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dax: fix NULL pointer dereference in __dax_dbg()

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In __dax_pmd_fault() we currently assume that get_block() will always
set bh.b_bdev and we unconditionally dereference it in __dax_dbg().

This assumption isn't always true - when called for reads of holes
ext4_dax_mmap_get_block() returns a buffer head where bh->b_bdev is
never set.  I hit this BUG while testing the DAX PMD fault path.

Instead, initialize bh.b_bdev before passing bh into get_block().  It is
possible that the filesystem's get_block() will update bh.b_bdev, and
this is fine - we just want to initialize bh.b_bdev to something
reasonable so that the calls to __dax_dbg() work and print something
useful.

Signed-off-by: Ross Zwisler <[email protected]>
Reported-by: Dan Williams <[email protected]>
Cc: Jan Kara <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>
This commit is contained in:
Ross Zwisler 2016-01-22 15:10:31 -08:00 • committed by Linus Torvalds
commit d4bbe7068b
1 changed files with 1 additions and 0 deletions
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fs/dax.c
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@ -624,6 +624,7 @@ int __dax_pmd_fault(struct vm_area_struct *vma, unsigned long address,
}
memset(&bh, 0, sizeof(bh));
bh.b_bdev = inode->i_sb->s_bdev;
block = (sector_t)pgoff << (PAGE_SHIFT - blkbits);
bh.b_size = PMD_SIZE;