Read data from fd which was received from fusermount failed - c

Question

I want to test fuse and copy some code from libfuse like this:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/mount.h>
#include <sys/socket.h>
#include <sys/wait.h>
#include <unistd.h>
#define xlog(_fmt_, ...) \
printf("\033[0;33m[%s:%s:%d]\033[0m " _fmt_ "\n", __FILE__, __func__, \
__LINE__, ##__VA_ARGS__)
static int receive_fd(int fd) {
struct msghdr msg;
struct iovec iov;
char buf[1];
int rv;
size_t ccmsg[CMSG_SPACE(sizeof(int)) / sizeof(size_t)];
struct cmsghdr *cmsg;
iov.iov_base = buf;
iov.iov_len = 1;
memset(&msg, 0, sizeof(msg));
msg.msg_name = 0;
msg.msg_namelen = 0;
msg.msg_iov = &iov;
msg.msg_iovlen = 1;
msg.msg_control = ccmsg;
msg.msg_controllen = sizeof(ccmsg);
while (((rv = recvmsg(fd, &msg, 0)) == -1) && errno == EINTR) {
}
if (rv == -1) {
xlog("recvmsg: %s", strerror(errno));
return -1;
}
if (!rv) {
return -1;
}
cmsg = CMSG_FIRSTHDR(&msg);
if (cmsg->cmsg_type != SCM_RIGHTS) {
xlog("got control message of unknown type %d", cmsg->cmsg_type);
return -1;
}
return *(int *)CMSG_DATA(cmsg);
}
static int fuse_mount(const char *mountpoint) {
int fds[2];
int res = socketpair(PF_UNIX, SOCK_STREAM, 0, fds);
if (res != 0) {
xlog("socketpair: %s", strerror(errno));
return -1;
}
pid_t pid = fork();
if (pid == -1) {
close(fds[0]);
close(fds[1]);
return -1;
}
if (pid == 0) {
const char *argv[32];
unsigned argc = 0;
argv[argc++] = "--";
argv[argc++] = mountpoint;
argv[argc++] = NULL;
char env[16];
snprintf(env, sizeof(env), "%i", fds[0]);
setenv("_FUSE_COMMFD", env, 1);
execvp("fusermount", (char **)argv);
xlog("exec: %s", strerror(errno));
exit(1);
}
int rv = receive_fd(fds[1]);
if (rv >= 0) {
fcntl(rv, F_SETFD, FD_CLOEXEC);
}
waitpid(pid, NULL, 0);
close(fds[0]);
close(fds[1]);
return rv;
}
#define MOUNT_POINT "/tmp/xx"
#define BUFFER_SIZE 1024
int main(int argc, char **argv) {
int fd = fuse_mount(MOUNT_POINT);
if (fd < 0) {
xlog("fuse mount fail");
return -1;
}
char buf[BUFFER_SIZE];
ssize_t n = read(fd, buf, sizeof(buf));
if (n < 0) {
xlog("read: %s", strerror(errno));
} else {
xlog("ok");
}
return 0;
}
It received the /dev/fuse filehandle from fusermount and read data from it.
The code run ok on most platforms, but when I run it at virtualbox, I got the error read: Invalid argument at line 107:
ssize_t n = read(fd, buf, sizeof(buf));
Then I check the code of libfuse, it set the buffer size like this:
se->bufsize = FUSE_MAX_MAX_PAGES * getpagesize() + FUSE_BUFFER_HEADER_SIZE;
So I set a larger BUFFER_SIZE like 10240 in my code and it run ok. I feel confused because I think 1024 was large enough and the data size I read from fd was only 56.
My question is in system call:
ssize_t read(int fd, void *buf, size_t count);
Why the size of count would cause Invalid argument error?
Why it was ok when I set count to 1024 on normal platforms, but it was failed on virtualbox?

Answer 1

Thanks for #user253751 tip, I read the code fs/fuse/dev.c and found this:
static ssize_t fuse_dev_do_read(struct fuse_dev *fud, struct file *file,
struct fuse_copy_state *cs, size_t nbytes) {
// ...
if (nbytes < max_t(size_t, FUSE_MIN_READ_BUFFER,
sizeof(struct fuse_in_header) +
sizeof(struct fuse_write_in) + fc->max_write))
return -EINVAL;
// ...
}
The nbytes check was added since linux v5.4, so if the kernel version newer than v5.3.18, the small nbytes would cause EINVAL error.