CVE-2026-80734: btrfs: initialize inode mapping flags for cached inodes

In the Linux kernel, the following vulnerability has been resolved:

btrfs: initialize inode mapping flags for cached inodes

[BUG] When running generic/795 with 8K block size, 4K page size, the test always fails, triggering some ASSERT()s related to folio size:

795 (241074): dropcaches: 3 assertion failed: ISALIGNED(start, blocksize) && ISALIGNED(end + 1, blocksize), in extentio.c:1404 (blocksize=8192 root=262 ino=258 start=16826368 end=16830463 mapping min order=0) ------------[ cut here ]------------ kernel BUG at extentio.c:1404! Oops: invalid opcode: 0000 [#1] SMP CPU: 8 UID: 0 PID: 241105 Comm: fsstress Tainted: G OE 7.2.0-rc5-custom+ #442 PREEMPT(full) f4bfb352566f3949f29c233ce6f735050a03b245 Tainted: [O]=OOTMODULE, [E]=UNSIGNEDMODULE Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS unknown 02/02/2022 RIP: 0010:assertfoliorange.cold+0x3d/0x3f [btrfs] Call Trace: <TASK> btrfsreadfolio+0x9e/0x170 [btrfs 4cd1dd93b341b8ef766643f9512f4a86259567a3] prepareonefolio.constprop.0+0x104/0x2a0 [btrfs 4cd1dd93b341b8ef766643f9512f4a86259567a3] btrfsbufferedwrite+0x285/0xa50 [btrfs 4cd1dd93b341b8ef766643f9512f4a86259567a3] btrfsdowriteiter+0x1aa/0x210 [btrfs 4cd1dd93b341b8ef766643f9512f4a86259567a3] iterfilesplicewrite+0x31a/0x540 directspliceactor+0x53/0x170 splicedirecttoactor+0xe9/0x240 dosplicedirect+0x76/0xb0 vfscopyfilerange+0x1fd/0x630 x64syscopyfilerange+0xf9/0x220 dosyscall64+0xe1/0x790 entrySYSCALL64afterhwframe+0x4b/0x53 </TASK> ---[ end trace 0000000000000000 ]---

The ASSERT() itself is added by a later patch. The crash is triggered with that new debug patch, and without this fix.

[CAUSE] In the above case, the start 16826368 is properly 8K aligned, but the end (16830463 + 1) is not 8K aligned. Furthermore the mapping's minimal folio order is 0, not the expected 1 for 8K block size with 4K page size.

So this means some inodes do not have btrfssetinodemappingorder() called on it.

The missing btrfssetinodemappingorder() call happens for cached inodes, through the following events:

- btrfscreatenewinode() called for inode X Which properly sets minimal folio order for the VFS inode.

- btrfsupdateinode() called for inode X Which calls btrfsdelayedupdateinode() to create a delayednode into root->delayednodes xarray.

- Drop cache/memory pressure, evicting in-memory inode X Which evicted the inode X, but delayednode is still in root->delayednodes for future reuse.

- btrfsiget() for inode X called again

btrfsiget() |- btrfsigetlocked() | |- iget5lockedrcu() | Which creates a new vfsinode for btrfs, whose mapping still | has the minimal order as 0. | |- btrfsreadlockedinode() |- btrfsfillinode() | |- btrfsgetdelayednode() | Which found out the previous node, and use that delayed | node to initialize the new inode. | |- filled = true; |- if (filled) goto cacheindex; Which skips the btrfsupdateinodemappingflags() and btrfssetinodemappingorder() calls. So the inode still has minimal folio order set as 0, not the required 1.

Thus later page cache read will get a folio whose size is smaller than block size, as the mapping has its minimal folio order set as 0 not 1, then trigger the ASSERT().

[FIX] Move the btrfsupdateinodemappingflags() and btrfssetinodemappingorder() calls under cacheindex label, so that the mapping flags and minimal folio order is always set no matter if we have a cached inode.