[Xen-devel] The hypercall will fail and return EFAULT when the page becomes COW by forking process in linux

[Wangzhenguo <wangzhenguo@xxxxxxxxxx>]

Hi, everybody
    I meet a trouble. in dom0, I call the xc_domain_set_pod_target hypercall in 
one thread, and meanwhile fork a process in another thread, it will return 
EFAULT by the function of copy_to_user failed in hypervisor. I see that when 
forking a process, the page will become COW, copy_to_user will cause a wirte 
protection page fault and return EFAULT. 
    Is There any ideas for it?

    I write a simple codes to reproduce it:

typedef unsigned long xen_pfn_t;
typedef unsigned long uint64_t;
typedef unsigned short domid_t;
#include <unistd.h>
#include <sys/ioctl.h>
#include <stdlib.h>
#include <assert.h>
#include <string.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <stdio.h>
#include <sys/types.h>
#include <xen/sys/privcmd.h>
#include <errno.h>

#define BUFFER_SIZE 4096

struct xen_pod_target {
    /* IN */
    uint64_t target_pages;
    /* OUT */
    uint64_t tot_pages;
    uint64_t pod_cache_pages;
    uint64_t pod_entries;
    /* IN */
    domid_t domid;
};
#define PRIVCMD_INTF "/proc/xen/privcmd"
#define XENMEM_get_pod_target       17
#define __HYPERVISOR_memory_op            12

int main (int argc, char *argv[])
{
    int err;
    domid_t domid;
    struct xen_pod_target *pod = NULL;
    
    int fd; 
    privcmd_hypercall_t hypercall;
    
    fd = open (PRIVCMD_INTF, O_RDWR);
    assert (0 < fd);

    pod = (struct xen_pod_target *) memalign(4096, BUFFER_SIZE);
    memset (pod, 0, BUFFER_SIZE);
    pod->domid = 0;
    printf("pid = %d, pod=0x%p\n",getpid(),  pod);

    hypercall.op     = __HYPERVISOR_memory_op;
    hypercall.arg[0] = (unsigned long)XENMEM_get_pod_target;
    hypercall.arg[1] = (unsigned long)pod;

    if (0 == fork())
    {
        exit(0);
    }

    err = ioctl (fd, IOCTL_PRIVCMD_HYPERCALL, &hypercall);
    if (0 > err) {
        printf("Failed get_pod_target dom0, errno = %d\n", errno);
    }

    return 0;
}

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