Re: BUG: unable to handle kernel paging request in fuse_copy_do

[Miklos Szeredi]

On Fri, 22 Mar 2024 at 16:41, David Hildenbrand <david@xxxxxxxxxx> wrote:

> But at least the vmsplice() just seems to work. Which is weird, because
> GUP-fast should not apply (page not faulted in?)

But it is faulted in, and that indeed seems to be the root cause.
Improved repro:

#define _GNU_SOURCE

#include <fcntl.h>
#include <unistd.h>
#include <stdio.h>
#include <errno.h>
#include <sys/mman.h>
#include <sys/syscall.h>

int main(void)
{
        int fd1, fd2;
        int pip[2];
        struct iovec iov;
        char *addr;
        int ret;

        fd1 = syscall(__NR_memfd_secret, 0);
        addr = mmap(NULL, 4096, PROT_READ | PROT_WRITE, MAP_SHARED, fd1, 0);
        ftruncate(fd1, 7);
        addr[0] = 1; /* fault in page */
        pipe(pip);
        iov.iov_base = addr;
        iov.iov_len = 0x50;
        ret = vmsplice(pip[1], &iov, 1, 0);
        if (ret == -1 && errno == EFAULT) {
                printf("Success\n");
                return 0;
        }

        fd2 = open("/tmp/repro-secretmem.test", O_RDWR | O_CREAT, 0x600);
        splice(pip[0], NULL, fd2, NULL, 0x50, 0);

        return 0;
}