prog: pkg/compiler: docs: introduce the `no_squash` attribute

The `no_squash` per-syscall attribute prevents the fuzzer from generating
squashed arguments to a particular syscall.

This is particularly helpful for pseudo-syscalls with elaborate
arguments that are hard to reason about when they are squashed - e.g.
for syz_kvm_add_vcpu() that takes a SYZOS program as an input.

I've considered an alternative solution that prohibits ANY for all
pseudo-syscalls. But there is a bunch of existing programs (both
the tests and the repros) for syscalls like syz_mount_image() for which
the benefit of not passing ANY is not immediately obvious.

I therefore decided to go with an explicit attribute that can later
be enforced for every pseudo-syscall at compile time.

1 files changed, 9 insertions, 0 deletions

diff --git a/sys/test/test.txt b/sys/test/test.txt
index 71ec2a011..54cf96f9b 100644
--- a/sys/test/test.txt
+++ b/sys/test/test.txt
@@ -799,6 +799,15 @@ hint_ints {
 }
 
 # Mutation tests.
+no_squash_struct {
+	f0	int32
+	f1	int32
+	f2	int32
+	f3	int32
+	f4	array[int8]
+} [packed]
+
+mutate_no_squash(a ptr[in, no_squash_struct]) (no_squash)
 
 mutate0()
 mutate1()