SchOS/cmd/shmtest/main.c

// shmtest -- prove that shared memory is actually shared, and that a
// file-backed mmap returns the file.
//
// Both of these are things you cannot check by reading a return value. shmget
// and shmat returned -ENOSYS until now, and mmap of a file "succeeded" while
// handing back zeroed anonymous pages -- a lie that looks exactly like
// success. The only convincing test is for one process to write and a
// different one to read.
//
//   shmtest        run every check
//
// Prints one line per check, and a final verdict a script can grep for.

#include "core/types.h"
#include "uapi/user.h"
#include "uapi/fcntl.h"

#define PROT_READ  0x1
#define PROT_WRITE 0x2
#define MAP_PRIVATE 0x02
#define MAP_SHARED  0x01
#define IPC_PRIVATE 0
#define IPC_CREAT   01000

static int failures;

static void
puts_(const char *s)
{
    int n = 0;
    while (s[n]) n++;
    write(1, (void*)s, n);
}

static void
check(int ok, const char *what)
{
    puts_(ok ? "  [ok] " : "  FAIL: ");
    puts_(what);
    puts_("\n");
    if (!ok) failures++;
}

int
main(void)
{
    // --- shared memory ----------------------------------------------------
    int id = shmget(IPC_PRIVATE, 8192, IPC_CREAT | 0600);
    check(id >= 0, "shmget created a segment");
    if (id < 0) { puts_("shmtest: FAILED\n"); return 1; }

    char *p = shmat(id, 0, 0);
    check((long)p > 0, "shmat mapped it");
    if ((long)p <= 0) { puts_("shmtest: FAILED\n"); return 1; }

    // A fresh segment must be zeroed, or it leaks whatever the previous
    // tenant left in those pages to whoever gets them next.
    int zeroed = 1;
    for (int i = 0; i < 8192; i++)
        if (p[i] != 0) { zeroed = 0; break; }
    check(zeroed, "a new segment reads as zeros");

    p[0] = 'P';
    p[1] = 0;

    int pid = fork();
    if (pid == 0) {
        // The child inherited the attachment through fork. If the pages had
        // been copied instead of shared, the parent would never see this.
        p[0] = 'C';
        p[100] = 'X';
        exit();
    }
    wait();

    check(p[0] == 'C',   "a write by the child is visible in the parent");
    check(p[100] == 'X', "and so is one further into the segment");

    // --- file-backed mmap --------------------------------------------------
    // Written and compared through one constant, and never by hand-counted
    // offsets. Those offsets used to spell out SCHUNIX-MMAP-OK a character at a
    // time -- m[3]=='U' and m[14]=='K' -- with the length 15 written out
    // separately. Renaming the marker to SCHOS shortened it to 13 and left both
    // behind: the write ran two bytes off the end of the literal and the
    // compare read past the data, which failed a file-backed mmap that was
    // working correctly.
    static const char MARK[] = "SCHOS-MMAP-OK";
    const int MARKLEN = (int)(sizeof(MARK) - 1);

    int fd = open("/tmp/shmtest.dat", O_CREATE | O_RDWR);
    check(fd >= 0, "created a file to map");
    if (fd >= 0) {
        write(fd, MARK, MARKLEN);
        close(fd);
    }

    fd = open("/tmp/shmtest.dat", O_RDONLY);
    if (fd >= 0) {
        char *m = mmap(0, 4096, PROT_READ, MAP_PRIVATE, fd, 0);
        int ok = ((long)m > 0);
        for (int i = 0; ok && i < MARKLEN; i++)
            ok = (m[i] == MARK[i]);
        check(ok, "mmap of a file returns the file, not zeros");
        close(fd);
    } else {
        check(0, "mmap of a file returns the file, not zeros");
    }

    // MAP_SHARED + PROT_WRITE on a file cannot be honoured without a page
    // cache, and is refused rather than quietly downgraded to a private copy.
    fd = open("/tmp/shmtest.dat", O_RDWR);
    if (fd >= 0) {
        void *m = mmap(0, 4096, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
        check((long)m < 0, "writable MAP_SHARED on a file is refused, not faked");
        close(fd);
    }

    check(shmdt(p) == 0, "shmdt released the attachment");

    puts_(failures ? "shmtest: FAILED\n" : "shmtest: all checks passed\n");
    return failures ? 1 : 0;
}