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Subject[PATCH v2 2/6] Btrfs: remove all subvol options before mounting top-level
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Currently, setup_root_args() substitutes 's/subvol=[^,]*/subvolid=0/'.
But, this means that if the user passes both a subvol and subvolid for
some reason, we won't actually mount the top-level when we recursively
mount. For example, consider:

mkfs.btrfs -f /dev/sdb
mount /dev/sdb /mnt
btrfs subvol create /mnt/subvol1 # subvolid=257
btrfs subvol create /mnt/subvol2 # subvolid=258
umount /mnt
mount -osubvol=/subvol1,subvolid=258 /dev/sdb /mnt

In the final mount, subvol=/subvol1,subvolid=258 becomes
subvolid=0,subvolid=258, and the last option takes precedence, so we
mount subvol2 and try to look up subvol1 inside of it, which fails.

So, instead, do a thorough scan through the argument list and remove any
subvol= and subvolid= options, then append subvolid=0 to the end. This
implicitly makes subvol= take precedence over subvolid=, but we're about
to add a stricter check for that. This also makes setup_root_args() more
generic, which we'll need soon.

Signed-off-by: Omar Sandoval <osandov@osandov.com>
---
fs/btrfs/super.c | 56 ++++++++++++++++++++------------------------------------
1 file changed, 20 insertions(+), 36 deletions(-)

diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
index d38be09..bac3c9a 100644
--- a/fs/btrfs/super.c
+++ b/fs/btrfs/super.c
@@ -1129,52 +1129,36 @@ static inline int is_subvolume_inode(struct inode *inode)
}

/*
- * This will strip out the subvol=%s argument for an argument string and add
- * subvolid=0 to make sure we get the actual tree root for path walking to the
- * subvol we want.
+ * This will add subvolid=0 to the argument string while removing any subvol=
+ * and subvolid= arguments to make sure we get the top-level root for path
+ * walking to the subvol we want.
*/
static char *setup_root_args(char *args)
{
- unsigned len = strlen(args) + 2 + 1;
- char *src, *dst, *buf;
+ char *buf, *dst, *sep;

- /*
- * We need the same args as before, but with this substitution:
- * s!subvol=[^,]+!subvolid=0!
- *
- * Since the replacement string is up to 2 bytes longer than the
- * original, allocate strlen(args) + 2 + 1 bytes.
- */
-
- src = strstr(args, "subvol=");
- /* This shouldn't happen, but just in case.. */
- if (!src)
- return NULL;
+ if (!args)
+ return kstrdup("subvolid=0", GFP_NOFS);

- buf = dst = kmalloc(len, GFP_NOFS);
+ /* The worst case is that we add ",subvolid=0" to the end. */
+ buf = dst = kmalloc(strlen(args) + strlen(",subvolid=0") + 1, GFP_NOFS);
if (!buf)
return NULL;

- /*
- * If the subvol= arg is not at the start of the string,
- * copy whatever precedes it into buf.
- */
- if (src != args) {
- *src++ = '\0';
- strcpy(buf, args);
- dst += strlen(args);
+ while (1) {
+ sep = strchrnul(args, ',');
+ if (!strstarts(args, "subvol=") &&
+ !strstarts(args, "subvolid=")) {
+ memcpy(dst, args, sep - args);
+ dst += sep - args;
+ *dst++ = ',';
+ }
+ if (*sep)
+ args = sep + 1;
+ else
+ break;
}
-
strcpy(dst, "subvolid=0");
- dst += strlen("subvolid=0");
-
- /*
- * If there is a "," after the original subvol=... string,
- * copy that suffix into our buffer. Otherwise, we're done.
- */
- src = strchr(src, ',');
- if (src)
- strcpy(dst, src);

return buf;
}
--
2.3.5


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