Nnpchecker error–how to debug?

Learning
1

I am getting an nnpchecker error in a build: [new release] dream-html (0.0.2) by yawaramin · Pull Request #23786 · ocaml/opam-repository · GitHub

The message is:

#=== ERROR while compiling dream-html.0.0.2 ===================================#
# context              2.2.0~alpha~dev | linux/x86_64 | ocaml-options-only-nnpchecker.1 | pinned(https://github.com/yawaramin/dream-html/releases/download/v0.0.2/dream-html-0.0.2.tbz)
# path                 ~/.opam/4.14/.opam-switch/build/dream-html.0.0.2
# command              ~/.opam/opam-init/hooks/sandbox.sh build dune build -p dream-html -j 127 @install @runtest
# exit-code            1
# env-file             ~/.opam/log/dream-html-7-5933ac.env
# output-file          ~/.opam/log/dream-html-7-5933ac.out
### output ###
# File "test/dune", line 2, characters 7-22:
# 2 |  (name dream_html_test)
#            ^^^^^^^^^^^^^^^
# (cd _build/default/test && ./dream_html_test.exe)
# <!DOCTYPE html>
# <html lang="en"><head><title>Dream_html Test</title>
# </head>
# <body id="test-content"><main spellcheck="true"><article id="article-1" class="story"><p data-hx-get="/p1?a%20b" data-hx-target="closest article > p">Test para 1.</p>
# <p>Test para 2.</p>
# <a href="/a?b=cd:efg/hij">cd:efg/hij</a>
# <a href="/%F0%9F%98%89">wink</a>
# </article>
# <input type="text" autocomplete="name" onblur="if (1 > 0) alert(this.value)"><!-- oops --&gt;&lt;script&gt;alert(&#x27;lol&#x27;)&lt;/script&gt; -->
# <dialog open><div></div>
# </dialog>
# <template id="idtmpl"><p>Template</p>
# </template>
# <div translate="no"><p translate="yes"></p>
# </div>
# <textarea required data-hx-trigger="keyup[target.value.trim() != '']" autocapitalize="words">super</textarea>
# <hr class="super"><p id="greet-Bob">Hello, Bob!</p>
# </main>
# </body>
# </html>
# 
# Out-of-heap pointer at 0x5616480e3210 of value 0x561647f87180 has non-black head (tag=65)
# 
# Out-of-heap pointers were detected by the runtime.
# The process would otherwise have terminated normally.
# File "test/dune", line 5, characters 0-65:
# 5 | (rule
# 6 |  (with-stdout-to
# 7 |   got.html
# 8 |   (run ./dream_html_test.exe)))
# (cd _build/default/test && ./dream_html_test.exe) > _build/default/test/got.html
# Out-of-heap pointer at 0x55dd1c32e210 of value 0x55dd1c1d2180 has non-black head (tag=65)
# 
# Out-of-heap pointers were detected by the runtime.
# The process would otherwise have terminated normally.

The test is defined here: dream-html/dune at v0.0.2 · yawaramin/dream-html · GitHub

It is a dune diff action. The test binary writes to standard output, dune captures it in a file and diffs it with another file. If they are the same then the test passes. This is working for me locally with OCaml 4.14 stock version. But it’s failing in Opam-CI with the ‘no naked pointers’ check.

Any ideas how to debug?

2

Easiest to debug if you have access to the binary and rr [ANN] A dynamic checker for detecting naked pointers

1 Like
3

Thanks. It looks like the binary is dune itself so I guess I can scan it with the nnpchecker. Kinda surprising that dune would be messing with C pointers though. I’ll try later today.

4

If the binary at fault is dune, then it would be great to make an issue on the dune issue tracker. Naked pointers are bugs in OCaml 5.

5

According to the log, I think I don’t think the issue is in dune itself. It’s in dream_html_test.exe and dune is displaying the command that failed:

# (cd _build/default/test && ./dream_html_test.exe) > _build/default/test/got.html
# Out-of-heap pointer at 0x55dd1c32e210 of value 0x55dd1c1d2180 has non-black head (tag=65)
6

Thanks for the analysis. If you have rr installed on your machine, it is fairly easy to get the source of this out-of-heap pointer.

7

Thanks for the pointer (no pun intended). Although, that is puzzling because the test is just building an HTML data structure, converting it to a string, and printing it to standard output:

let node = ...
let () = node |> to_string |> print_endline

In fact, we can see that it ran successfully because we see the entire test HTML output in the logs:

<!DOCTYPE html>
<html ...>
...
</html>

Only after it successfully finishes printing, does it throw the out-of-heap pointer error. So, I am actually hesitant to conclude that the issue is even in my test executable. Will look into it further.

8

Your executable is probably fine, but one of the libraries you’re linking against might use naked pointers.

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