xcp.ustkptr is only assigned once in up_initial_state() when a task is
created and, since the syscall fast path was optimized in
e6973c764c, is never updated afterwards. up_backtrace() used
"ustkptr != NULL" to decide whether a task is currently blocked
inside a syscall, and *(ustkptr + 1) as the frame pointer to resume
tracing from. Since ustkptr is now a dead value fixed at task
creation time, the check is always true and the "frame pointer" it
derives points at stale data near the initial stack top, unrelated
to where the task is actually blocked.
Use rtcb->flags & TCB_FLAG_SYSCALL together with xcp.sregs, which
dispatch_syscall() maintains precisely across the entire syscall
execution window (including any nested context switches caused by
blocking), to locate the frame pointer/return address saved at
syscall entry instead.
Signed-off-by: liang.huang <liang.huang@houmo.ai>
bdfa666f3e