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, 28 insertions, 0 deletions

diff --git a/prog/mutation.go b/prog/mutation.go
index ca53b1c7b..147f75ac1 100644
--- a/prog/mutation.go
+++ b/prog/mutation.go
@@ -374,6 +374,30 @@ func (t *BufferType) mutate(r *randGen, s *state, arg Arg, ctx ArgCtx) (calls []
 	case BufferText:
 		data := append([]byte{}, a.Data()...)
 		a.data = r.mutateText(t.Text, data)
+	case BufferCompressed:
+		data := a.Data()
+		if len(data) == 0 {
+			return
+		}
+		data, err := Decompress(data)
+		if err != nil {
+			panic(fmt.Sprintf("could not decompress data: %v", err))
+		}
+		if len(data) == 0 {
+			return // Do not mutate empty data.
+		}
+		hm := MakeGenericHeatmap(data)
+		// At least two mutations, up to about one mutation every 128 KB of heatmap size.
+		numMutations := r.Intn(hm.Size()/(1<<17)+1) + 2
+		for i := 0; i < numMutations; i++ {
+			index := hm.ChooseLocation(r.Rand)
+			width := 1 << uint(r.Intn(4))
+			if index+width > len(data) {
+				width = 1
+			}
+			storeInt(data[index:], r.Uint64(), width)
+		}
+		a.data = Compress(data)
 	default:
 		panic("unknown buffer kind")
 	}
@@ -663,6 +687,10 @@ func (t *BufferType) getMutationPrio(target *Target, arg Arg, ignoreSpecial bool
 		// These are effectively consts (and frequently file names).
 		return dontMutate, false
 	}
+	if t.Kind == BufferCompressed {
+		// Prioritise mutation of compressed buffers, e.g. disk images (`compressed_image`).
+		return maxPriority, false
+	}
 	return 0.8 * maxPriority, false
 }