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When the MB_CHECK_ASSERT macro is enabled, we found that the
current validation logic in __mb_check_buddy has a gap in
detecting certain invalid buddy states, particularly related
to order-0 (bitmap) bits.
The original logic consists of three steps:
1. Validates higher-order buddies: if a higher-order bit is
set, at most one of the two corresponding lower-order bits
may be free; if a higher-order bit is clear, both lower-order
bits must be allocated (and their bitmap bits must be 0).
2. For any set bit in order-0, ensures all corresponding
higher-order bits are not free.
3. Verifies that all preallocated blocks (pa) in the group
have pa_pstart within bounds and their bitmap bits marked as
allocated.
However, this approach fails to properly validate cases where
order-0 bits are incorrectly cleared (0), allowing some invalid
configurations to pass:
corrupt integral
order 3 1 1
order 2 1 1 1 1
order 1 1 1 1 1 1 1 1 1
order 0 0 0 1 1 1 1 1 1 1 1 1 1 1 1 1 1
Here we get two adjacent free blocks at order-0 with inconsistent
higher-order state, and the right one shows the correct scenario.
The root cause is insufficient validation of order-0 zero bits.
To fix this and improve completeness without significant performance
cost, we refine the logic:
1. Maintain the top-down higher-order validation, but we no longer
check the cases where the higher-order bit is 0, as this case will
be covered in step 2.
2. Enhance order-0 checking by examining pairs of bits:
- If either bit in a pair is set (1), all corresponding
higher-order bits must not be free.
- If both bits are clear (0), then exactly one of the
corresponding higher-order bits must be free
3. Keep the preallocation (pa) validation unchanged.
This change closes the validation gap, ensuring illegal buddy states
involving order-0 are correctly detected, while removing redundant
checks and maintaining efficiency.
Fixes: c9de560ded ("ext4: Add multi block allocator for ext4")
Suggested-by: Jan Kara <jack@suse.cz>
Signed-off-by: Yongjian Sun <sunyongjian1@huawei.com>
Reviewed-by: Baokun Li <libaokun1@huawei.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Message-ID: <20251106060614.631382-3-sunyongjian@huaweicloud.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the reStructuredText markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.