runtime/pprof: theoretical appendLocsForStack panic with SIGPROF between cgocallback and exitsyscall

[prattmic]

runtime/pprof.appendLocsForStack asserts that an inline-expanded PC always expands to the same number of logical PCs (inlined frames). This is a static property of a given PC, so it should always be true.

When handling SIGPROF, if we are on an extra M running in C, we don't try to traceback since this is C anyway, and just add a sample with stack {pc, runtime._ExternalCode}.

"We are on an extra M running in C" is defined as gp.m.isExtraInC. In cgocallbackg, we clear this field after exitsyscall returns. This leaves a fairly long window when we are in fact running Go code, but the SIGPROF handler will think it is in C.

A lot of this code (particularly in exitsyscall) is reachable from normal Go code as well. If any of this code has more than 2 inlined frames at a single PC, then a SIGPROF from a normal Go context followed by a SIGPROF in this cgocallback context could trigger this appendLocsForStack panic.

I do not know if any code reachable in this window actually has more than 2 inlined frames. Only 2 frames is insufficient, as appendLocsForStack wouldn't actually care that the second frame is runtime._ExternalCode instead of the proper frame.

One potential fix is to attempt to do inline expansion in sigprofNonGoPC in case it actually is a Go PC.

[gabyhelp]

Related Issues

• runtime: "invalid pc-encoded table" throw caused by bad cgo traceback #44971 (closed)
• runtime: sigprof handler crashes backtracing runtime.nanotime #24925 (closed)
• gccgo: seg fault if profiling signal arrives when collecting backtrace #29448 (closed)
• runtime/pprof: inline frames may not use combined location #37446 (closed)

Related Code Changes

• runtime: record current PC for SIGPROF on non-Go thread
• runtime/pprof: increment fake overflow record PC
• runtime/pprof: expand final stack frame to avoid truncation
• runtime/pprof: correctly encode inlined functions in CPU profile
• runtime: collect stack trace if SIGPROF arrives on non-Go thread
• runtime: fail silently if we unwind over sigpanic into C code

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