在我之前文章提到Linux 4.1内核支持单用户模式（传送门：https://blog.csdn.net/cui841923894/article/details/81568351），此模式下用户UID和GID均为0同时不再区分用户权限（类root权限），应用于在某些小系统（例如嵌入式系统）。
接下来我们看下这个patch是如何实现内核单用户的。
内核patch解析
1.commit说明
kernel: conditionally support non-root users, groups and capabilities
There are a lot of embedded systems that run most or all of their
functionality in init, running as root:root. For these systems,
supporting multiple users is not necessary.
在很多嵌入式系统中，他们始终使用root:root用户进行操作。这些系统中，多用户功能显得不是很必需（鸡肋了~）。
This patch adds a new symbol, CONFIG_MULTIUSER, that makes support for
non-root users, non-root groups, and capabilities optional. It is enabled
under CONFIG_EXPERT menu.
这个patch添加了新的CONFIG_MULTIUSER内核开关，支持non-root users，, non-root groups, and capabilities。
When this symbol is not defined, UID and GID are zero in any possible case
and processes always have all capabilities.
当CONFIG_MULTIUSER关闭（关闭多用户模式），UID和GID均是0，进程拥有所有capabilities拥有的功能。
The following syscalls are compiled out: setuid, setregid, setgid,
setreuid, setresuid, getresuid, setresgid, getresgid, setgroups,
getgroups, setfsuid, setfsgid, capget, capset.
同时系统调用setuid, setregid, setgid,
setreuid, setresuid, getresuid, setresgid, getresgid, setgroups,
getgroups, setfsuid, setfsgid, capget, capset将不再编译（和支持）。
Also, groups.c is compiled out completely.
同时group.c文件不再编译。
In kernel/capability.c, capable function was moved in order to avoid
adding two ifdef blocks.
kernel/capability.c中的capable相关函数也将移除（其实是采用#ifdef来判断进入正常处理还是直接返回）。
This change saves about 25 KB on a defconfig build. The most minimal
kernels have total text sizes in the high hundreds of kB rather than
low MB. (The 25k goes down a bit with allnoconfig, but not that much.
这项修改在使用defconfig（内核的默认config）可以节省25KB的内核二进制大小。在小内核的config场景可以节省数百KB空间（小于1MB）。在allnoconfig下节省稍微小于25KB的空间。
The kernel was booted in Qemu. All the common functionalities work.
Adding users/groups is not possible, failing with -ENOSYS.
在虚拟机启动的系统（验证），基本系统调用都可以正常运行，所有设计添加users/groups的操作都无效，返回-ENOSYS。
Bloat-o-meter output:
add/remove: 7/87 grow/shrink: 19/397 up/down: 1675/-26325 (-24650)
[[email protected]: coding-style fixes]
Signed-off-by: Iulia Manda <[email protected]>
Reviewed-by: Josh Triplett <[email protected]>
Acked-by: Geert Uytterhoeven <[email protected]>
Tested-by: Paul E. McKenney <[email protected]>
Reviewed-by: Paul E. McKenney <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>2.patch修改内容解析
因为patch涉及修改行多，并且很多目的相同，所以挑重点介绍。
a.某些功能和架构中添加对MULTIUSER config的支持：
diff --git a/arch/s390/Kconfig b/arch/s390/Kconfig
index a5ced5c..de2726a 100644
--- a/arch/s390/Kconfig
+++ b/arch/s390/Kconfig
@@ -328,6 +328,7 @@ config COMPAT
select COMPAT_BINFMT_ELF if BINFMT_ELF
select ARCH_WANT_OLD_COMPAT_IPC
select COMPAT_OLD_SIGACTION
+ depends on MULTIUSER
diff --git a/drivers/staging/lustre/lustre/Kconfig b/drivers/staging/lustre/lustre/Kconfig
index 6725467..62c7bba 100644
--- a/drivers/staging/lustre/lustre/Kconfig
+++ b/drivers/staging/lustre/lustre/Kconfig
@@ -10,6 +10,7 @@ config LUSTRE_FS
select CRYPTO_SHA1
select CRYPTO_SHA256
select CRYPTO_SHA512
+ depends on MULTIUSER
…b.通过#ifdef CONFIG_MULTIUSER设置函数分支
diff --git a/include/linux/capability.h b/include/linux/capability.h
index aa93e5e..af9f0b9 100644
--- a/include/linux/capability.h
+++ b/include/linux/capability.h
@@ -205,6 +205,7 @@ static inline kernel_cap_t cap_raise_nfsd_set(const kernel_cap_t a,
cap_intersect(permitted, __cap_nfsd_set));
}
+#ifdef CONFIG_MULTIUSER //如果定义多用户，则执行正常功能函数
extern bool has_capability(struct task_struct *t, int cap);
extern bool has_ns_capability(struct task_struct *t,
struct user_namespace *ns, int cap);
@@ -213,6 +214,34 @@ extern bool has_ns_capability_noaudit(struct task_struct *t,
struct user_namespace *ns, int cap);
extern bool capable(int cap);
extern bool ns_capable(struct user_namespace *ns, int cap);
+#else // 如果non-root模式，则capability等操作不支持
+static inline bool has_capability(struct task_struct *t, int cap)
+{
+ return true;
+}
…
+static inline bool ns_capable(struct user_namespace *ns, int cap)
+{
+ return true;
+}
+#endif /* CONFIG_MULTIUSER *
diff --git a/include/linux/cred.h b/include/linux/cred.h
index 2fb2ca2..8b6c083 100644
--- a/include/linux/cred.h
+++ b/include/linux/cred.h
@@ -62,9 +62,27 @@ do { \
groups_free(group_info); \
} while (0)
-extern struct group_info *groups_alloc(int);
extern struct group_info init_groups;
+#ifdef CONFIG_MULTIUSER //non-root模式屏蔽in_group_p和in_egroup_p等函数
+extern struct group_info *groups_alloc(int);
extern void groups_free(struct group_info *);
+
+extern int in_group_p(kgid_t);
+extern int in_egroup_p(kgid_t);
+#else
+static inline void groups_free(struct group_info *group_info)
+{
+}
+
+static inline int in_group_p(kgid_t grp)
+{
+ return 1;
+}
+static inline int in_egroup_p(kgid_t grp)
+{
+ return 1;
+}
+#endif
diff --git a/include/linux/uidgid.h b/include/linux/uidgid.h
index 2d1f9b6..0ee05da 100644
--- a/include/linux/uidgid.h
+++ b/include/linux/uidgid.h
@@ -29,6 +29,7 @@ typedef struct {
#define KUIDT_INIT(value) (kuid_t){ value }
#define KGIDT_INIT(value) (kgid_t){ value }
+#ifdef CONFIG_MULTIUSER //屏蔽__kuid_val和__kuid_val
static inline uid_t __kuid_val(kuid_t uid)
{
return uid.val;
@@ -38,6 +39,17 @@ static inline gid_t __kgid_val(kgid_t gid)
{
return gid.val;
}
+#else
+static inline uid_t __kuid_val(kuid_t uid)
+{
+ return 0;
+}
+
+static inline gid_t __kgid_val(kgid_t gid)
+{
+ return 0;
+}
+#endifc. init/Kconfig添加MULTIUSER支持，这样内核make menuconfig可以看到MULTIUSER
…
+config MULTIUSER
+ bool "Multiple users, groups and capabilities support" if EXPERT
+ default y
+ help
+ This option enables support for non-root users, groups and
+ capabilities.
+
+ If you say N here, all processes will run with UID 0, GID 0, and all
+ possible capabilities. Saying N here also compiles out support for
+ system calls related to UIDs, GIDs, and capabilities, such as setuid,
+ setgid, and capset.
+
+ If unsure, say Y here.
+d. kernel/Makefile添加MULTIUSER支持
diff --git a/kernel/Makefile b/kernel/Makefile
index 1408b33..0f8f8b0 100644
--- a/kernel/Makefile
+++ b/kernel/Makefile
@@ -9,7 +9,9 @@ obj-y = fork.o exec_domain.o panic.o \
extable.o params.o \
kthread.o sys_ni.o nsproxy.o \
notifier.o ksysfs.o cred.o reboot.o \
- async.o range.o groups.o smpboot.o
+ async.o range.o smpboot.o
+
+obj-$(CONFIG_MULTIUSER) += groups.o //这里，选择CONFIG_MULTIUSER后才会编译group.ce.这里在capability.c中，第35行添加ifdef CONFIG_MULTIUSER，第386行添加+#endif /* CONFIG_MULTIUSER */，说明只有选择CONFIG_MULTIUSER，文件第35行——386行中包括的函数，才可以生效（定义，实现）。
diff --git a/kernel/capability.c b/kernel/capability.c
index 989f5bf..45432b5 100644
--- a/kernel/capability.c
+++ b/kernel/capability.c
@@ -35,6 +35,7 @@ static int __init file_caps_disable(char *str)
}
__setup("no_file_caps", file_caps_disable);
+#ifdef CONFIG_MULTIUSER
/*
* More recent versions of libcap are available from:
*
@@ -386,6 +387,24 @@ bool ns_capable(struct user_namespace *ns, int cap)
}
EXPORT_SYMBOL(ns_capable);
+
+/**
+ * capable - Determine if the current task has a superior capability in effect
+ * @cap: The capability to be tested for
+ *
+ * Return true if the current task has the given superior capability currently
+ * available for use, false if not.
+ *
+ * This sets PF_SUPERPRIV on the task if the capability is available on the
+ * assumption that it's about to be used.
+ */
+bool capable(int cap)
+{
+ return ns_capable(&init_user_ns, cap);
+}
+EXPORT_SYMBOL(capable);
+#endif /* CONFIG_MULTIUSER */f.sys_ni.c中添加以上处理函数。
这里提一下sys_ni.c作用，如果一个系统调用被淘汰，它所对应的服务例程就要被指定为sys_ni_syscall。sys_ni_syscall中的”ni”即表示”not implemented（没有实现）”。
diff --git a/kernel/sys_ni.c b/kernel/sys_ni.c
index 5adcb0a..7995ef5 100644
--- a/kernel/sys_ni.c
+++ b/kernel/sys_ni.c
@@ -159,6 +159,20 @@ cond_syscall(sys_uselib);
cond_syscall(sys_fadvise64);
cond_syscall(sys_fadvise64_64);
cond_syscall(sys_madvise);
+cond_syscall(sys_setuid);
+cond_syscall(sys_setregid);
+cond_syscall(sys_setgid);
+cond_syscall(sys_setreuid);
+cond_syscall(sys_setresuid);
+cond_syscall(sys_getresuid);
+cond_syscall(sys_setresgid);
+cond_syscall(sys_getresgid);
+cond_syscall(sys_setgroups);
+cond_syscall(sys_getgroups);
+cond_syscall(sys_setfsuid);
+cond_syscall(sys_setfsgid);
+cond_syscall(sys_capget);
+cond_syscall(sys_capset);以上，patch简单来说，就是实现了：
The following syscalls are compiled out: setuid, setregid, setgid,
setreuid, setresuid, getresuid, setresgid, getresgid, setgroups,
getgroups, setfsuid, setfsgid, capget, capset.Also, groups.c is compiled out completely.
In kernel/capability.c, capable function was moved in order to avoid
adding two ifdef blocks.
运行效果
1.使用v4.18内核编译bzImage
#git branch
* (HEAD detached at v4.18)
#cp arch/x86/configs/x86_64_defconfig ./.config
#make menuconfig (关闭MULTIUSER)
#make bzImage -j8
编译好后，内核在目录arch/x86/boot/bzImage
2.使用qemu启动
/ # adduser cuibixuan （这里为什么还能添加用户？）
adduser: /home/cuibixuan: No such file or directory
passwd: unknown uid 0
/ # su cuibixuan
su: can't set groups: Function not implemented可以看到，groups相关操作，已经” Function not implemented”。说明添加到kernel/sys_ni.c的函数sys_setgroups已经生效（+cond_syscall(sys_setgroups);）。
后续
Linux对single-user system的支持，个人认为仅仅不支持uid/gid、group和等capability等相关函数是不够的。比如，启动前fs已经配置多个用户(/etc/passwd和/etc/group)怎么处理；以及某些（安全相关）系统调用建议运行在个人用户权限下怎么办?以及https://lwn.net/Articles/631853/讨论提到：
multiple processes, scheduling等问题：
Come to think of it, I look forward to the next tinification patch
that removes support for multiple processes, scheduling, and makes the
only running process always have pid 1.
或者针对threads讨论：
The problem is then that the single userspace task can prevent
necessary kernel threads from running.
