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From: Miklos Szeredi <miklos@szeredi.hu>
Date: Fri, 22 Mar 2024 20:46:40 +0100
Message-ID: <CAJfpegt3UCsMmxd0taOY11Uaw5U=eS1fE5dn0wZX3HF0oy8-oQ@mail.gmail.com>
Subject: Re: BUG: unable to handle kernel paging request in fuse_copy_do
To: David Hildenbrand <david@redhat.com>
Cc: xingwei lee <xrivendell7@gmail.com>, linux-fsdevel@vger.kernel.org, 
	linux-kernel@vger.kernel.org, samsun1006219@gmail.com, 
	syzkaller-bugs@googlegroups.com, linux-mm <linux-mm@kvack.org>, 
	Mike Rapoport <rppt@kernel.org>
Content-Type: text/plain; charset="UTF-8"

On Fri, 22 Mar 2024 at 16:41, David Hildenbrand <david@redhat.com> 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;
}