[PATCH bpf-next v1 4/4] selftests/bpf: Test modified syscall ctx for global subprog

[Kumar Kartikeya Dwivedi <memxor@gmail.com>]

Ensure that global subprogs can only accept an unmodified PTR_TO_CTX for
syscall programs. For all other program types, fixed or variable offsets
on PTR_TO_CTX is rejected when passed into an argument of any call
instruction type, through the unified logic of check_func_arg_reg_off.

Finally, add a positive example of a case that should succeed with all
our previous changes.

Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
---
 .../bpf/progs/verifier_global_subprogs.c      | 42 +++++++++++++++++++
 1 file changed, 42 insertions(+)

diff --git a/tools/testing/selftests/bpf/progs/verifier_global_subprogs.c b/tools/testing/selftests/bpf/progs/verifier_global_subprogs.c
index 2250fc31574d..0c14adaddb90 100644
--- a/tools/testing/selftests/bpf/progs/verifier_global_subprogs.c
+++ b/tools/testing/selftests/bpf/progs/verifier_global_subprogs.c
@@ -357,6 +357,48 @@ int arg_tag_ctx_syscall(void *ctx)
 	return tracing_subprog_void(ctx) + tracing_subprog_u64(ctx) + tp_whatever(ctx);
 }
 
+__weak int syscall_array_bpf_for(void *ctx __arg_ctx)
+{
+	int *arr = ctx;
+	int i;
+
+	bpf_for(i, 0, 100)
+		arr[i] *= i;
+
+	return 0;
+}
+
+SEC("?syscall")
+__success __log_level(2)
+int arg_tag_ctx_syscall_bpf_for(void *ctx)
+{
+	return syscall_array_bpf_for(ctx);
+}
+
+SEC("?syscall")
+__failure __log_level(2)
+__msg("arg#0 of syscall prog must have zero offset")
+int arg_tag_ctx_syscall_fixed_off_bad(void *ctx)
+{
+	char *p = ctx;
+
+	p += 8;
+	return subprog_ctx_tag(p);
+}
+
+SEC("?syscall")
+__failure __log_level(2)
+__msg("arg#0 of syscall prog must have zero offset")
+int arg_tag_ctx_syscall_var_off_bad(void *ctx)
+{
+	__u64 off = bpf_get_prandom_u32();
+	char *p = ctx;
+
+	off &= 4;
+	p += off;
+	return subprog_ctx_tag(p);
+}
+
 __weak int subprog_dynptr(struct bpf_dynptr *dptr)
 {
 	long *d, t, buf[1] = {};
-- 
2.52.0