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Andy Chiu
9ddfc3cd80
Runtime code patching must be done at a naturally aligned address, or we
may execute on a partial instruction.
We have encountered problems traced back to static jump functions during
the test. We switched the tracer randomly for every 1~5 seconds on a
dual-core QEMU setup and found the kernel sucking at a static branch
where it jumps to itself.
The reason is that the static branch was 2-byte but not 4-byte aligned.
Then, the kernel would patch the instruction, either J or NOP, with two
half-word stores if the machine does not have efficient unaligned
accesses. Thus, moments exist where half of the NOP mixes with the other
half of the J when transitioning the branch. In our particular case, on
a little-endian machine, the upper half of the NOP was mixed with the
lower part of the J when enabling the branch, resulting in a jump that
jumped to itself. Conversely, it would result in a HINT instruction when
disabling the branch, but it might not be observable.
ARM64 does not have this problem since all instructions must be 4-byte
aligned.
Fixes: ebc00dde8a ("riscv: Add jump-label implementation")
Link: https://lore.kernel.org/linux-riscv/20220913094252.3555240-6-andy.chiu@sifive.com/
Reviewed-by: Greentime Hu <greentime.hu@sifive.com>
Signed-off-by: Andy Chiu <andy.chiu@sifive.com>
Signed-off-by: Guo Ren <guoren@kernel.org>
Link: https://lore.kernel.org/r/20230206090440.1255001-1-guoren@kernel.org
Cc: stable@vger.kernel.org
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>
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 Restructured Text 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.