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

["Emil Tsalapatis" <emil@etsalapatis.com>]

On Tue Mar 17, 2026 at 7:18 AM EDT, Kumar Kartikeya Dwivedi wrote:
> 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>
> ---

Reviewed-by: Emil Tsalapatis <emil@etsalapatis.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] = {};