Skip to content
New issue

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement. We’ll occasionally send you account related emails.

Already on GitHub? Sign in to your account

Fix cache segfaults due to unaligned atomics on 32b architectures #75

Merged
merged 1 commit into from Oct 18, 2019
Merged

Fix cache segfaults due to unaligned atomics on 32b architectures #75

merged 1 commit into from Oct 18, 2019

Conversation

nickbp
Copy link
Contributor

@nickbp nickbp commented Oct 12, 2019

Environment where the issue was encountered:

  • Raspberry Pi 4: Linux pi-03 4.19.66-v7l+ #1253 SMP Thu Aug 15 12:02:08 BST 2019 armv7l GNU/Linux
  • Go version 1.13.1

Per the atomic library docs:

On ARM, x86-32, and 32-bit MIPS, it is the caller's responsibility to arrange for 64-bit alignment of 64-bit words accessed atomically. The first word in a variable or in an allocated struct, array, or slice can be relied upon to be 64-bit aligned.


I was able to reproduce the segfault with the following test code (main.go):

package main

import (
	"fmt"
	"time"
	"istio.io/pkg/cache"
)

func set(c cache.Cache, k string, v string) {
	fmt.Printf("Setting %v => %v\n", k, v)
	c.Set(k, v)
}

func get(c cache.Cache, k string) {
	v, ok := c.Get(k)
    if ok {
        fmt.Printf("%v=%v\n", k, v)
    } else {
        fmt.Printf("%v not found\n", k)
    }
}

func test(c cache.Cache) {
	set(c, "one", "foo")

	get(c, "one")
	get(c, "two")

	// Trigger size-based eviction
	set(c, "two", "bar")

	get(c, "one")
	get(c, "two")

	// Trigger TTL-based eviction
	fmt.Printf("Waiting for expiry...\n")
	time.Sleep(1500 * time.Millisecond)

	get(c, "one")
	get(c, "two")
}

func main() {
	fmt.Printf("*** LRU ***\n")
	test(cache.NewLRU(1 * time.Second, 1 * time.Second, 1))

	fmt.Printf("\n*** TTL ***\n")
	test(cache.NewTTL(1 * time.Second, 1 * time.Second))

	fmt.Printf("\n*** PASS ***\n")
}

The above can be built as a static ARM7 binary via the following go build command:

CGO_ENABLED=0 GOARCH=arm GOOS=linux go build -ldflags="-s"

Error 1: atomic increment of unaligned within lock in lruCache.go, fix was to just remove the atomic increment since it's within a lock and none of the other stats do this:

# ./main 
*** LRU ***
Setting one => foo
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x11da0]

goroutine 1 [running]:
runtime/internal/atomic.goXadd64(0x188402c, 0x1, 0x0, 0x186a4a8, 0x1866100)
	/usr/lib/go/src/runtime/internal/atomic/atomic_arm.go:103 +0x1c
istio.io/pkg/cache.(*lruCache).SetWithExpiration(0x1884000, 0xc1280, 0x1866128, 0xc1280, 0x1866130, 0x3b9aca00, 0x0)
	/home/nick/code/pkg/.gopath/src/istio.io/pkg/cache/lruCache.go:247 +0x1bc
istio.io/pkg/cache.(*lruCache).Set(0x1884000, 0xc1280, 0x1866128, 0xc1280, 0x1866130)
	/home/nick/code/pkg/.gopath/src/istio.io/pkg/cache/lruCache.go:224 +0x4c
main.set(0xa6d22008, 0x1866110, 0xd6946, 0x3, 0xd6931, 0x3)
	/home/nick/code/pkg/.gopath/src/istio.io/pkg/main/main.go:11 +0x108
main.test(0xa6d22008, 0x1866110)
	/home/nick/code/pkg/.gopath/src/istio.io/pkg/main/main.go:24 +0x40
main.main()
	/home/nick/code/pkg/.gopath/src/istio.io/pkg/main/main.go:45 +0xa0

Error 2: unaligned baseTimeNanos in ttlCache.go, fix was to move the value to the start of the type to ensure alignment as recommended by atomic docs:

# ./main 
*** LRU ***
Setting one => foo
one=foo
two not found
Setting two => bar
one not found
two=bar
Waiting for expiry...
one not found
two not found

*** TTL ***
Setting one => foo
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x11f68]

goroutine 1 [running]:
runtime/internal/atomic.goLoad64(0x89e04c, 0x16c618, 0xd1850)
	/usr/lib/go/src/runtime/internal/atomic/atomic_arm.go:131 +0x1c
istio.io/pkg/cache.(*ttlCache).SetWithExpiration(0x89e000, 0xc1280, 0x88e040, 0xc1280, 0x88e048, 0x3b9aca00, 0x0)
	/home/nick/code/pkg/.gopath/src/istio.io/pkg/cache/ttlCache.go:173 +0x24
istio.io/pkg/cache.(*ttlCache).Set(0x89e000, 0xc1280, 0x88e040, 0xc1280, 0x88e048)
	/home/nick/code/pkg/.gopath/src/istio.io/pkg/cache/ttlCache.go:167 +0x4c
main.set(0xa6c37008, 0x88e028, 0xd6946, 0x3, 0xd6931, 0x3)
	/home/nick/code/pkg/.gopath/src/istio.io/pkg/main/main.go:11 +0x108
main.test(0xa6c37008, 0x88e028)
	/home/nick/code/pkg/.gopath/src/istio.io/pkg/main/main.go:24 +0x40
main.main()
	/home/nick/code/pkg/.gopath/src/istio.io/pkg/main/main.go:48 +0x12c

After both fixes:

# ./main 
*** LRU ***
Setting one => foo
one=foo
two not found
Setting two => bar
one not found
two=bar
Waiting for expiry...
one not found
two not found

*** TTL ***
Setting one => foo
one=foo
two not found
Setting two => bar
one=foo
two=bar
Waiting for expiry...
one not found
two not found

*** PASS ***

@nickbp nickbp requested a review from as a code owner Oct 12, 2019
@googlebot
Copy link

@googlebot googlebot commented Oct 12, 2019

Thanks for your pull request. It looks like this may be your first contribution to a Google open source project (if not, look below for help). Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

📝 Please visit https://cla.developers.google.com/ to sign.

Once you've signed (or fixed any issues), please reply here with @googlebot I signed it! and we'll verify it.


What to do if you already signed the CLA

Individual signers
Corporate signers

ℹ️ Googlers: Go here for more info.

@googlebot googlebot added the cla: no label Oct 12, 2019
@istio-testing istio-testing added size/XS needs-ok-to-test labels Oct 12, 2019
@istio-testing
Copy link
Contributor

@istio-testing istio-testing commented Oct 12, 2019

Hi @nickbp. Thanks for your PR.

I'm waiting for a istio member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

Once the patch is verified, the new status will be reflected by the ok-to-test label.

I understand the commands that are listed here.

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository.

@nickbp
Copy link
Contributor Author

@nickbp nickbp commented Oct 12, 2019

@googlebot I signed it!

@googlebot
Copy link

@googlebot googlebot commented Oct 12, 2019

CLAs look good, thanks!

ℹ️ Googlers: Go here for more info.

@googlebot googlebot added cla: yes and removed cla: no labels Oct 12, 2019
@@ -244,7 +244,7 @@ func (c *lruCache) SetWithExpiration(key interface{}, value interface{}, expirat
ent.value = value
ent.expiration = exp

atomic.AddUint64(&c.stats.Writes, 1)
c.stats.Writes++
Copy link
Contributor Author

@nickbp nickbp Oct 12, 2019

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

This was the only place that was using an atomic increment against c.stats. It wasn't clear why the atomic increment was necessary.

Copy link
Contributor

@geeknoid geeknoid Oct 12, 2019

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

Hmmm, seems like a leftover, probably from cloning the ttlCache.go code initially.

Copy link
Contributor Author

@nickbp nickbp Oct 12, 2019

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

Forgot to point out earlier that this is also within c.Lock()

@geeknoid geeknoid added the do-not-merge label Oct 12, 2019
Copy link
Contributor

@geeknoid geeknoid left a comment

Thanks for the improvements.

@geeknoid
Copy link
Contributor

@geeknoid geeknoid commented Oct 13, 2019

/ok-to-test

@istio-testing istio-testing added ok-to-test and removed needs-ok-to-test labels Oct 13, 2019
@nickbp
Copy link
Contributor Author

@nickbp nickbp commented Oct 18, 2019

Anything else needed here? This has been idle for a while.

@geeknoid geeknoid removed the do-not-merge label Oct 18, 2019
@istio-testing istio-testing merged commit 030bacb into istio:master Oct 18, 2019
5 checks passed
Sign up for free to join this conversation on GitHub. Already have an account? Sign in to comment
Labels
cla: yes ok-to-test size/XS
Projects
None yet
5 participants