gcc generated long read out of bounds segfault

[David Fries <David at Fries dot net>]

I was using valgrind and found an out of bounds error reading 8
bytes off an array of 3 byte data structures where the extra 5 bytes
being read were out of the array bounds.  I attached a program that
ends the array at the end of a page so reading beyond the end of the
array would cause a crash, and this is.  x86-64 crashes, x86-32
doesn't.

Before I file a bug report I wanted to check to see if my expectations
are wrong or if this is a compiler bug.  Is there anything that allows
the compiler to generate instructions that would read beyond the end
of an array potentially causing a crash if the page isn't accessible?
Or is this program somehow invalid?

tested gcc and g++ 4.7.2
and from svn, gcc (GCC) 4.9.0 20140221 (experimental)

While both lines read an array entry, only the second crashes.
dup = c[i];
fun(c[i]);

The attached program sets up and reads through the array with extra
padding at the of the array from 8 bytes to 0 bytes.  Padding from 4
to 0 crashes.  There are some #defines to make it easy to use malloc
vs mmap, use munmap or mprotect to make the next page not accessible.

-- 
David Fries <david@fries.net>    PGP pub CB1EE8F0
http://fries.net/~david/

//#define __USE_MISC
//#define __USE_GNU
#include <sys/mman.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <string.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

typedef struct
{
	char r, g, b;
	/*
	char a;
	char w;
	*/
} rgb;

static char rtn;

char fun(rgb r)
{
	rtn = r.r/2 + r.g/3 + r.b/3;
	return rtn;
}

//#define USE_MALLOC
#define SPECIFY_ADDRESS
//#define MPROTECT

/* g++ 4.7.2 generates an 8 byte read for rgb in fun(c[i]); which for the
 * last 2 pixels (3 bytes each), is out of the array bounds.
 * valgrind will flag the problem in all cases.  
 * just mmap runs without crashing, /proc/pid/maps shows there happens to
 * be an adjacent mapping and additional mmaps expands down
 * with SPECIFY_ADDRESS, it maps, unmaps, and maps one page down to leave
 * an whole, which results in a crash
 * with MPROTECT out of bounds is protected and it crashes
 * Linux 3.2.0 x86-64
 * libc6 2.13
 * g++ 4.7.2 and 4.9.0 20140221 (experimental)
 *
 * movq    (%rax), %rdi
 * call    _Z3fun3rgb
 */
char test(int buffer)
{
	const int page = sysconf(_SC_PAGE_SIZE);
	const int count = 3216360;
	printf("buffer %d\n", buffer);
	#ifdef USE_MALLOC
	rgb *c = (rgb*)malloc(sizeof(rgb) * count + buffer);
	#else
	// 3 extra pages worth, 1 rounding, 1 mprotect, 1 align
	size_t size = (sizeof(rgb)*count + buffer + 3*page)/page * page;
	char *p = (char*)mmap(NULL, size, PROT_READ | PROT_WRITE,
		MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);

	#ifdef SPECIFY_ADDRESS
	char *p1 = p;
	munmap(p1, size);
	p = (char*)mmap(p1 - page, size, PROT_READ | PROT_WRITE,
		MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
	#endif

	char *none = NULL;
	#ifdef MPROTECT
	none = p + size - page;
	buffer += page;
	if(mprotect(none, page, PROT_NONE) == -1)
		perror("mprotect failed");
	#endif
	rgb *c = (rgb*)(p + size - sizeof(rgb)*count - buffer);

	printf("mmap size %llu, start %p end %p rgb * %p protect at %p\n",
		(long long)size, p, p+size, c, none);
	#endif
	memset(c, 0, sizeof(rgb)*count);
	rgb dup;
	int i;
	for(i=0; i<count; ++i)
	{
		dup = c[i];
		//fflush(stdout);
		fun(c[i]);
		//fflush(stdout);
	}
	#ifdef CLEANUP
		#ifdef USE_MALLOC
		free(c);
		#else
		munmap(p, size);
		#endif
	#endif
	return dup.r;
}

int main(int argc, char **argv)
{
	if(argc == 2)
		test(atoi(argv[1]));
	int i;
	for(i=8; i>=0; --i)
		test(i);

	return rtn;
}