Re: [DTrace-devel] [PATCH 1/4] parser, cg: Implement the return() action

[Nick Alcock <nick.alcock@oracle.com>]

On 15 Jul 2025, Kris Van Hees via DTrace-devel outgrape:

> The return(n) action can be used for error injection by forcing a
> given return value for a kernel function.

I am amazed this is even possible! I can see all sorts of uses.

Presumably we need to tell people to turn on CONFIG_BPF_KPROBE_OVERRIDE
somewhere... UEK already has it on, good. Not many functions
covered... I suppose it'll let them trace a *few* functions, and if they
use this they're probably used to hacking in more error-injection points
anyway.

> Signed-off-by: Kris Van Hees <kris.van.hees@oracle.com>

Reviewed-by: Nick Alcock <nick.alcock@oracle.com>

modulo the tiny nits below.

> diff --git a/libdtrace/dt_lex.l b/libdtrace/dt_lex.l
> index cc165c1e..bdcca415 100644
> --- a/libdtrace/dt_lex.l
> +++ b/libdtrace/dt_lex.l
> @@ -104,7 +104,6 @@ if (yypcb->pcb_token != 0) {
>  <S0>provider	return DT_KEY_PROVIDER;
>  <S0>register	return DT_KEY_REGISTER;
>  <S0>restrict	return DT_KEY_RESTRICT;
> -<S0>return	return DT_KEY_RETURN;
>  <S0>self	return DT_KEY_SELF;
>  <S0>short	return DT_KEY_SHORT;
>  <S0>signed	return DT_KEY_SIGNED;

I guess these are here for C/C++ header parsing :) a somewhat hopeful
approach. I wonder if this change will cause breakage for non-bracketed
returns in C code in static inlines... and returns of non-integral
types, and just plain return... ugh. I suppose inlines barely work
anyway and the right way to do that is to simply have the parser ignore
static inlines in C headers anyway, not have it try to parse them :)

> diff --git a/libdtrace/dt_open.c b/libdtrace/dt_open.c
> index c67455e7..08394a11 100644
> --- a/libdtrace/dt_open.c
> +++ b/libdtrace/dt_open.c
> @@ -269,6 +269,8 @@ static const dt_ident_t _dtrace_globals[] = {
>  	&dt_idops_func, "void(int)" },
>  { "rand", DT_IDENT_FUNC, 0, DIF_SUBR_RAND, DT_ATTR_STABCMN, DT_VERS_1_0,
>  	&dt_idops_func, "int()" },
> +{ "return", DT_IDENT_ACTFUNC, 0, DT_ACT_RETURN, DT_ATTR_STABCMN, DT_VERS_2_0,
> +	&dt_idops_func, "void(int)" },

The "problem" (if this actually is a problem) is that this is not the C
prototype, which is not expressible in C :(

> diff --git a/test/unittest/actions/return/err.D_PROTO_LEN.missing_arg.d b/test/unittest/actions/return/err.D_PROTO_LEN.missing_arg.d
> new file mode 100644
> index 00000000..97ed0762
> --- /dev/null
> +++ b/test/unittest/actions/return/err.D_PROTO_LEN.missing_arg.d
> @@ -0,0 +1,20 @@
> +/*
> + * Oracle Linux DTrace.
> + * Copyright (c) 2025, 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.
> + */
> +
> +/*
> + * ASSERTION: The return() action takes exactly one argument.
> + *
> + * SECTION: Actions and Subroutines/return()
> + */
> +
> +#pragma D option quiet
> +
> +BEGIN
> +{
> +	return();
> +	exit(0);
> +}

You might want to try a straight 'return;' as well :)

> diff --git a/test/unittest/actions/return/err.D_PROTO_LEN.too_many_args.r b/test/unittest/actions/return/err.D_PROTO_LEN.too_many_args.r
> new file mode 100644
> index 00000000..37ff4d9d
> --- /dev/null
> +++ b/test/unittest/actions/return/err.D_PROTO_LEN.too_many_args.r
> @@ -0,0 +1,2 @@
> +-- @@stderr --
> +dtrace: failed to compile script test/unittest/actions/return/err.D_PROTO_LEN.too_many_args.d: [D_PROTO_LEN] line 18: return( ) prototype mismatch: 2 args passed, 1 expected
> diff --git a/test/unittest/actions/return/err.destructive.d b/test/unittest/actions/return/err.destructive.d
> new file mode 100644
> index 00000000..4c4a246a
> --- /dev/null
> +++ b/test/unittest/actions/return/err.destructive.d
> @@ -0,0 +1,26 @@
> +/*
> + * Oracle Linux DTrace.
> + * Copyright (c) 2025, 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.
> + */
> +
> +/*
> + * ASSERTION: return() is allowed when destructive execution is allowed

return() is forbidden when destructive execution is not allowed.