropriate file: + - Various `sys/.txt` files hold system calls for particular kernel + subsystems, for example `bpf` or `socket`. + - [sys/sys.txt](/sys/sys.txt) holds descriptions for more general system calls. + - An entirely new subsystem can be added as a new `sys/.txt` file. + +The description format is described [above](#syntax). + +If the subsystem is present in the mainline kernel, add the new txt file to `extract.sh` +file and run `make extract LINUX=$KSRC` with `KSRC` set to the location of a kernel +source tree. This will generate const files. + +If the subsystem is not present in the mainline kernel, then you need to manually +run `syz-extract` binary: +``` +make bin/syz-extract +bin/syz-extract -arch $ARCH -linux "$LINUX" -linuxbld "$LINUXBLD" sys/.txt +``` +`$ARCH` is one of `amd64`, `arm64`, `ppc64le`. If the subsystem is supported on several architectures, +then run `syz-exctact` for each arch. +`$LINUX` should point to kernel source checkout, which is configured for the corresponding arch +(i.e. you need to run `make someconfig && make` there first). If the kernel was built into a separate +directory (with `make O=...`) then also set `$LINUXBLD` to the location of the +build directory. + +Then, run `make generate` which will update generated code. + +Rebuild syzkaller (`make clean all`) to force use of the new system call definitions. + +Optionally, adjust the `enable_syscalls` configuration value for syzkaller to specifically target the +new system calls. -- cgit mrf-deployment