[DTrace-devel] [PATCH] Provide set_upper_bound() macro for C-to-BPF source code

[Eugene Loh]

Reviewed-by: Eugene Loh <eugene.loh at oracle.com>

On 7/29/21 11:39 PM, Kris Van Hees wrote:
> The BPF verifier obtains (and needs) information about upper and  lower
> bounds for values based on conditional clauses.  The BPF GCC cross
> compiler generates conditionals in a variety of forms.  The BPF verifier
> sypports only a limited set of conditionals, so we need to be able to
> enforce a specific form.  More specifically, the BPF verifier requires
> the bound (a constant value) to be stored in the src register while the
> value to be validated is in the dst register.
>
> Signed-off-by: Kris Van Hees <kris.van.hees at oracle.com>
> ---
>   include/bpf-lib.h | 30 ++++++++++++++++++++++++++++++
>   1 file changed, 30 insertions(+)
>   create mode 100644 include/bpf-lib.h
>
> diff --git a/include/bpf-lib.h b/include/bpf-lib.h
> new file mode 100644
> index 00000000..70783fe0
> --- /dev/null
> +++ b/include/bpf-lib.h
> @@ -0,0 +1,30 @@
> +/*
> + * Oracle Linux DTrace.
> + * Copyright (c) 2021, Oracle and/or its affiliates. All rights reserved.
> + * Licensed under the Universal Permissive License v 1.0 as shown at
> + * http://oss.oracle.com/licenses/upl.
> + */
> +
> +#ifndef BPF_LIB_H
> +#define BPF_LIB_H
> +
> +/*
> + * Explicit inline assembler to implement an upper bound check:
> + *
> + *	if (var > bnd)
> + *		var = bnd;
> + *
> + * The BPF GCC compiler does not guarantee that the bound (expected to be a
> + * variable that holds a constant value) will be encoded in the source register
> + * while the BPF verifier does require that (for now).
> + */
> +#define set_upper_bound(var, bnd) \
> +	asm ("jle %0, %1, 1f\n\t" \
> +	     "mov %0, %1\n\t" \
> +	     "1:" \
> +		: "+r" (var) \
> +		: "r" (bnd) \
> +		: /* no clobbers */ \
> +	);
> +
> +#endif /* BPF_LIB_H */