prog, pkg/compiler: add `BufferCompressed` buffer type & `compressed_image` builtin

Create the `BufferCompressed` kind of `BufferType`, which will be used
to represent compressed data. Create the corresponding `compressed_image`
syzlang builtin, which is backed by `BufferCompressed`. For now, no
syscalls use this feature - this will be introduced in future commits.

We have to be careful to decompress the data before mutating, and
re-compress before storing. We make sure that any deserialised
`BufferCompressed` data is valid too.

`BufferCompressed` arguments are mutated using a generic heatmap. In
future, we could add variants of `BufferCompressed` or populate the
`BufferType` sub-kind, using it to choose different kinds of heatmap for
different uncompressed data formats.

Various operations on compressed data must be forbidden, so we check for
`BufferCompressed` in key places. We also have to ensure `compressed_image`
can only be used in syscalls that are marked `no_{generate,minimize}`.
Therefore, we add a generic compiler check which allows type
descriptions to require attributes on the syscalls which use them.

1 files changed, 2 insertions, 0 deletions

diff --git a/prog/rand.go b/prog/rand.go
index 8337e7af4..c91ffeee4 100644
--- a/prog/rand.go
+++ b/prog/rand.go
@@ -768,6 +768,8 @@ func (a *BufferType) generate(r *randGen, s *state, dir Dir) (arg Arg, calls []*
 			return MakeOutDataArg(a, dir, uint64(r.Intn(100))), nil
 		}
 		return MakeDataArg(a, dir, r.generateText(a.Text)), nil
+	case BufferCompressed:
+		panic(fmt.Sprintf("can't generate compressed type %v", a))
 	default:
 		panic("unknown buffer kind")
 	}