linux-kernel - Re: [PATCH v3 1/3] x86/uprobes: Fix not using prefixes.nbytes for loop over prefixes.bytes

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Message-ID: <20201204150522.GG31534@zn.tnic>
Date:   Fri, 4 Dec 2020 16:05:22 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Masami Hiramatsu <mhiramat@...nel.org>
Cc:     x86@...nel.org, Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        "H . Peter Anvin" <hpa@...or.com>, Joerg Roedel <jroedel@...e.de>,
        Tom Lendacky <thomas.lendacky@....com>,
        "Gustavo A . R . Silva" <gustavoars@...nel.org>,
        Jann Horn <jannh@...gle.com>,
        Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
        Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 1/3] x86/uprobes: Fix not using prefixes.nbytes for
 loop over prefixes.bytes

On Fri, Dec 04, 2020 at 07:55:20PM +0900, Masami Hiramatsu wrote:
> +/**
> + * for_each_insn_prefix() -- Iterate prefixes in the instruction
> + * @insn: Pointer to struct insn.
> + * @idx:  Index storage.
> + * @prefix: Prefix byte.
> + *
> + * Iterate prefix bytes of given @insn. Each prefix byte is stored in @prefix
> + * and the index is stored in @idx (note that this @idx is just for a cursor,
> + * do not change it.)
> + * Since prefixes.nbytes can be bigger than NUM_INSN_FIELD_BYTES when some
> + * prefixes are repeated, it can not be used for looping over the prefixes.
> + */
> +#define for_each_insn_prefix(insn, idx, prefix)				\
> +	for (idx = 0;							\
> +	     idx < MAX_LEGACY_PREFIX_GROUPS &&				\

The problem I see here is that you check for the index limit to be
< MAX_LEGACY_PREFIX_GROUPS but the array itself is defined using
NUM_INSN_FIELD_BYTES, and that is confusing.

I guess this should be:

#define MAX_LEGACY_PREFIX_GROUPS	4
#define NUM_INSN_FIELD_BYTES		MAX_LEGACY_PREFIX_GROUPS

and later, iff the legacy prefixes array size needs separating from the
insn field array size, then the defines would need to change too.

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette