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runtime: add note that Callers never returns an entry PC

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The presence of a pc > entry check in CallersFrame implies we might
actually see pc == entry, when in reality Callers will never return such
a PC. This check is actually just a safety check for avoid reporting
completely nonsensical from bad input.

all.bash reports two violations to this invariant:

TestCallersFromWrapper, which explicitly constructs a CallersFrame input
with an entry PC.

runtime/pprof.printStackRecord, which passes pprof stacks to
CallersFrame (technically not a valid use of CallersFrames!).
runtime/pprof.(*Profile).Add can add the entry PC of
runtime/pprof.lostProfileEvent to samples.

(CPU profiles do lostProfileEvent + 1. I will send a second CL to fix
Add.)

Change-Id: Iac2a2f0c15117d4a383bd84cddf0413b2d7dd3ef
Reviewed-on: https://go-review.googlesource.com/c/go/+/634315
Auto-Submit: Michael Pratt <mpratt@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
This commit is contained in:
Michael Pratt 2024-12-06 15:37:34 -05:00 • committed by Gopher Robot
parent 0d453232db
commit 312f7c1bd3
1 changed files with 9 additions and 4 deletions
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7
src/runtime/symtab.go
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@ -118,11 +118,16 @@ func (ci *Frames) Next() (frame Frame, more bool) {
}
f := funcInfo._Func()
entry := f.Entry()
if pc > entry {
// We store the pc of the start of the instruction following
// the instruction in question (the call or the inline mark).
// This is done for historical reasons, and to make FuncForPC
// work correctly for entries in the result of runtime.Callers.
// Decrement to get back to the instruction we care about.
//
// It is not possible to get pc == entry from runtime.Callers,
// but if the caller does provide one, provide best-effort
// results by avoiding backing out of the function entirely.
if pc > entry {
pc--
}
// It's important that interpret pc non-strictly as cgoTraceback may