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Start for freeriscv: trace: fix snapshot deadlock with sbi ecall
In the Linux kernel, the following vulnerability has been resolved:
riscv: trace: fix snapshot deadlock with sbi ecall
If sbi_ecall.c's functions are traceable,
echo "__sbi_ecall:snapshot" > /sys/kernel/tracing/set_ftrace_filter
may get the kernel into a deadlock.
(Functions in sbi_ecall.c are excluded from tracing if CONFIG_RISCV_ALTERNATIVE_EARLY is set.)
__sbi_ecall triggers a snapshot of the ringbuffer. The snapshot code raises an IPI interrupt, which results in another call to __sbi_ecall and another snapshot...
All it takes to get into this endless loop is one initial __sbi_ecall. On RISC-V systems without SSTC extension, the clock events in timer-riscv.c issue periodic sbi ecalls, making the problem easy to trigger.
Always exclude the sbi_ecall.c functions from tracing to fix the potential deadlock.
sbi ecalls can easiliy be logged via trace events, excluding ecall functions from function tracing is not a big limitation.