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<base href="https://llvm.org/bugs/" />
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<body><table border="1" cellspacing="0" cellpadding="8">
<tr>
<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - Copy construction performs unsolicited reads of volatile derived-class members in padding"
href="https://llvm.org/bugs/show_bug.cgi?id=25474">25474</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>Copy construction performs unsolicited reads of volatile derived-class members in padding
</td>
</tr>
<tr>
<th>Product</th>
<td>clang
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
<tr>
<th>Hardware</th>
<td>All
</td>
</tr>
<tr>
<th>OS</th>
<td>Linux
</td>
</tr>
<tr>
<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>normal
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
<tr>
<th>Component</th>
<td>C++
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>hstong@ca.ibm.com
</td>
</tr>
<tr>
<th>CC</th>
<td>dgregor@apple.com, llvm-bugs@lists.llvm.org
</td>
</tr>
<tr>
<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>It is observed that Clang generates reads into the tail padding of non-POD
classes, which is unsafe because said tail padding may (in the complete object)
be associated with volatile members.
The following code demonstrates the issue on typical x86 Linux systems. A
segmentation fault does not occur when the members of the A subobject are
"manually" copied, but the code generated by Clang for the copy construction
causes a segmentation fault.
Online compiler: <a href="http://melpon.org/wandbox/permlink/91iSbt94RCoqH9d8">http://melpon.org/wandbox/permlink/91iSbt94RCoqH9d8</a>
### REPRODUCTION SCRIPT:
clang++ -x c++ -std=c++11 -<<'EOF' &&
#include <sys/mman.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>
#include <stdlib.h>
#include <stdio.h>
#include <new>
constexpr long pgsz = 4096;
struct A {
~A() {}
struct AA {
alignas(pgsz * 2) char aa[pgsz * 2];
} aa;
struct AB {
char ab[pgsz];
} ab;
};
struct B : A {
struct BA {
volatile char ba[pgsz];
} ba;
};
static_assert(sizeof(A) == pgsz * 4, "");
static_assert(sizeof(B) == sizeof(A), "");
int main(void) {
void *paLHS, *pbLHS;
if (posix_memalign(&paLHS, alignof(A), sizeof(A)))
abort();
if (posix_memalign(&pbLHS, alignof(B), sizeof(B)))
abort();
B &bLHS = *new (pbLHS) B;
int fd = open("pgszX6", O_RDONLY);
if (fd < 0)
abort();
void *pb1 = mmap(0, pgsz * 6, PROT_READ, MAP_PRIVATE, fd, 0);
if (!pb1)
abort();
void *pb0 = reinterpret_cast<void *>(
(reinterpret_cast<intptr_t>(pb1) + (pgsz * 2 - 1)) / (pgsz * 2) *
(pgsz * 2));
B *pb = new (pb0) B;
if (mprotect(&pb->ba, pgsz, PROT_NONE))
abort();
fprintf(stderr, "Page size: %ld\n", sysconf(_SC_PAGESIZE));
fprintf(stderr, "About to perform assignment member-by-member...\n");
bLHS.aa = pb->aa;
bLHS.ab = pb->ab;
fprintf(stderr, "About to perform copy construction...\n");
new (paLHS) A(*static_cast<A *>(pb));
}
EOF
dd ibs=$(( 4096 * 6 )) count=1 if=/dev/zero of=pgszX6 && ./a.out
### ACTUAL OUTPUT:
Page size: 4096
About to perform assignment member-by-member...
About to perform copy construction...
prog.sh: line 64: 30 Segmentation fault ./a.out
### EXPECTED OUTPUT (no segmentation fault):
Page size: 4096
About to perform assignment member-by-member...
About to perform copy construction...
### COMPILER VERSION INFO (clang++ -v):
clang version 3.8.0 (trunk 252469)
Target: x86_64-unknown-linux-gnu
Thread model: posix
InstalledDir: /usr/local/llvm-head/bin
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/4.6
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/4.6.3
Selected GCC installation: /usr/lib/gcc/x86_64-linux-gnu/4.6
Candidate multilib: .;@m64
Candidate multilib: 32;@m32
Selected multilib: .;@m64</pre>
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