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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:
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| .. | ||
| alpha | ||
| arc | ||
| arm | ||
| arm64 | ||
| csky | ||
| hexagon | ||
| ia64 | ||
| loongarch | ||
| m68k | ||
| microblaze | ||
| mips | ||
| nios2 | ||
| openrisc | ||
| parisc | ||
| powerpc | ||
| riscv | ||
| s390 | ||
| sh | ||
| sparc | ||
| um | ||
| x86 | ||
| xtensa | ||
| .gitignore | ||
| Kconfig | ||