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mount-setup: add a comment that the character/block device nodes are "optional" (#8893) 

if we lack privs to create device nodes that's fine, and creating
/run/systemd/inaccessible/chr or /run/systemd/inaccessible/blk won't
work then. Document this in longer comments.

Fixes: #4484
This commit is contained in:
Lennart Poettering 2018-05-03 16:10:35 +02:00 committed by Yu Watanabe
parent 3e692b58d0
commit fe80fcc7e8
2 changed files with 21 additions and 6 deletions
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15
src/basic/mount-util.c
View File

@ -700,24 +700,33 @@ int repeat_unmount(const char *path, int flags) {
}
const char* mode_to_inaccessible_node(mode_t mode) {
/* This function maps a node type to the correspondent inaccessible node type.
* Character and block inaccessible devices may not be created (because major=0 and minor=0),
* in such case we map character and block devices to the inaccessible node type socket. */
/* This function maps a node type to a corresponding inaccessible file node. These nodes are created during
* early boot by PID 1. In some cases we lacked the privs to create the character and block devices (maybe
* because we run in an userns environment, or miss CAP_SYS_MKNOD, or run with a devices policy that excludes
* device nodes with major and minor of 0), but that's fine, in that case we use an AF_UNIX file node instead,
* which is not the same, but close enough for most uses. And most importantly, the kernel allows bind mounts
* from socket nodes to any non-directory file nodes, and that's the most important thing that matters. */
switch(mode & S_IFMT) {
case S_IFREG:
return "/run/systemd/inaccessible/reg";
case S_IFDIR:
return "/run/systemd/inaccessible/dir";
case S_IFCHR:
if (access("/run/systemd/inaccessible/chr", F_OK) == 0)
return "/run/systemd/inaccessible/chr";
return "/run/systemd/inaccessible/sock";
case S_IFBLK:
if (access("/run/systemd/inaccessible/blk", F_OK) == 0)
return "/run/systemd/inaccessible/blk";
return "/run/systemd/inaccessible/sock";
case S_IFIFO:
return "/run/systemd/inaccessible/fifo";
case S_IFSOCK:
return "/run/systemd/inaccessible/sock";
}

12
src/core/mount-setup.c
View File

@ -457,14 +457,20 @@ int mount_setup(bool loaded_policy) {
(void) mkdir_label("/run/systemd", 0755);
(void) mkdir_label("/run/systemd/system", 0755);
/* Set up inaccessible items */
/* Set up inaccessible (and empty) file nodes of all types */
(void) mkdir_label("/run/systemd/inaccessible", 0000);
(void) mknod("/run/systemd/inaccessible/reg", S_IFREG | 0000, 0);
(void) mkdir_label("/run/systemd/inaccessible/dir", 0000);
(void) mknod("/run/systemd/inaccessible/chr", S_IFCHR | 0000, makedev(0, 0));
(void) mknod("/run/systemd/inaccessible/blk", S_IFBLK | 0000, makedev(0, 0));
(void) mkfifo("/run/systemd/inaccessible/fifo", 0000);
(void) mknod("/run/systemd/inaccessible/sock", S_IFSOCK | 0000, 0);
/* The following two are likely to fail if we lack the privs for it (for example in an userns environment, if
* CAP_SYS_MKNOD is missing, or if a device node policy prohibit major/minor of 0 device nodes to be
* created). But that's entirely fine. Consumers of these files should carry fallback to use a different node
* then, for example /run/systemd/inaccessible/sock, which is close enough in behaviour and semantics for most
* uses. */
(void) mknod("/run/systemd/inaccessible/chr", S_IFCHR | 0000, makedev(0, 0));
(void) mknod("/run/systemd/inaccessible/blk", S_IFBLK | 0000, makedev(0, 0));
return 0;
}