pkg/csource: add description of the generation process

pkg/csource/csource.go

@@ -2,6 +2,25 @@
 // Use of this source code is governed by Apache 2 LICENSE that can be found in the LICENSE file.
 
 // Package csource generates [almost] equivalent C programs from syzkaller programs.
+//
+// Outline of the process:
+// - inputs to the generation are the program and options
+// - options control multiple aspects of the resulting C program,
+//   like if we want a multi-threaded program or a single-threaded,
+//   what type of sandbox we want to use, if we want to setup net devices or not, etc
+// - we use actual executor sources as the base
+// - gen.go takes all executor/common*.h headers and bundles them into generated.go
+// - during generation we tear executor headers apart and take only the bits
+//   we need for the current program/options, this is done by running C preprocessor
+//   with particular set of defines so that the preprocessor removes unneeded
+//   #ifdef SYZ_FOO sections
+// - then we generate actual syscall calls with the given arguments
+//   based on the binary "encodingexec" representation of the program
+//   (the same representation executor uses for interpretation)
+// - then we glue it all together
+// - as the last step we run some text post-processing on the resulting source code:
+//   remove debug calls, replace exitf/fail with exit, hoist/sort/dedup includes,
+//   remove duplicate empty lines, etc
 package csource
 
 import (
@@ -15,6 +34,7 @@ import (
 	"github.com/google/syzkaller/sys/targets"
 )
 
+// Write generates C source for program p based on the provided options opt.
 func Write(p *prog.Prog, opts Options) ([]byte, error) {
 	if err := opts.Check(p.Target.OS); err != nil {
 		return nil, fmt.Errorf("csource: invalid opts: %v", err)