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<a href="https://gitlab.com/alexander-us">Alexander Us</a> created a merge request:
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Project:Branches: alexander-us/libtasn1:clang_10_ptr_arith_ub to gnutls/libtasn1:master
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Author: Alexander Us
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<p dir="auto"><code>asn1_der_coding</code> contained unchecked pointer arithmetics.</p>
<p dir="auto">As source code shows, <code>ider</code> can be <code>NULL</code> pointer. C standard states that arithmetic using <code>NULL</code> pointers gives undefined behavior (C99, 6.5.6, clause 8). LLVM (beginning from version 10) started to optimize pointer arithmetic in comparisons with <code>NULL</code> with assumptions that addition of <code>NULL</code> and non-zero value will produce undefined behavior (<a href="https://reviews.llvm.org/D66608" rel="nofollow noreferrer noopener" target="_blank">https://reviews.llvm.org/D66608</a>). This means that code like <code>ptr + x == NULL</code> will be turned to <code>ptr == NULL</code> because if ptr is <code>NULL</code> then <code>NULL + x</code> will be UB. Short summary of what happened in <code>asn1_der_coding</code> is in this code: <a href="https://godbolt.org/z/DAo0Bt" rel="nofollow noreferrer noopener" target="_blank">https://godbolt.org/z/DAo0Bt</a>.</p>
<p dir="auto">Without my patch clang UBSan reports problems in <strong>copynode</strong> and <strong>Test_tree</strong> tests and these tests fail with <code>-O2</code> flag (<code>CC=clang-10 CFLAGS="-O2 -fsanitize=undefined"</code>):</p>
<pre class="code highlight js-syntax-highlight plaintext" lang="plaintext" v-pre="true"><code><span id="LC1" class="line" lang="plaintext">../../libtasn1/lib/coding.c:1207:56: runtime error: applying non-zero offset 7 to null pointer</span>
<span id="LC2" class="line" lang="plaintext">SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior ../../libtasn1/lib/coding.c:1207:56 in</span>
<span id="LC3" class="line" lang="plaintext">../../libtasn1/lib/coding.c:1225:54: runtime error: applying non-zero offset 287 to null pointer</span>
<span id="LC4" class="line" lang="plaintext">SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior ../../libtasn1/lib/coding.c:1225:54 in</span>
<span id="LC5" class="line" lang="plaintext">LIBTASN1 ERROR: VALUE_NOT_VALID</span>
<span id="LC6" class="line" lang="plaintext">Cannot copy node</span>
<span id="LC7" class="line" lang="plaintext">FAIL copynode (exit status: 1)</span></code></pre>
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