[PULL 048/111] virtiofsd: use /proc/self/fd/ O_PATH file descriptor

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

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>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
---
 tools/virtiofsd/passthrough_ll.c | 130 ++++++++++++++++++++++++-------
 1 file changed, 103 insertions(+), 27 deletions(-)

diff --git a/tools/virtiofsd/passthrough_ll.c b/tools/virtiofsd/passthrough_ll.c
index e3d65c3676..e2e2211ea1 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, "%s: readlink failed: %m\n", __func__);
         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;
@@ -874,9 +889,9 @@ 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);
@@ -1404,8 +1419,8 @@ 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);
     }
@@ -1458,7 +1473,6 @@ static void lo_fsync(fuse_req_t req, fuse_ino_t ino, int datasync,
                      struct fuse_file_info *fi)
 {
     int res;
-    (void)ino;
     int fd;
     char *buf;
 
@@ -1466,12 +1480,14 @@ static void lo_fsync(fuse_req_t req, fuse_ino_t ino, int datasync,
              (void *)fi);
 
     if (!fi) {
-        res = asprintf(&buf, "/proc/self/fd/%i", lo_fd(req, ino));
+        struct lo_data *lo = lo_data(req);
+
+        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);
@@ -1587,11 +1603,13 @@ 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) {
@@ -1616,7 +1634,11 @@ 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);
@@ -1624,7 +1646,7 @@ 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;
         }
@@ -1635,7 +1657,7 @@ 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;
         }
@@ -1644,6 +1666,10 @@ 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:
@@ -1655,11 +1681,13 @@ 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) {
@@ -1683,7 +1711,11 @@ 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);
@@ -1691,7 +1723,7 @@ 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;
         }
@@ -1702,7 +1734,7 @@ 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;
         }
@@ -1711,6 +1743,10 @@ 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:
@@ -1724,9 +1760,11 @@ 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) {
@@ -1751,21 +1789,31 @@ 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) {
@@ -1789,12 +1837,20 @@ 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);
 }
 
@@ -1887,12 +1943,25 @@ 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;
 
@@ -2003,6 +2072,9 @@ 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);
 
@@ -2018,6 +2090,10 @@ 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);
     }
-- 
2.24.1