Subject: Re: [PATCH 04/64] stddef: Introduce struct_group() helper macro

From: Kees Cook <[email-masked]>
Date: Fri, 30 Jul 2021 19:59:04 -0700

On Fri, Jul 30, 2021 at 10:19:20PM +0000, Williams, Dan J wrote:
> On Wed, 2021-07-28 at 14:59 -0700, Kees Cook wrote:
> > On Wed, Jul 28, 2021 at 12:54:18PM +0200, Rasmus Villemoes wrote:
> > > On 27/07/2021 22.57, Kees Cook wrote:
> > > 
> > > > In order to have a regular programmatic way to describe a struct
> > > > region that can be used for references and sizing, can be examined for
> > > > bounds checking, avoids forcing the use of intermediate identifiers,
> > > > and avoids polluting the global namespace, introduce the struct_group()
> > > > macro. This macro wraps the member declarations to create an anonymous
> > > > union of an anonymous struct (no intermediate name) and a named struct
> > > > (for references and sizing):
> > > > 
> > > >         struct foo {
> > > >                 int one;
> > > >                 struct_group(thing,
> > > >                         int two,
> > > >                         int three,
> > > >                 );
> > > >                 int four;
> > > >         };
> > > 
> > > That example won't compile, the commas after two and three should be
> > > semicolons.
> > 
> > Oops, yes, thanks. This is why I shouldn't write code that doesn't first
> > go through a compiler. ;)
> > 
> > > And your implementation relies on MEMBERS not containing any comma
> > > tokens, but as
> > > 
> > >   int a, b, c, d;
> > > 
> > > is a valid way to declare multiple members, consider making MEMBERS
> > > variadic
> > > 
> > > #define struct_group(NAME, MEMBERS...)
> > > 
> > > to have it slurp up every subsequent argument and make that work.
> > 
> > Ah! Perfect, thank you. I totally forgot I could do it that way.
> 
> This is great Kees. It just so happens it would clean-up what we are
> already doing in drivers/cxl/cxl.h for anonymous + named register block
> pointers. However in the cxl case it also needs the named structure to
> be typed. Any appetite for a typed version of this?

Oh cool! Yeah, totally I can expand it. Thanks for the suggestion!

> 
> Here is a rough idea of the cleanup it would induce in drivers/cxl/:
> 
> diff --git a/drivers/cxl/cxl.h b/drivers/cxl/cxl.h
> index 53927f9fa77e..a2308c995654 100644
> --- a/drivers/cxl/cxl.h
> +++ b/drivers/cxl/cxl.h
> @@ -75,52 +75,19 @@ static inline int cxl_hdm_decoder_count(u32 cap_hdr)
>  #define CXLDEV_MBOX_BG_CMD_STATUS_OFFSET 0x18
>  #define CXLDEV_MBOX_PAYLOAD_OFFSET 0x20
>  
> -#define CXL_COMPONENT_REGS() \
> -       void __iomem *hdm_decoder
> -
> -#define CXL_DEVICE_REGS() \
> -       void __iomem *status; \
> -       void __iomem *mbox; \
> -       void __iomem *memdev
> -
> -/* See note for 'struct cxl_regs' for the rationale of this organization */
>  /*
> - * CXL_COMPONENT_REGS - Common set of CXL Component register block base pointers
>   * @hdm_decoder: CXL 2.0 8.2.5.12 CXL HDM Decoder Capability Structure
> - */
> -struct cxl_component_regs {
> -       CXL_COMPONENT_REGS();
> -};
> -
> -/* See note for 'struct cxl_regs' for the rationale of this organization */
> -/*
> - * CXL_DEVICE_REGS - Common set of CXL Device register block base pointers
>   * @status: CXL 2.0 8.2.8.3 Device Status Registers
>   * @mbox: CXL 2.0 8.2.8.4 Mailbox Registers
>   * @memdev: CXL 2.0 8.2.8.5 Memory Device Registers
>   */
> -struct cxl_device_regs {
> -       CXL_DEVICE_REGS();
> -};
> -
> -/*
> - * Note, the anonymous union organization allows for per
> - * register-block-type helper routines, without requiring block-type
> - * agnostic code to include the prefix.
> - */
>  struct cxl_regs {
> -       union {
> -               struct {
> -                       CXL_COMPONENT_REGS();
> -               };
> -               struct cxl_component_regs component;
> -       };
> -       union {
> -               struct {
> -                       CXL_DEVICE_REGS();
> -               };
> -               struct cxl_device_regs device_regs;
> -       };
> +       struct_group_typed(cxl_component_regs, component,
> +               void __iomem *hdm_decoder;
> +       );
> +       struct_group_typed(cxl_device_regs, device_regs,
> +               void __iomem *status, *mbox, *memdev;
> +       );
>  };
>  
>  struct cxl_reg_map {
> diff --git a/include/linux/stddef.h b/include/linux/stddef.h
> index cf7f866944f9..84b7de24ffb5 100644
> --- a/include/linux/stddef.h
> +++ b/include/linux/stddef.h
> @@ -49,12 +49,18 @@ enum {
>   * @ATTRS: Any struct attributes (normally empty)
>   * @MEMBERS: The member declarations for the mirrored structs
>   */
> -#define struct_group_attr(NAME, ATTRS, MEMBERS) \
> +#define struct_group_attr(NAME, ATTRS, MEMBERS...) \
>         union { \
>                 struct { MEMBERS } ATTRS; \
>                 struct { MEMBERS } ATTRS NAME; \
>         }
>  
> +#define struct_group_attr_typed(TYPE, NAME, ATTRS, MEMBERS...) \
> +       union { \
> +               struct { MEMBERS } ATTRS; \
> +               struct TYPE { MEMBERS } ATTRS NAME; \
> +       }
> +
>  /**
>   * struct_group(NAME, MEMBERS)
>   *
> @@ -67,7 +73,10 @@ enum {
>   * @NAME: The name of the mirrored sub-struct
>   * @MEMBERS: The member declarations for the mirrored structs
>   */
> -#define struct_group(NAME, MEMBERS)    \
> +#define struct_group(NAME, MEMBERS...) \
>         struct_group_attr(NAME, /* no attrs */, MEMBERS)
>  
> +#define struct_group_typed(TYPE, NAME, MEMBERS...) \
> +       struct_group_attr_typed(TYPE, NAME, /* no attrs */, MEMBERS)
> +
>  #endif

Awesome! My instinct is to expose the resulting API as:

__struct_group(type, name, attrs, members...)

struct_group(name, members...)
struct_group_attr(name, attrs, members...)
struct_group_typed(type, name, members...)

-- 
Kees Cook