[llvm-bugs] [Bug 25474] New: Copy construction performs unsolicited reads of volatile derived-class members in padding

[via llvm-bugs]

https://llvm.org/bugs/show_bug.cgi?id=25474

            Bug ID: 25474
           Summary: Copy construction performs unsolicited reads of
                    volatile derived-class members in padding
           Product: clang
           Version: trunk
          Hardware: All
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: C++
          Assignee: unassignedclangbugs at nondot.org
          Reporter: hstong at ca.ibm.com
                CC: dgregor at apple.com, llvm-bugs at lists.llvm.org
    Classification: Unclassified

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: http://melpon.org/wandbox/permlink/91iSbt94RCoqH9d8

### 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

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