## Background
The current verison of NodeJS hardcodes the path where perf data is written when the `--perf-basic-prof` flag is used.

https://github.com/v8/v8/blob/061c2ab23a1d4cd192b935e7912e7dfb1fed845d/src/log.cc#L236

At Pinterest, `/tmp` has limited capacity and so we weren't able to utilize perf data to troubleshoot NodeJS without filling the disk and crashing the running system.

## How to patch

First I'll copy the system node binary to my local directory so this set of actions isn't destructive to the running system:

```
$ cp $(which node) .
```

To demonstrate the behavior before patching, I'll run node with the `--perf-basic-prof` flag:

```
$ ./node --perf-basic-prof
````

NodeJS is indeed writing perf files to `/tmp/`:

```
$ ls -l /tmp/perf-*
-rw-r--r-- 1 root root 70363 Dec  7 17:37 /tmp/perf-33630.map
```

We'll search in the binary for the offset of the hardcoded value:

```
$ xxd ./node | grep -- '/tmp/perf-'
0b62ed0: 2f74 6d70 2f70 6572 662d 2564 2e6d 6170  /tmp/perf-%d.map
```

The string "tmp" that we want to patch starts at hex `0b62ed1`, decimal `11939537`. `mnt` is conveniently three bytes and has a lot of space in our environment, so let's patch with that:

```
$ dd if=<(printf 'mnt') of=./node obs=1 seek=11939537 conv=notrunc
0+1 records in
3+0 records out
3 bytes (3 B) copied, 3.8673e-05 s, 77.6 kB/s
```

Now let's verify the patch was successful:

```
$ diff -u <(xxd $(which node)) <(xxd ./node)
--- /dev/fd/63	2016-12-07 17:38:00.364054335 +0000
+++ /dev/fd/62	2016-12-07 17:38:00.364054335 +0000
@@ -746219,7 +746219,7 @@
 0b62ea0: 50cb af00 0000 0000 e088 6e00 0000 0000  P.........n.....
 0b62eb0: e088 6e00 0000 0000 7379 6d62 6f6c 2868  ..n.....symbol(h
 0b62ec0: 6173 6820 0000 0000 0000 0000 0000 0000  ash ............
-0b62ed0: 2f74 6d70 2f70 6572 662d 2564 2e6d 6170  /tmp/perf-%d.map
+0b62ed0: 2f6d 6e74 2f70 6572 662d 2564 2e6d 6170  /mnt/perf-%d.map
 0b62ee0: 0000 0000 1000 0000 0000 0000 0000 0000  ................
 0b62ef0: 0000 0000 0000 0000 0000 0000 0000 0000  ................
 0b62f00: e07f af00 0000 0000 907f af00 0000 0000  ................
```

Looks good, let's test it out:

```
$ ./node --perf-basic-prof
```

Voila!

```
$ ls -l /mnt/perf-*
-rw-r--r-- 1 root root 70302 Dec  7 17:38 /mnt/perf-33979.map
```

In order to make this "production ready" you'll just need to perform this patch each time a new version of NodeJS is installed.

## Troubleshooting

Q: I got the following error message, what did I do wrong?

```
#
# Fatal error in ../deps/v8/src/log.cc, line 272
# Check failed: (perf_output_handle_) != nullptr.
#

==== C stack trace ===============================

 1: V8_Fatal
 2: v8::internal::PerfBasicLogger::PerfBasicLogger()
 3: v8::internal::Logger::SetUp(v8::internal::Isolate*)
 4: v8::internal::Isolate::Init(v8::internal::Deserializer*)
 5: v8::internal::Snapshot::Initialize(v8::internal::Isolate*)
 6: v8::Isolate::New(v8::Isolate::CreateParams const&)
 7: node::Start(int, char**)
 8: __libc_start_main
 9: 0x6e907d
Illegal instruction
```

A: The path that you patched in is not writable by the NodeJS process. Either escalate the privileges of the NodeJS process or `chown`/`chmod` the directory.

## Feedback
What NodeJS developers are saying:

> This is the first application of hex outside of DOM colors I've seen!
>
> — Jessica Chan