runtime: badmorestackg0 is never called on wasm

[mauri870]

While investigating crash stack on wasm, i found that badmorestackg0 is behind a condition that is never true.

go/src/runtime/asm_wasm.s

Lines 217 to 230 in d72f454

	TEXT runtime·morestack(SB), NOSPLIT, $0-0
	// R1 = g.m
	MOVD g_m(g), R1
	// R2 = g0
	MOVD m_g0(R1), R2
	// Cannot grow scheduler stack (m->g0).
	Get g
	Get R1
	I64Eq
	If
	CALLNORESUME runtime·badmorestackg0(SB)
	End

As you can see above, it is comparing g == R1, R1 being g.m. We should be comparing it to g0, which is stored in R2.

It is also present in 1.20 and 1.21, but I'm not sure if we need to backport this given that it is not a significant issue.

[mauri870]

Original behavior:

RuntimeError: memory access out of bounds
    at runtime.morestack (wasm://wasm/0370d90e:wasm-function[1436]:0x134e6f)
    at runtime.morestack_noctxt (wasm://wasm/0370d90e:wasm-function[1437]:0x134eab)
    at runtime.newstack (wasm://wasm/0370d90e:wasm-function[977]:0xe4596)
    at runtime.morestack (wasm://wasm/0370d90e:wasm-function[1436]:0x134e8d)
    at runtime.morestack_noctxt (wasm://wasm/0370d90e:wasm-function[1437]:0x134eab)
    at runtime.newstack (wasm://wasm/0370d90e:wasm-function[977]:0xe4596)
    at runtime.morestack (wasm://wasm/0370d90e:wasm-function[1436]:0x134e8d)
    at runtime.morestack_noctxt (wasm://wasm/0370d90e:wasm-function[1437]:0x134eab)
    at runtime.newstack (wasm://wasm/0370d90e:wasm-function[977]:0xe4596)
    at runtime.morestack (wasm://wasm/0370d90e:wasm-function[1436]:0x134e8d)

With the fix applied:

RuntimeError: memory access out of bounds
    at runtime.morestack (wasm://wasm/0370d90e:wasm-function[1436]:0x134e6f)
    at runtime.morestack_noctxt (wasm://wasm/0370d90e:wasm-function[1437]:0x134eab)
    at runtime.write1 (wasm://wasm/0370d90e:wasm-function[662]:0x9c78f)
    at runtime.write (wasm://wasm/0370d90e:wasm-function[1056]:0xfbefc)
    at runtime.badmorestackg0 (wasm://wasm/0370d90e:wasm-function[766]:0xad590)
    at runtime.morestack (wasm://wasm/0370d90e:wasm-function[1436]:0x134df1)
    at runtime.morestack_noctxt (wasm://wasm/0370d90e:wasm-function[1437]:0x134eab)
    at runtime.write1 (wasm://wasm/0370d90e:wasm-function[662]:0x9c78f)
    at runtime.write (wasm://wasm/0370d90e:wasm-function[1056]:0xfbefc)
    at runtime.badmorestackg0 (wasm://wasm/0370d90e:wasm-function[766]:0xad590)

[gopherbot]

Change https://go.dev/cl/539840 mentions this issue: runtime: fix badmorestackg0 never called on wasm