runtime error: index out of range [4294967295] with length 16 in github.com/klauspost/compress@v1.15.1/flate

[noxer]

We've seen a sentry crash in version 1.15.1 of klauspost/compress. The code seems to be essentially the same on master as on v1.15.1 and the history doesn't indicate a fix for this, so I'm opening a new issue.

runtime.boundsError: runtime error: index out of range [4294967295] with length 16
  File "github.com/klauspost/compress@v1.15.1/flate/huffman_code.go", line 270, in (*huffmanEncoder).bitCounts
  File "github.com/klauspost/compress@v1.15.1/flate/huffman_code.go", line 351, in (*huffmanEncoder).generate
  File "github.com/klauspost/compress@v1.15.1/flate/huffman_bit_writer.go", line 823, in (*huffmanBitWriter).generate
  File "github.com/klauspost/compress@v1.15.1/flate/huffman_bit_writer.go", line 722, in (*huffmanBitWriter).writeBlockDynamic
  File "github.com/klauspost/compress@v1.15.1/flate/deflate.go", line 688, in (*compressor).storeFast
  File "github.com/klauspost/compress@v1.15.1/flate/deflate.go", line 704, in (*compressor).write
  File "github.com/klauspost/compress@v1.15.1/flate/deflate.go", line 865, in (*Writer).Write
  File "github.com/klauspost/compress@v1.15.1/gzip/gzip.go", line 212, in (*Writer).Write
  File "github.com/mattermost/gziphandler@v0.0.1/gzip.go", line 104, in (*GzipResponseWriter).Write
  File "io/io.go", line 428, in copyBuffer
  File "io/io.go", line 385, in Copy
  File "io/io.go", line 361, in CopyN
  File "net/http/fs.go", line 337, in serveContent
  File "net/http/fs.go", line 664, in serveFile
  File "net/http/fs.go", line 850, in (*fileHandler).ServeHTTP
  File "net/http/server.go", line 2127, in StripPrefix.func1
  File "net/http/server.go", line 2084, in HandlerFunc.ServeHTTP
  File "github.com/mattermost/mattermost-server/v6/web/static.go", line 91, in staticFilesHandler.func1
  File "net/http/server.go", line 2084, in HandlerFunc.ServeHTTP
  File "github.com/mattermost/gziphandler@v0.0.1/gzip.go", line 337, in GzipHandlerWithOpts.func1.1
  File "net/http/server.go", line 2084, in HandlerFunc.ServeHTTP
  File "github.com/gorilla/mux@v1.8.0/mux.go", line 210, in (*Router).ServeHTTP
  File "net/http/server.go", line 2084, in HandlerFunc.ServeHTTP
  File "net/http/server.go", line 2916, in serverHandler.ServeHTTP
  File "net/http/server.go", line 1966, in (*conn).serve

OS / Arch

Linux / AMD64

Go version

1.18.1

[noxer]

Problem may be on our side. Sorry. Will reopen when we've investigated further and it's not on our side after all.

[klauspost]

👍🏼 It could be more than one goroutine writing at once. Let me know what you find.

[noxer]

Will do, thanks.

[hanzei]

Heads up that this is still happening with v1.15.4:

runtime.boundsError: runtime error: index out of range [4294967295] with length 16
  File "github.com/klauspost/compress@v1.15.14/flate/huffman_code.go", line 287, in (*huffmanEncoder).bitCounts
  File "github.com/klauspost/compress@v1.15.14/flate/huffman_code.go", line 368, in (*huffmanEncoder).generate
  File "github.com/klauspost/compress@v1.15.14/flate/huffman_bit_writer.go", line 825, in (*huffmanBitWriter).generate
  File "github.com/klauspost/compress@v1.15.14/flate/huffman_bit_writer.go", line 722, in (*huffmanBitWriter).writeBlockDynamic
  File "github.com/klauspost/compress@v1.15.14/flate/deflate.go", line 688, in (*compressor).storeFast
  File "github.com/klauspost/compress@v1.15.14/flate/deflate.go", line 704, in (*compressor).write
  File "github.com/klauspost/compress@v1.15.14/flate/deflate.go", line 864, in (*Writer).Write
  File "github.com/klauspost/compress@v1.15.14/gzip/gzip.go", line 212, in (*Writer).Write
  File "github.com/mattermost/gziphandler@v0.0.1/gzip.go", line 104, in (*GzipResponseWriter).Write

[klauspost]

@hanzei Upgrade and (more likely) make sure you don't do concurrent writes.

[hanzei]

We use compress as part of our gziphandler. AFAIK there should be no concurrent writes, as it's just a stack of http.Handlers.

[klauspost]

You can easily do concurrent writes to a ResponseWriter.

Also check out https://github.com/klauspost/compress/tree/master/gzhttp#gzip-middleware

[klauspost]

There are literally millions of uses of this per day, so the chance of you doing concurrent writes is a bit bigger than an unseen bug, that hasn't been caught or shown up in fuzz testing.

[hanzei]

Thanks for the swift response. 👍

[agnivade]

Hey @klauspost - apologies for commenting on a closed issue. But I took a good look at the code from our side. Atleast the staticFilesHandler from where these panics are originating. And I could not see anything wrong.

Looking a bit closely at the huffman_code.go file however, I see that the panic is happening from:

} else {
	// If we stole from below, move down temporarily to replenish it.
	for levels[level-1].needed > 0 { // <--- here
		level--
	}
}

And every time, it's the same error runtime error: index out of range [4294967295] with length 16, which points to an integer overflow issue.

I am wondering if there could be a case where all the needed fields are greater than zero, and this for loop gets stuck in this, which makes level reach 0, and therefore cause level - 1 to crash? Both the levels and level variable are strictly local, so from my very brief look it doesn't look like memory corruption due to concurrent writes.

I have also been running some tests with the binary running in -race mode and that does not show up anything as well.

Curious to know your thoughts on this.

[klauspost]

Without a reproducer it is quite hard to get further. Would it be feasible to wrap the encoder in a defer func() { if r == recover(); r != nil {... dump file... }} or similar?

[agnivade]

Without a reproducer it is quite hard to get further.

Couldn't agree more.

Would it be feasible to wrap the encoder in a defer func() { if r == recover(); r != nil {... dump file... }} or similar?

The issue is that, there's no way for us to actually get the dump file back from people running into this. It's just that we get the stack trace in the Sentry dashboard. And that's about the only info we have. There is no way for us to reach out to the affected people.

I apologize for not able to give you anything more solid than this. I spent quite a few hours on this trying to get it to repro but failed to. But we do keep seeing this crash from time to time. :(

[klauspost]

I know the issue. My hunch is that if this is triggered, simply slapping a "if level == 0 { something}" will just cause something else to crash - or worse - be wrong. I will see what I can cook up.