linux-kernel - Re: [PATCH resend] fs/namei.c: micro-optimize acl_permission

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Message-ID: <20200608020522.GN23230@ZenIV.linux.org.uk>
Date:   Mon, 8 Jun 2020 03:05:22 +0100
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Rasmus Villemoes <linux@...musvillemoes.dk>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH resend] fs/namei.c: micro-optimize acl_permission_check

On Sun, Jun 07, 2020 at 12:48:53PM -0700, Linus Torvalds wrote:

> Rasmus, say the word and I'll mark you for authorship on the first one.
> 
> Comments? Can you find something else wrong here, or some other fixup to do?
> 
> Al, any reaction?

It's correct, but this

> +	if (mask & (mode ^ (mode >> 3))) {
> +		if (in_group_p(inode->i_gid))
> +			mode >>= 3;
> +	}
> +
> +	/* Bits in 'mode' clear that we require? */
> +	return (mask & ~mode) ? -EACCES : 0;

might be easier to follow if we had, from the very beginning done
	unsigned int deny = ~inode->i_mode;
and turned that into

	// for group the bits 3..5 apply, for others - 0..2
	// we only care which to use when they do not
	// agree anyway.
	if (mask & (deny ^ (deny >> 3))) // mask & deny != mask & (deny >> 3)
		if (in_...
			deny >>= 3;
	return mask & deny ? -EACCES : 0;

Hell knows...