fs/sysv: v7: adjust sanity checks for some volumes

Newly mkfs-ed filesystems from Seventh Edition have last modification time
set to zero, but are otherwise perfectly valid.

Also, tighten up other sanity checks to filter out most filesystems with

[[e-mail]: coding-style fixes]
Signed-off-by: Lubomir Rintel <[e-mail]>
Cc: Christoph Hellwig <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

fs/sysv/super.c

···
 	v7sb = (struct v7_super_block *) bh->b_data;
 	if (fs16_to_cpu(sbi, v7sb->s_nfree) > V7_NICFREE ||
 	    fs16_to_cpu(sbi, v7sb->s_ninode) > V7_NICINOD ||
-	    fs32_to_cpu(sbi, v7sb->s_time) == 0)
+	    fs32_to_cpu(sbi, v7sb->s_fsize) > V7_MAXSIZE)
 		goto failed;
 
 	/* plausibility check on root inode: it is a directory,
···
 	v7i = (struct sysv_inode *)(bh2->b_data + 64);
 	if ((fs16_to_cpu(sbi, v7i->i_mode) & ~0777) != S_IFDIR ||
 	    (fs32_to_cpu(sbi, v7i->i_size) == 0) ||
-	    (fs32_to_cpu(sbi, v7i->i_size) & 017) != 0)
+	    (fs32_to_cpu(sbi, v7i->i_size) & 017) ||
+	    (fs32_to_cpu(sbi, v7i->i_size) > V7_NFILES *
+	     sizeof(struct sysv_dir_entry)))
 		goto failed;
 	brelse(bh2);
 	bh2 = NULL;

include/linux/sysv_fs.h

···
 	char    s_fname[6];     /* file system name */
 	char    s_fpack[6];     /* file system pack name */
 };
+/* Constants to aid sanity checking */
+/* This is not a hard limit, nor enforced by v7 kernel. It's actually just
+ * the limit used by Seventh Edition's ls, though is high enough to assume
+ * that no reasonable file system would have that much entries in root
+ * directory. Thus, if we see anything higher, we just probably got the
+ * endiannes wrong. */
+#define V7_NFILES	1024
+/* The disk addresses are three-byte (despite direct block addresses being
+ * aligned word-wise in inode). If the most significant byte is non-zero,
+ * something is most likely wrong (not a filesystem, bad bytesex). */
+#define V7_MAXSIZE	0x00ffffff
 
 /* Coherent super-block data on disk */
 #define COH_NICINOD	100	/* number of inode cache entries */