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syscall_descriptions.md

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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. For example see the sys/sys.txt file.

Syntax

The description of the syntax can be found here.

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 is translated into 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 and in 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/<subsystem>.txt files hold system calls for particular kernel subsystems, for example bpf or socket.
  • sys/sys.txt holds descriptions for more general system calls.
  • An entirely new subsystem can be added as a new sys/<new>.txt file.

The description of the syntax can be found here.

If the subsystem is present in the mainline kernel, add the new txt file to sys/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. Not, that this will overwrite .config file you have in $KSRC.

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/<new>.txt

$ARCH is one of amd64, arm64, ppc64le. If the subsystem is supported on several architectures, then run syz-extract 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.