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authorDavid Oberhollenzer <david.oberhollenzer@sigma-star.at>2021-07-21 09:41:47 +0200
committerDavid Oberhollenzer <david.oberhollenzer@sigma-star.at>2021-07-21 09:56:31 +0200
commit248494992442fbde7eb6ca3979a778d82fa86016 (patch)
tree3ba8d5100aef7cf0d1f1aeae9f306fd5e6053503 /include/sqfs/inode.h
parent5333fbe46bcbf70b4888bcc6655681f2cd0f161b (diff)
Fix libsquashfs directory writer size accounting
The squashfs readdir() implementation in the Linux kernel returns non-existing "." and ".." entries for offsets 0 and 1, and after that reads from disk. For convenience, it was decided to store an off-by-3 value on disk instead of doing complex primary school math to adjust for this. This didn't show up until now, because the kernel implementation trusts the value from the directory header more than the actual size in the inode and happily reads 3 more than the inode would allow it to. This only showed up with 7-zip which subtracts 3 from the size and expects the result to be exact and bails if the directory headers suggest otherwise. And yes, I did consider making a "Holy Hand Granade of Antioch" reference, but consciously decided not to. Signed-off-by: David Oberhollenzer <david.oberhollenzer@sigma-star.at>
Diffstat (limited to 'include/sqfs/inode.h')
-rw-r--r--include/sqfs/inode.h11
1 files changed, 10 insertions, 1 deletions
diff --git a/include/sqfs/inode.h b/include/sqfs/inode.h
index 0bc33db..8df7158 100644
--- a/include/sqfs/inode.h
+++ b/include/sqfs/inode.h
@@ -400,6 +400,15 @@ struct sqfs_inode_dir_t {
/**
* @brief Combined size of all directory entries and headers in bytes.
+ *
+ * The value stored here is off by 3 bytes, i.e. 3 bytes larger than
+ * the actual listing on disk. The Linux implementation of SquashFS
+ * uses readdir() offsets 0 and 1 to synthesize "." and ".." entries,
+ * and after that looks into the actual directory.
+ *
+ * Why store an off-by-3 value on disk instead of faking it in the
+ * kernel and have something consistent here? Beats me, but that's
+ * how it is implemented.
*/
sqfs_u16 size;
@@ -428,7 +437,7 @@ struct sqfs_inode_dir_ext_t {
sqfs_u32 nlink;
/**
- * @brief Combined size of all directory entries and headers in bytes.
+ * @brief Size of all directory entries and headers in bytes plus 3.
*/
sqfs_u32 size;