selftests/bpf: Test modified syscall ctx for ARG_PTR_TO_CTX

Ensure that global subprogs and tail calls 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.

Reviewed-by: Emil Tsalapatis <emil@etsalapatis.com>
Acked-by: Puranjay Mohan <puranjay@kernel.org>
Acked-by: Mykyta Yatsenko <yatsenko@meta.com>
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Link: https://lore.kernel.org/r/20260406194403.1649608-6-memxor@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

1 files changed, 94 insertions, 0 deletions

diff --git a/tools/testing/selftests/bpf/progs/verifier_global_subprogs.c b/tools/testing/selftests/bpf/progs/verifier_global_subprogs.c
index 2250fc31574d..1e08aff7532e 100644
--- a/tools/testing/selftests/bpf/progs/verifier_global_subprogs.c
+++ b/tools/testing/selftests/bpf/progs/verifier_global_subprogs.c
@@ -357,6 +357,100 @@ 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")
+__auxiliary
+int syscall_tailcall_target(void *ctx)
+{
+	return syscall_array_bpf_for(ctx);
+}
+
+struct {
+	__uint(type, BPF_MAP_TYPE_PROG_ARRAY);
+	__uint(max_entries, 1);
+	__uint(key_size, sizeof(__u32));
+	__array(values, int (void *));
+} syscall_prog_array SEC(".maps") = {
+	.values = {
+		[0] = (void *)&syscall_tailcall_target,
+	},
+};
+
+SEC("?syscall")
+__success __log_level(2)
+int arg_tag_ctx_syscall_tailcall(void *ctx)
+{
+	bpf_tail_call(ctx, &syscall_prog_array, 0);
+	return 0;
+}
+
+SEC("?syscall")
+__failure __log_level(2)
+__msg("dereference of modified ctx ptr R1 off=8 disallowed")
+int arg_tag_ctx_syscall_tailcall_fixed_off_bad(void *ctx)
+{
+	char *p = ctx;
+
+	p += 8;
+	bpf_tail_call(p, &syscall_prog_array, 0);
+	return 0;
+}
+
+SEC("?syscall")
+__failure __log_level(2)
+__msg("variable ctx access var_off=(0x0; 0x4) disallowed")
+int arg_tag_ctx_syscall_tailcall_var_off_bad(void *ctx)
+{
+	__u64 off = bpf_get_prandom_u32();
+	char *p = ctx;
+
+	off &= 4;
+	p += off;
+	bpf_tail_call(p, &syscall_prog_array, 0);
+	return 0;
+}
+
+SEC("?syscall")
+__failure __log_level(2)
+__msg("dereference of modified ctx ptr R1 off=8 disallowed")
+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("variable ctx access var_off=(0x0; 0x4) disallowed")
+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] = {};