frame point trace is in loop

[wangwss]

when I use rust write os in qemu, I wrote a stack trace function to resolve frame point trace, but found qemu in loop.
my code is like this:
pub unsafe fn print_stack_trace() {
let mut fp: *const usize;

asm!("mv {}, fp", out(reg) fp);

let mut saved_ra = *fp.sub(1);
let mut saved_fp = *fp.sub(2);

println!("== Begin stack trace ==");

while (!fp.is_null() && saved_fp != 0) || (saved_ra == *fp) {
    println!("0x{:016x}, fp = 0x{:016x}", saved_ra, saved_fp);

    fp = saved_fp as *const usize;

    saved_ra = *fp.sub(1);
    saved_fp = *fp.sub(2);
}

println!("== End stack trace ==");

}

but with "-bios opensbi.bin" ,the console out like this:
ra =0x0000000080200010, fp = 0x000000008020fa60
ra =0x0000000080210158, fp = 0x000000008020fcd0
ra =0x0000000080213c3c, fp = 0x000000008020fd10
ra =0x0000000080212190, fp = 0x0000000080210010
ra =0x0000000080200010, fp = 0x000000008020fa60
.........
when I use "-bios rustsbi.bin" (version 0.0.4") run the same code ,the console out like this:
== Begin stack trace ==
0x0000000080210158, fp = 0x000000008020fcd0
0x0000000080213c3c, fp = 0x000000008020fd10
0x0000000080212190, fp = 0x0000000080210010
== End stack trace ==

the result is correct.

Then I write out the ra & fp ,found when
use rustsbi : ra = 0x0000000080210010,fp=0,
so that the code is correct over.
but use opensbi :ra =0x0000000080200010, fp = 0x000000008020fa60
fp is not null, so that is in loop.

where opensbi code has bug?