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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Swapping a closed filebuf runs into undefined behavior"
href="https://bugs.llvm.org/show_bug.cgi?id=49923">49923</a>
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<th>Summary</th>
<td>Swapping a closed filebuf runs into undefined behavior
</td>
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<th>Product</th>
<td>libc++
</td>
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<th>Version</th>
<td>11.0
</td>
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<th>Hardware</th>
<td>All
</td>
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<th>OS</th>
<td>All
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<th>Status</th>
<td>NEW
</td>
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<th>Severity</th>
<td>normal
</td>
</tr>
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<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>All Bugs
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<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
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<th>Reporter</th>
<td>alex@grundis.de
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</tr>
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<th>CC</th>
<td>llvm-bugs@lists.llvm.org, mclow.lists@gmail.com
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<p>
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<pre>In the following Code UBsan is triggered:
std::ofstream f_old(filename);
assert(f_old << "Hello ");
std::ofstream f_new(filename2);
assert(f_new << "Foo ");
// After swapping both are valid and where they left
f_new.swap(f_old);
assert(f_old << "Bar");
assert(f_new << "World");
f_new.close();
swap(f_new, f_old);
The problem is as follows:
- __extbuf_ is set to some buffer
- during operation of the filebug __extbufnext_ and __extbufend_ are set
relative to that
- filebuf.close calls setbuf(0, 0) which ends up setting __extbuf_ to the
internal min buffer:
<a href="https://github.com/llvm/llvm-project/blob/82e537a9d28a2c18bd1637e2eac0e0af658ed829/libcxx/include/fstream#L896">https://github.com/llvm/llvm-project/blob/82e537a9d28a2c18bd1637e2eac0e0af658ed829/libcxx/include/fstream#L896</a>
- Note that it does not reset the next/end members
- filebuf.swap then uses the difference between __extbufnext_/__extbufend_ to
__extbuf_ which is invalid as they don't point to the same memory anymore:
<a href="https://github.com/llvm/llvm-project/blob/release/12.x/libcxx/include/fstream#L428-L429">https://github.com/llvm/llvm-project/blob/release/12.x/libcxx/include/fstream#L428-L429</a>
- This then triggers UBsan in the following pointer addition with that invalid
offset:
<a href="https://github.com/llvm/llvm-project/blob/release/12.x/libcxx/include/fstream#L442-L445">https://github.com/llvm/llvm-project/blob/release/12.x/libcxx/include/fstream#L442-L445</a></pre>
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</p>
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