linux-ext4 - Re: [PATCH 01/17] mm: introduce MAP_SHARED_VALIDATE, a mechanism to safely define new mmap flags

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Message-ID: <20171020072707.GA18000@infradead.org>
Date:   Fri, 20 Oct 2017 00:27:07 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Jan Kara <jack@...e.cz>
Cc:     linux-fsdevel@...r.kernel.org,
        Christoph Hellwig <hch@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Arnd Bergmann <arnd@...db.de>, linux-nvdimm@...ts.01.org,
        linux-api@...r.kernel.org, linux-xfs@...r.kernel.org,
        Andy Lutomirski <luto@...nel.org>, linux-ext4@...r.kernel.org
Subject: Re: [PATCH 01/17] mm: introduce MAP_SHARED_VALIDATE, a mechanism to
 safely define new mmap flags

>  	if (file) {
>  		struct inode *inode = file_inode(file);
> +		unsigned long flags_mask = file->f_op->mmap_supported_flags;
> +
> +		if (!flags_mask)
> +			flags_mask = LEGACY_MAP_MASK;
>  
>  		switch (flags & MAP_TYPE) {
>  		case MAP_SHARED:
> +			/*
> +			 * Silently ignore unsupported flags - MAP_SHARED has
> +			 * traditionally behaved like that and we don't want
> +			 * to break compatibility.
> +			 */
> +			flags &= flags_mask;
> +			/*
> +			 * Force use of MAP_SHARED_VALIDATE with non-legacy
> +			 * flags. E.g. MAP_SYNC is dangerous to use with
> +			 * MAP_SHARED as you don't know which consistency model
> +			 * you will get.
> +			 */
> +			flags &= LEGACY_MAP_MASK;
> +			/* fall through */
> +		case MAP_SHARED_VALIDATE:
> +			if (flags & ~flags_mask)
> +				return -EOPNOTSUPP;

Hmmm.  I'd expect this to worth more like:

		case MAP_SHARED:
			/* Ignore all new flags that need validation: */
			flags &= LEGACY_MAP_MASK;
			/*FALLTHROUGH*/
		case MAP_SHARED_VALIDATE:
			if (flags & ~file->f_op->mmap_supported_flags)
				return -EOPNOTSUPP;

with the legacy mask always implicitly support as indicated in my
comment to the XFS patch.

Although even the ignoring in MAP_SHARED seems dangerous, but I guess
we need that to keep strict backwards compatibility.  In world I'd
rather do

	case MAP_SHARED:
		if (flags & ~LEGACY_MAP_MASK)
			return -EINVAL;