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fuse: Mix inode hashes in a non-symmetric way

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Since 0.15 (#4020), inodes are generated as hashes of names, xor'd with
the parent inode. That means that the inode of a/b/b is

	h(a/b/b) = h(a) ^ h(b) ^ h(b) = h(a).

I.e., the grandchild has the same inode as the grandparent. GNU find
trips over this because it thinks it has encountered a loop in the
filesystem, and fails to search a/b/b. This happens more generally when
the same name occurs an even number of times.

Fix this by multiplying the parent by a large prime, so the combining
operation is not longer symmetric in its arguments. This is what the FNV
hash does, which we used prior to 0.15. The hash is now

	h(a/b/b) = h(b) ^ p*(h(b) ^ p*h(a))

Note that we already ensure that h(x) is never zero.

Collisions can still occur, but they should be much less likely to occur
within a single path.

Fixes #4253.
This commit is contained in:
greatroar 2023-03-21 17:33:18 +01:00
parent f646406822
commit a0885d5d69
3 changed files with 33 additions and 2 deletions
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11
internal/fuse/fuse_test.go
View file

@ -226,6 +226,17 @@ func TestInodeFromNode(t *testing.T) {
ino1 = inodeFromNode(1, node)
ino2 = inodeFromNode(2, node)
rtest.Assert(t, ino1 != ino2, "same inode %d but different parent", ino1)
// Regression test: in a path a/b/b, the grandchild should not get the
// same inode as the grandparent.
a := &restic.Node{Name: "a", Type: "dir", Links: 2}
ab := &restic.Node{Name: "b", Type: "dir", Links: 2}
abb := &restic.Node{Name: "b", Type: "dir", Links: 2}
inoA := inodeFromNode(1, a)
inoAb := inodeFromNode(inoA, ab)
inoAbb := inodeFromNode(inoAb, abb)
rtest.Assert(t, inoA != inoAb, "inode(a/b) = inode(a)")
rtest.Assert(t, inoA != inoAbb, "inode(a/b/b) = inode(a)")
}
var sink uint64