add new system file

hc

2024-02-20 102a0743326a03cd1a1202ceda21e175b7d3575c

 kernel/fs/verity/enable.c
..
..
@@ -391,25 +391,27 @@
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 		goto out_drop_write;
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 	err = enable_verity(filp, &arg);
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-	if (err)
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-		goto out_allow_write_access;
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 	/*
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-	 * Some pages of the file may have been evicted from pagecache after
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-	 * being used in the Merkle tree construction, then read into pagecache
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-	 * again by another process reading from the file concurrently.  Since
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-	 * these pages didn't undergo verification against the file digest which
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-	 * fs-verity now claims to be enforcing, we have to wipe the pagecache
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-	 * to ensure that all future reads are verified.
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+	 * We no longer drop the inode's pagecache after enabling verity.  This
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+	 * used to be done to try to avoid a race condition where pages could be
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+	 * evicted after being used in the Merkle tree construction, then
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+	 * re-instantiated by a concurrent read.  Such pages are unverified, and
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+	 * the backing storage could have filled them with different content, so
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+	 * they shouldn't be used to fulfill reads once verity is enabled.
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+	 *
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+	 * But, dropping the pagecache has a big performance impact, and it
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+	 * doesn't fully solve the race condition anyway.  So for those reasons,
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+	 * and also because this race condition isn't very important relatively
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+	 * speaking (especially for small-ish files, where the chance of a page
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+	 * being used, evicted, *and* re-instantiated all while enabling verity
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+	 * is quite small), we no longer drop the inode's pagecache.
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 	 */
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-	filemap_write_and_wait(inode->i_mapping);
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-	invalidate_inode_pages2(inode->i_mapping);
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 	/*
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 	 * allow_write_access() is needed to pair with deny_write_access().
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 	 * Regardless, the filesystem won't allow writing to verity files.
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 	 */
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-out_allow_write_access:
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 	allow_write_access(filp);
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 out_drop_write:
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 	mnt_drop_write_file(filp);