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Fix thread_profile_frames crashing due to uninitialized PC

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ZJIT never sets `cfp->jit_return`, so to avoid crashing while
profiling, we need to explicitly validate the PC of the top most frame.
Particularly pertinent for profilers that call rb_profile_frames() from
within a signal handler such as Vernier and Stackprof since they
can sample at any time and observe an invalid PC.
This commit is contained in:
Alan Wu authored and GitHub committed 2025-09-23 16:20:04 -04:00
1 parent 39d764ed80
commit 06b7a70837
1 file changed
+13 -6
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vm_backtrace.c
+13 -6
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@@ -1761,15 +1761,22 @@ thread_profile_frames(rb_execution_context_t *ec, int start, int limit, VALUE *b
}
if (lines) {
// The topmost frame may not have an updated PC because the JIT
// may not have set one. The JIT compiler will update the PC
// before entering a new function (so that `caller` will work),
// so only the topmost frame could possibly have an out of date PC
if (cfp == top && cfp->jit_return) {
const VALUE *pc = cfp->pc;
VALUE *iseq_encoded = ISEQ_BODY(cfp->iseq)->iseq_encoded;
VALUE *pc_end = iseq_encoded + ISEQ_BODY(cfp->iseq)->iseq_size;
// The topmost frame may have an invalid PC because the JIT
// may leave it uninitialized for speed. JIT code must update the PC
// before entering a non-leaf method (so that `caller` will work),
// so only the topmost frame could possibly have an out-of-date PC.
// ZJIT doesn't set `cfp->jit_return`, so it's not a reliable signal.
//
// Avoid passing invalid PC to calc_lineno() to avoid crashing.
if (cfp == top && (pc < iseq_encoded || pc > pc_end)) {
lines[i] = 0;
}
else {
lines[i] = calc_lineno(cfp->iseq, cfp->pc);
lines[i] = calc_lineno(cfp->iseq, pc);
}
}