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<a href="https://gitlab.com/berrange">Daniel P. Berrangé</a> created a merge request: <a href="https://gitlab.com/gnutls/gnutls/-/merge_requests/2032">!2032</a>
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Project:Branches: berrange/gnutls:dep-malloc-free to gnutls/gnutls:master
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<div class="author">
Author: Daniel P. Berrangé
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Assignees:
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Reviewers:
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<div class="md gl-mt-5" style="position: relative; z-index: 1; color: #3a383f; word-wrap: break-word; margin-top: 1rem;">
<ul dir="auto" style="text-align: initial; list-style-type: disc; margin: 0 0 1rem; padding: 0;">
<li style="margin-top: 0; line-height: 1.6em; margin-left: 25px; padding-left: 3px;">doc: discourage use of gnutls_malloc/gnutls_free by applications</li>
</ul>
<p dir="auto" style="color: #3a383f; margin: 0 0 1rem;" align="initial">Version 3.3.0 turned gnutls_global_set_mem_functions() into a no-op,
so the C library system allocator functions are guaranteed to always
be used.</p>
<p dir="auto" style="color: #3a383f; margin: 0 0 1rem;" align="initial">For application code, this has turned gnutls_malloc & gnutls_free
into a trivial indirection to the C library malloc / free functions.</p>
<p dir="auto" style="color: #3a383f; margin: 0 0 1rem;" align="initial">Unfortunately when an application is built with Control Flow Integrity
enabled, use of gnutls_malloc / gnutls_free may result in termination
of the program with SIGILL. We just hit this in QEMU, with our code
crashing when QEMU is built with CFI when we call gnutls_free().</p>
<p dir="auto" style="color: #3a383f; margin: 0 0 1rem;" align="initial">The problem can be seen standalone with the following example</p>
<div class="gl-relative markdown-code-block js-markdown-code">
<pre class="code highlight js-syntax-highlight language-plaintext" v-pre="true" style='display: block; font-size: 14px; color: #3a383f; line-height: 1.6em; overflow-x: auto; border-radius: .25rem; position: relative; font-family: "GitLab Mono", "JetBrains Mono", "Menlo", "DejaVu Sans Mono", "Liberation Mono", "Consolas", "Ubuntu Mono", "Courier New", "andale mono", "lucida console", monospace; font-variant-ligatures: none; word-break: break-all; word-wrap: break-word; background-color: #fbfafd; margin: 0 0 1rem; padding: 12px; border: 1px solid #dcdcde;'><code style='font-size: inherit; color: inherit; word-wrap: normal; word-break: keep-all; background-color: inherit; border-radius: .25rem; white-space: pre; margin-top: 0; font-family: "GitLab Mono", "JetBrains Mono", "Menlo", "DejaVu Sans Mono", "Liberation Mono", "Consolas", "Ubuntu Mono", "Courier New", "andale mono", "lucida console", monospace; font-variant-ligatures: none; overflow-wrap: normal; padding: unset;'><span id="LC1" class="line" lang="plaintext" style="margin-top: 0;">$ cat g.c</span>
<span id="LC2" class="line" lang="plaintext">#include <gnutls/gnutls.h></span>
<span id="LC3" class="line" lang="plaintext"></span>
<span id="LC4" class="line" lang="plaintext">void foo() {</span>
<span id="LC5" class="line" lang="plaintext"> gnutls_datum_t v;</span>
<span id="LC6" class="line" lang="plaintext"> v.data = gnutls_malloc(10);</span>
<span id="LC7" class="line" lang="plaintext"> gnutls_free(v.data);</span>
<span id="LC8" class="line" lang="plaintext">}</span>
<span id="LC9" class="line" lang="plaintext"></span>
<span id="LC10" class="line" lang="plaintext">int main(int argc, char **argv) {</span>
<span id="LC11" class="line" lang="plaintext"> foo();</span>
<span id="LC12" class="line" lang="plaintext"> return 0;</span>
<span id="LC13" class="line" lang="plaintext">}</span>
<span id="LC14" class="line" lang="plaintext">$ clang -fsanitize=cfi-icall -flto -Wall -I /usr/include/gnutls -lgnutls -o g g.c</span>
<span id="LC15" class="line" lang="plaintext">$ ./g</span>
<span id="LC16" class="line" lang="plaintext">Illegal instruction (core dumped)</span></code></pre>
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<p dir="auto" style="color: #3a383f; margin: 0 0 1rem;" align="initial">I've tested this on Fedora 42, but I would expect same results anywhere
with modern enough clang to support CFI.</p>
<p dir="auto" style="color: #3a383f; margin: 0 0 1rem;" align="initial">In this initial patch I kept the references to gnutls_free/malloc as conditional
recommendations depending on the apps need for back compat with 3.3.0, and didn't
touch the example programs.</p>
<p dir="auto" style="color: #3a383f; margin: 0 0 1rem;" align="initial">The 3.3.0 release of gnutls was 11 years ago now though. Given that long timeframe,
perhaps it is acceptable to entirely remove any reference to gnutls_free/malloc in
the public API docs now ? Then likewise purge their usage in the example C programs
too, with plain free/malloc used instead.</p>
<p dir="auto" style="color: #3a383f; margin: 0 0 1rem;" align="initial">Let me know if I should do that more comprehensive change....</p>
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