linux-hardening - [RFC] Approaches to deal with a struct with multiple fake flexible arrays members

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Message-ID: <Y2siZmiTD40mTYpJ@mail.google.com>
Date:   Wed, 9 Nov 2022 16:45:42 +1300
From:   Paulo Miguel Almeida <paulo.miguel.almeida.rodenas@...il.com>
To:     linux-hardening@...r.kernel.org
Subject: [RFC] Approaches to deal with a struct with multiple fake flexible
 arrays members

Hi KSPP community,

I've been working on replacing 1-element arrays with flex array members
on the drm/amdgpu files. I came across one insteresting case which I
may need to pick your brains to find a solution for it.

The structure below has two fake flexible arrays but I would get an
error if I try make them both FAM. How should/could I deal with the
asRegIndexBuf in this case? In theory, DECLARE_FLEX_ARRAY would "work"
but that doesn't seem to be its intended usage as far I've searched.
(unless I got it wrong, if that's the case, feel free to set me straight)

Any ideas? 

struct _ATOM_INIT_REG_BLOCK {
	USHORT                     usRegIndexTblSize;    /*     0     2 */
	USHORT                     usRegDataBlkSize;     /*     2     2 */
	ATOM_INIT_REG_INDEX_FORMAT asRegIndexBuf[1];     /*     4     3 */
	ATOM_MEMORY_SETTING_DATA_BLOCK asRegDataBuf[1];  /*     7     8 */

	/* size: 15, cachelines: 1, members: 4 */
	/* last cacheline: 15 bytes */
} __attribute__((__packed__));

thanks!

- Paulo A.