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runtime/pprof: increment fake overflow record PC

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gentraceback generates PCs which are usually following the CALL
instruction. For those that aren't, it fixes up the PCs so that
functions processing the output can unconditionally decrement the PC.

runtime_expandInlineFrames does this unconditional decrement when
looking up the function. However, the fake stack frame generated for
overflow records fails to meet the contract, and decrementing the PC
results in a PC in the previous function. If that function contains
inlined call, runtime_expandInlineFrames will not short-circuit and will
panic trying to look up a PC that doesn't exist.

Note that the added test does not fail at HEAD. It will only fail (with
a panic) if the function preceeding lostProfileEvent contains inlined
function calls. At the moment (on linux/amd64), that is
runtime/pprof.addMaxRSS, which does not.

Fixes #38096

Change-Id: Iad0819f23c566011c920fd9a5b1254719228da0b
Reviewed-on: https://go-review.googlesource.com/c/go/+/225661
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>
Reviewed-by: Heschi Kreinick <heschi@google.com>
Reviewed-by: Keith Randall <khr@golang.org>
Run-TryBot: Michael Pratt <mpratt@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
This commit is contained in:
Michael Pratt 2020-03-26 15:10:21 -04:00
parent a5d1a9df81
commit 4a8b9bd264
2 changed files with 16 additions and 1 deletions
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12
src/runtime/pprof/pprof_test.go
View file

@ -1171,6 +1171,18 @@ func TestTryAdd(t *testing.T) {
{Value: []int64{10, 10 * period}, Location: []*profile.Location{{ID: 1}, {ID: 1}}},
{Value: []int64{20, 20 * period}, Location: []*profile.Location{{ID: 1}}},
},
}, {
name: "bug38096",
input: []uint64{
3, 0, 500, // hz = 500. Must match the period.
// count (data[2]) == 0 && len(stk) == 1 is an overflow
// entry. The "stk" entry is actually the count.
4, 0, 0, 4242,
},
wantLocs: [][]string{{"runtime/pprof.lostProfileEvent"}},
wantSamples: []*profile.Sample{
{Value: []int64{4242, 4242 * period}, Location: []*profile.Location{{ID: 1}}},
},
}, {
// If a function is called recursively then it must not be
// inlined in the caller.

5
src/runtime/pprof/proto.go
View file

@ -322,7 +322,10 @@ func (b *profileBuilder) addCPUData(data []uint64, tags []unsafe.Pointer) error
// overflow record
count = uint64(stk[0])
stk = []uint64{
uint64(funcPC(lostProfileEvent)),
// gentraceback guarantees that PCs in the
// stack can be unconditionally decremented and
// still be valid, so we must do the same.
uint64(funcPC(lostProfileEvent)+1),
}
}
b.m.lookup(stk, tag).count += int64(count)