docs: new file pseudo_syscalls.md

Add a brief description about pseudo-syscalls and how to create new
ones.

docs/pseudo_syscalls.md

@@ -0,0 +1,72 @@
+# Syzkaller pseudo-syscalls
+
+Besides regular system calls, a [syscall
+description](syscall_descriptions.md) file can also contain
+pseudo-syscalls. These are C functions defined in the syzkaller
+executor. When a syzkaller program uses a pseudo-syscall, the executor
+will generate the pseudo-syscall function code in the resulting C program. 
+
+This allows a test program to have specific code blocks to perform
+certain actions, they may also be used as more test-friendly wrappers
+for primitive syscalls.
+
+## How to add a pseudo-syscall to the executor
+
+First, think about the scope of the pseudo-syscall and which systems and
+subsystems it will be related to. The executor includes a fixed set of C
+header files containing the code of the pseudo-syscalls. Check if the
+new one can fit in one of the existing files before creating a new
+one. These header files are defined in [gen.go](../pkg/csource/gen.go):
+
+    executorFilenames := []string{
+            "common_linux.h",
+            "common_akaros.h",
+            "common_bsd.h",
+            "common_fuchsia.h",
+            "common_windows.h",
+            "common_test.h",
+            "common_kvm_amd64.h",
+            "common_kvm_arm64.h",
+            "common_usb_linux.h",
+            "common_usb_netbsd.h",
+            "common_usb.h",
+            "android/android_seccomp.h",
+            "kvm.h",
+            "kvm.S.h",
+    }
+
+For instance, if our new pseudo-syscall is Linux-specific, then
+[common_linux.h](../executor/common_linux.h) would be the place to put it.
+
+The actual pseudo-syscall function may look something like this:
+
+    #if SYZ_EXECUTOR || __NR_syz_mycall
+    /* Add all the necessary #include and #define headers */
+
+    static volatile long syz_mycall(volatile long a0, volatile long a1)
+    {
+            /* Function body */
+    }
+    #endif
+
+Make sure that all the function requirements are met and that it can
+be compiled. Note that the function name must start with "syz_". It may
+also take a different number of arguments.
+
+Now, to handle the pseudo-syscall properly we have to update the
+`isSupportedSyzkall` in
+[syscalls_linux.go](../pkg/host/syscalls_linux.go) and add a particular
+case for this syscall, enabling it when necessary. If we want to enable
+it unconditionally we can simply make `isSupportedSyzkall` return `true,
+""` for it:
+
+    func isSupportedSyzkall(sandbox string, c *prog.Syscall) (bool, string) {
+            switch c.CallName {
+            ...
+            case "syz_mycall":
+                    return true, ""
+
+Finally, run `make generate`. Now you can use it in a syscall
+description file as if it was a regular system call:
+
+    syz_mycall(arg0 pid, arg1 const[0])