Re: [PATCH 046/104] virtiofsd: use /proc/self/fd/ O_PATH file descriptor

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Philippe Mathieu-Daudé (philmd@redhat.com) wrote:
> On 12/12/19 5:38 PM, Dr. David Alan Gilbert (git) wrote:
> > From: Stefan Hajnoczi <stefanha@redhat.com>
> > 
> > Sandboxing will remove /proc from the mount namespace so we can no
> > longer build string paths into "/proc/self/fd/...".
> > 
> > Keep an O_PATH file descriptor so we can still re-open fds via
> > /proc/self/fd.
> > 
> > Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
> > ---
> >   tools/virtiofsd/passthrough_ll.c | 129 ++++++++++++++++++++++++-------
> >   1 file changed, 102 insertions(+), 27 deletions(-)
> > 
> > diff --git a/tools/virtiofsd/passthrough_ll.c b/tools/virtiofsd/passthrough_ll.c
> > index 123f095990..006908f25a 100644
> > --- a/tools/virtiofsd/passthrough_ll.c
> > +++ b/tools/virtiofsd/passthrough_ll.c
> > @@ -110,6 +110,9 @@ struct lo_data {
> >       struct lo_map ino_map; /* protected by lo->mutex */
> >       struct lo_map dirp_map; /* protected by lo->mutex */
> >       struct lo_map fd_map; /* protected by lo->mutex */
> > +
> > +    /* An O_PATH file descriptor to /proc/self/fd/ */
> > +    int proc_self_fd;
> >   };
> >   static const struct fuse_opt lo_opts[] = {
> > @@ -379,9 +382,9 @@ static int lo_parent_and_name(struct lo_data *lo, struct lo_inode *inode,
> >       int res;
> >   retry:
> > -    sprintf(procname, "/proc/self/fd/%i", inode->fd);
> > +    sprintf(procname, "%i", inode->fd);
> > -    res = readlink(procname, path, PATH_MAX);
> > +    res = readlinkat(lo->proc_self_fd, procname, path, PATH_MAX);
> >       if (res < 0) {
> >           fuse_log(FUSE_LOG_WARNING, "lo_parent_and_name: readlink failed: %m\n");
> >           goto fail_noretry;
> > @@ -477,9 +480,9 @@ static int utimensat_empty(struct lo_data *lo, struct lo_inode *inode,
> >           }
> >           return res;
> >       }
> > -    sprintf(path, "/proc/self/fd/%i", inode->fd);
> > +    sprintf(path, "%i", inode->fd);
> > -    return utimensat(AT_FDCWD, path, tv, 0);
> > +    return utimensat(lo->proc_self_fd, path, tv, 0);
> >   fallback:
> >       res = lo_parent_and_name(lo, inode, path, &parent);
> > @@ -535,8 +538,8 @@ static void lo_setattr(fuse_req_t req, fuse_ino_t ino, struct stat *attr,
> >           if (fi) {
> >               res = fchmod(fd, attr->st_mode);
> >           } else {
> > -            sprintf(procname, "/proc/self/fd/%i", ifd);
> > -            res = chmod(procname, attr->st_mode);
> > +            sprintf(procname, "%i", ifd);
> > +            res = fchmodat(lo->proc_self_fd, procname, attr->st_mode, 0);
> >           }
> >           if (res == -1) {
> >               goto out_err;
> > @@ -552,11 +555,23 @@ static void lo_setattr(fuse_req_t req, fuse_ino_t ino, struct stat *attr,
> >           }
> >       }
> >       if (valid & FUSE_SET_ATTR_SIZE) {
> > +        int truncfd;
> > +
> >           if (fi) {
> > -            res = ftruncate(fd, attr->st_size);
> > +            truncfd = fd;
> >           } else {
> > -            sprintf(procname, "/proc/self/fd/%i", ifd);
> > -            res = truncate(procname, attr->st_size);
> > +            sprintf(procname, "%i", ifd);
> > +            truncfd = openat(lo->proc_self_fd, procname, O_RDWR);
> > +            if (truncfd < 0) {
> > +                goto out_err;
> > +            }
> > +        }
> > +
> > +        res = ftruncate(truncfd, attr->st_size);
> > +        if (!fi) {
> > +            saverr = errno;
> > +            close(truncfd);
> > +            errno = saverr;
> >           }
> >           if (res == -1) {
> >               goto out_err;
> > @@ -857,9 +872,9 @@ static int linkat_empty_nofollow(struct lo_data *lo, struct lo_inode *inode,
> >           return res;
> >       }
> > -    sprintf(path, "/proc/self/fd/%i", inode->fd);
> > +    sprintf(path, "%i", inode->fd);
> > -    return linkat(AT_FDCWD, path, dfd, name, AT_SYMLINK_FOLLOW);
> > +    return linkat(lo->proc_self_fd, path, dfd, name, AT_SYMLINK_FOLLOW);
> >   fallback:
> >       res = lo_parent_and_name(lo, inode, path, &parent);
> > @@ -1387,8 +1402,8 @@ static void lo_open(fuse_req_t req, fuse_ino_t ino, struct fuse_file_info *fi)
> >           fi->flags &= ~O_APPEND;
> >       }
> > -    sprintf(buf, "/proc/self/fd/%i", lo_fd(req, ino));
> > -    fd = open(buf, fi->flags & ~O_NOFOLLOW);
> > +    sprintf(buf, "%i", lo_fd(req, ino));
> > +    fd = openat(lo->proc_self_fd, buf, fi->flags & ~O_NOFOLLOW);
> >       if (fd == -1) {
> >           return (void)fuse_reply_err(req, errno);
> >       }
> > @@ -1440,8 +1455,8 @@ static void lo_flush(fuse_req_t req, fuse_ino_t ino, struct fuse_file_info *fi)
> >   static void lo_fsync(fuse_req_t req, fuse_ino_t ino, int datasync,
> >                        struct fuse_file_info *fi)
> >   {
> > +    struct lo_data *lo = lo_data(req);
> 
> We can initialize this one ...
> 
> >       int res;
> > -    (void)ino;
> >       int fd;
> >       char *buf;
> > @@ -1449,12 +1464,12 @@ static void lo_fsync(fuse_req_t req, fuse_ino_t ino, int datasync,
> >                (void *)fi);
> >       if (!fi) {
> 
> ... here:
> 
>            lo = lo_data(req);
> 
> Similarly in other functions, but I see this is the style used by this file.

I moved that one, I'll keep my out for others; it's nice to keep the
scope small.

> > -        res = asprintf(&buf, "/proc/self/fd/%i", lo_fd(req, ino));
> > +        res = asprintf(&buf, "%i", lo_fd(req, ino));
> >           if (res == -1) {
> >               return (void)fuse_reply_err(req, errno);
> >           }
> > -        fd = open(buf, O_RDWR);
> > +        fd = openat(lo->proc_self_fd, buf, O_RDWR);
> >           free(buf);
> >           if (fd == -1) {
> >               return (void)fuse_reply_err(req, errno);
> > @@ -1570,11 +1585,13 @@ static void lo_flock(fuse_req_t req, fuse_ino_t ino, struct fuse_file_info *fi,
> >   static void lo_getxattr(fuse_req_t req, fuse_ino_t ino, const char *name,
> >                           size_t size)
> >   {
> > +    struct lo_data *lo = lo_data(req);
> >       char *value = NULL;
> >       char procname[64];
> >       struct lo_inode *inode;
> >       ssize_t ret;
> >       int saverr;
> > +    int fd = -1;
> >       inode = lo_inode(req, ino);
> >       if (!inode) {
> > @@ -1599,7 +1616,11 @@ static void lo_getxattr(fuse_req_t req, fuse_ino_t ino, const char *name,
> >           goto out;
> >       }
> > -    sprintf(procname, "/proc/self/fd/%i", inode->fd);
> > +    sprintf(procname, "%i", inode->fd);
> > +    fd = openat(lo->proc_self_fd, procname, O_RDONLY);
> > +    if (fd < 0) {
> > +        goto out_err;
> > +    }
> >       if (size) {
> >           value = malloc(size);
> > @@ -1607,7 +1628,7 @@ static void lo_getxattr(fuse_req_t req, fuse_ino_t ino, const char *name,
> >               goto out_err;
> >           }
> > -        ret = getxattr(procname, name, value, size);
> > +        ret = fgetxattr(fd, name, value, size);
> >           if (ret == -1) {
> >               goto out_err;
> >           }
> > @@ -1618,7 +1639,7 @@ static void lo_getxattr(fuse_req_t req, fuse_ino_t ino, const char *name,
> >           fuse_reply_buf(req, value, ret);
> >       } else {
> > -        ret = getxattr(procname, name, NULL, 0);
> > +        ret = fgetxattr(fd, name, NULL, 0);
> >           if (ret == -1) {
> >               goto out_err;
> >           }
> > @@ -1627,6 +1648,10 @@ static void lo_getxattr(fuse_req_t req, fuse_ino_t ino, const char *name,
> >       }
> >   out_free:
> >       free(value);
> > +
> > +    if (fd >= 0) {
> > +        close(fd);
> > +    }
> >       return;
> >   out_err:
> > @@ -1638,11 +1663,13 @@ out:
> >   static void lo_listxattr(fuse_req_t req, fuse_ino_t ino, size_t size)
> >   {
> > +    struct lo_data *lo = lo_data(req);
> >       char *value = NULL;
> >       char procname[64];
> >       struct lo_inode *inode;
> >       ssize_t ret;
> >       int saverr;
> > +    int fd = -1;
> >       inode = lo_inode(req, ino);
> >       if (!inode) {
> > @@ -1666,7 +1693,11 @@ static void lo_listxattr(fuse_req_t req, fuse_ino_t ino, size_t size)
> >           goto out;
> >       }
> > -    sprintf(procname, "/proc/self/fd/%i", inode->fd);
> > +    sprintf(procname, "%i", inode->fd);
> > +    fd = openat(lo->proc_self_fd, procname, O_RDONLY);
> > +    if (fd < 0) {
> > +        goto out_err;
> > +    }
> >       if (size) {
> >           value = malloc(size);
> > @@ -1674,7 +1705,7 @@ static void lo_listxattr(fuse_req_t req, fuse_ino_t ino, size_t size)
> >               goto out_err;
> >           }
> > -        ret = listxattr(procname, value, size);
> > +        ret = flistxattr(fd, value, size);
> >           if (ret == -1) {
> >               goto out_err;
> >           }
> > @@ -1685,7 +1716,7 @@ static void lo_listxattr(fuse_req_t req, fuse_ino_t ino, size_t size)
> >           fuse_reply_buf(req, value, ret);
> >       } else {
> > -        ret = listxattr(procname, NULL, 0);
> > +        ret = flistxattr(fd, NULL, 0);
> >           if (ret == -1) {
> >               goto out_err;
> >           }
> > @@ -1694,6 +1725,10 @@ static void lo_listxattr(fuse_req_t req, fuse_ino_t ino, size_t size)
> >       }
> >   out_free:
> >       free(value);
> > +
> > +    if (fd >= 0) {
> > +        close(fd);
> > +    }
> >       return;
> >   out_err:
> > @@ -1707,9 +1742,11 @@ static void lo_setxattr(fuse_req_t req, fuse_ino_t ino, const char *name,
> >                           const char *value, size_t size, int flags)
> >   {
> >       char procname[64];
> > +    struct lo_data *lo = lo_data(req);
> >       struct lo_inode *inode;
> >       ssize_t ret;
> >       int saverr;
> > +    int fd = -1;
> >       inode = lo_inode(req, ino);
> >       if (!inode) {
> > @@ -1734,21 +1771,31 @@ static void lo_setxattr(fuse_req_t req, fuse_ino_t ino, const char *name,
> >           goto out;
> >       }
> > -    sprintf(procname, "/proc/self/fd/%i", inode->fd);
> > +    sprintf(procname, "%i", inode->fd);
> > +    fd = openat(lo->proc_self_fd, procname, O_RDWR);
> > +    if (fd < 0) {
> > +        saverr = errno;
> > +        goto out;
> > +    }
> > -    ret = setxattr(procname, name, value, size, flags);
> > +    ret = fsetxattr(fd, name, value, size, flags);
> >       saverr = ret == -1 ? errno : 0;
> >   out:
> > +    if (fd >= 0) {
> > +        close(fd);
> > +    }
> >       fuse_reply_err(req, saverr);
> >   }
> >   static void lo_removexattr(fuse_req_t req, fuse_ino_t ino, const char *name)
> >   {
> >       char procname[64];
> > +    struct lo_data *lo = lo_data(req);
> >       struct lo_inode *inode;
> >       ssize_t ret;
> >       int saverr;
> > +    int fd = -1;
> >       inode = lo_inode(req, ino);
> >       if (!inode) {
> > @@ -1772,12 +1819,20 @@ static void lo_removexattr(fuse_req_t req, fuse_ino_t ino, const char *name)
> >           goto out;
> >       }
> > -    sprintf(procname, "/proc/self/fd/%i", inode->fd);
> > +    sprintf(procname, "%i", inode->fd);
> > +    fd = openat(lo->proc_self_fd, procname, O_RDWR);
> > +    if (fd < 0) {
> > +        saverr = errno;
> > +        goto out;
> > +    }
> > -    ret = removexattr(procname, name);
> > +    ret = fremovexattr(fd, name);
> >       saverr = ret == -1 ? errno : 0;
> >   out:
> > +    if (fd >= 0) {
> > +        close(fd);
> > +    }
> >       fuse_reply_err(req, saverr);
> >   }
> > @@ -1870,12 +1925,25 @@ static void print_capabilities(void)
> >       printf("}\n");
> >   }
> > +static void setup_proc_self_fd(struct lo_data *lo)
> > +{
> > +    lo->proc_self_fd = open("/proc/self/fd", O_PATH);
> > +    if (lo->proc_self_fd == -1) {
> > +        fuse_log(FUSE_LOG_ERR, "open(/proc/self/fd, O_PATH): %m\n");
> > +        exit(1);
> > +    }
> > +}
> > +
> >   int main(int argc, char *argv[])
> >   {
> >       struct fuse_args args = FUSE_ARGS_INIT(argc, argv);
> >       struct fuse_session *se;
> >       struct fuse_cmdline_opts opts;
> > -    struct lo_data lo = { .debug = 0, .writeback = 0 };
> > +    struct lo_data lo = {
> > +        .debug = 0,
> > +        .writeback = 0,
> > +        .proc_self_fd = -1,
> > +    };
> >       struct lo_map_elem *root_elem;
> >       int ret = -1;
> > @@ -1986,6 +2054,9 @@ int main(int argc, char *argv[])
> >       fuse_daemonize(opts.foreground);
> > +    /* Must be after daemonize to get the right /proc/self/fd */
> > +    setup_proc_self_fd(&lo);
> > +
> >       /* Block until ctrl+c or fusermount -u */
> >       ret = virtio_loop(se);
> > @@ -2001,6 +2072,10 @@ err_out1:
> >       lo_map_destroy(&lo.dirp_map);
> >       lo_map_destroy(&lo.ino_map);
> > +    if (lo.proc_self_fd >= 0) {
> > +        close(lo.proc_self_fd);
> > +    }
> > +
> >       if (lo.root.fd >= 0) {
> >           close(lo.root.fd);
> >       }
> > 
> 
> Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>

Thanks!

--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK