cat /etc/passwd

struct task_struct {
	#ifdef CONFIG_THREAD_INFO_IN_TASK
	/*
	 * For reasons of header soup (see current_thread_info()), this
	 * must be the first element of task_struct.
	 */
	struct thread_info		thread_info;
#endif
	/* -1 unrunnable, 0 runnable, >0 stopped: */
	volatile long			state;

	/*
	 * This begins the randomizable portion of task_struct. Only
	 * scheduling-critical items should be added above here.
	 */
	randomized_struct_fields_start

	void				*stack;
	refcount_t			usage;
	/* Per task flags (PF_*), defined further below: */
	unsigned int			flags;
	unsigned int			ptrace;

  //...
  struct sched_info		sched_info;

	struct list_head		tasks;

	struct mm_struct		*mm;
	struct mm_struct		*active_mm;
  /* Real parent process: */
	struct task_struct __rcu	*real_parent;

	/* Recipient of SIGCHLD, wait4() reports: */
	struct task_struct __rcu	*parent;
	struct list_head		ptraced;
	struct list_head		ptrace_entry;

	/* PID/PID hash table linkage. */
	struct pid			*thread_pid;
	struct hlist_node		pid_links[PIDTYPE_MAX];
	struct list_head		thread_group;
	struct list_head		thread_node;
	/* Process credentials: */

	/* Tracer's credentials at attach: */
	const struct cred __rcu		*ptracer_cred;

	/* Objective and real subjective task credentials (COW): */
	const struct cred __rcu		*real_cred;

	/* Effective (overridable) subjective task credentials (COW): */
	const struct cred __rcu		*cred;
  char				comm[TASK_COMM_LEN];
  struct seccomp			seccomp;
  // ...
  	/*
	 * New fields for task_struct should be added above here, so that
	 * they are included in the randomized portion of task_struct.
	 */
	randomized_struct_fields_end

	/* CPU-specific state of this task: */
	struct thread_struct		thread;

	/*
	 * WARNING: on x86, 'thread_struct' contains a variable-sized
	 * structure.  It *MUST* be at the end of 'task_struct'.
	 *
	 * Do not put anything below here!
	 */
}

$ ls -l test
-rw-r--r-- 1 pan staff 0 Nov 21 14:35 test

$ setfacl -m u:evilpan:r /etc/passwd

struct cred {
	atomic_t	usage;
#ifdef CONFIG_DEBUG_CREDENTIALS
	atomic_t	subscribers;	/* number of processes subscribed */
	void		*put_addr;
	unsigned	magic;
#define CRED_MAGIC	0x43736564
#define CRED_MAGIC_DEAD	0x44656144
#endif
	kuid_t		uid;		/* real UID of the task */
	kgid_t		gid;		/* real GID of the task */
	kuid_t		suid;		/* saved UID of the task */
	kgid_t		sgid;		/* saved GID of the task */
	kuid_t		euid;		/* effective UID of the task */
	kgid_t		egid;		/* effective GID of the task */
	kuid_t		fsuid;		/* UID for VFS ops */
	kgid_t		fsgid;		/* GID for VFS ops */
	// ...
} __randomize_layout;

static void bprm_fill_uid(struct linux_binprm *bprm, struct file *file)
{
	/* Handle suid and sgid on files */
	struct inode *inode;
	unsigned int mode;
	kuid_t uid;
	kgid_t gid;

	if (!mnt_may_suid(file->f_path.mnt))
		return;

	if (task_no_new_privs(current))
		return;

	inode = file->f_path.dentry->d_inode;
	mode = READ_ONCE(inode->i_mode);
	if (!(mode & (S_ISUID|S_ISGID)))
		return;

	/* Be careful if suid/sgid is set */
	inode_lock(inode);

	/* reload atomically mode/uid/gid now that lock held */
	mode = inode->i_mode;
	uid = inode->i_uid;
	gid = inode->i_gid;
	inode_unlock(inode);

	/* We ignore suid/sgid if there are no mappings for them in the ns */
	if (!kuid_has_mapping(bprm->cred->user_ns, uid) ||
		 !kgid_has_mapping(bprm->cred->user_ns, gid))
		return;

	if (mode & S_ISUID) {
		bprm->per_clear |= PER_CLEAR_ON_SETID;
		bprm->cred->euid = uid;
	}

	if ((mode & (S_ISGID | S_IXGRP)) == (S_ISGID | S_IXGRP)) {
		bprm->per_clear |= PER_CLEAR_ON_SETID;
		bprm->cred->egid = gid;
	}
}

struct cred {
	// ...
	kernel_cap_t	cap_inheritable; /* caps our children can inherit */
	kernel_cap_t	cap_permitted;	/* caps we're permitted */
	kernel_cap_t	cap_effective;	/* caps we can actually use */
	kernel_cap_t	cap_bset;	/* capability bounding set */
	kernel_cap_t	cap_ambient;	/* Ambient capability set */
  // ...
}

#include <sys/capability.h>

int capget(cap_user_header_t hdrp, cap_user_data_t datap);
int capset(cap_user_header_t hdrp, const cap_user_data_t datap);

typedef struct __user_cap_header_struct {
  __u32 version;
  int pid;
} *cap_user_header_t;

typedef struct __user_cap_data_struct {
  __u32 effective;
  __u32 permitted;
  __u32 inheritable;
} *cap_user_data_t;

generic:/ # ls -lZ /
total 2424
dr-xr-xr-x  3 root   root   u:object_r:cgroup:s0                 0 1970-01-01 08:00 acct
lrwxrwxrwx  1 root   root   u:object_r:rootfs:s0                50 1970-01-01 08:00 bugreports -> /data/user_de/0/com.android.shell/files/bugreports
drwxrwx---  6 system cache  u:object_r:cache_file:s0          4096 2019-12-23 16:52 cache
lrwxrwxrwx  1 root   root   u:object_r:rootfs:s0                13 1970-01-01 08:00 charger -> /sbin/healthd
dr-x------  2 root   root   u:object_r:rootfs:s0                40 1970-01-01 08:00 config
lrwxrwxrwx  1 root   root   u:object_r:rootfs:s0                17 1970-01-01 08:00 d -> /sys/kernel/debug
drwxrwx--x 38 system system u:object_r:system_data_file:s0    4096 1970-01-01 08:00 data

generic:/ # ps -Z
LABEL                          USER      PID   PPID  VSIZE  RSS   WCHAN            PC  NAME
u:r:init:s0                    root      1     0     7856   1556  SyS_epoll_ 0008e458 S /init
u:r:kernel:s0                  root      2     0     0      0       kthreadd 00000000 S kthreadd
u:r:platform_app:s0:c512,c768  u0_a28    1642  885   1669496 105556 binder_thr ab777494 D com.android.systemui
u:r:fingerprintd:s0            system    901   1     8260   3536  binder_thr b12da494 S /system/bin/fingerprintd
u:r:gatekeeperd:s0             system    902   1     7244   2888  binder_thr b21a1494 S /system/bin/gatekeeperd
u:r:perfprofd:s0               root      903   1     4104   1740  hrtimer_na a72f0378 S /system/xbin/perfprofd
u:r:logd:s0                    logd      906   1     4644   2184  __skb_recv ac3d3584 S /system/bin/logcat
u:r:shell:s0                   shell     909   884   3544   1896  sigsuspend af915698 S /system/bin/sh

type=1400 audit(18.250:15): avc: denied { getattr } for pid=939 comm="ls" path="/ueventd.rc" dev="rootfs" ino=2842 scontext=u:r:shell:s0 tcontext=u:object_r:rootfs:s0 tclass=file permissive=0

rule_name source_type target_type:class perm_set;

allow initrc_t acct_exec_t:file { getattr read execute };

#define call_int_hook(FUNC, IRC, ...) ({			\
	int RC = IRC;						\
	do {							\
		struct security_hook_list *P;			\
								\
		list_for_each_entry(P, &security_hook_heads.FUNC, list) { \
			RC = P->hook.FUNC(__VA_ARGS__);		\
			if (RC != 0)				\
				break;				\
		}						\
	} while (0);						\
	RC;							\
})

int security_inode_permission(struct inode *inode, int mask)
{
	if (unlikely(IS_PRIVATE(inode)))
		return 0;
	return call_int_hook(inode_permission, 0, inode, mask);
}

struct security_hook_heads {
	struct list_head binder_set_context_mgr;
	struct list_head binder_transaction;
	struct list_head binder_transfer_binder;
	struct list_head binder_transfer_file;
	struct list_head ptrace_access_check;
	struct list_head ptrace_traceme;
	struct list_head capget;
	struct list_head capset;
  //...
  struct list_head inode_permission;
  // ...
}

static struct security_hook_list selinux_hooks[] = {
	LSM_HOOK_INIT(binder_set_context_mgr, selinux_binder_set_context_mgr),
	LSM_HOOK_INIT(binder_transaction, selinux_binder_transaction),
	LSM_HOOK_INIT(binder_transfer_binder, selinux_binder_transfer_binder),
	LSM_HOOK_INIT(binder_transfer_file, selinux_binder_transfer_file),

	LSM_HOOK_INIT(ptrace_access_check, selinux_ptrace_access_check),
	LSM_HOOK_INIT(ptrace_traceme, selinux_ptrace_traceme),
	LSM_HOOK_INIT(capget, selinux_capget),
	LSM_HOOK_INIT(capset, selinux_capset),
	// ...
	LSM_HOOK_INIT(inode_permission, selinux_inode_permission),
	// ...
}

static int selinux_inode_permission(struct inode *inode, int mask)
{
	const struct cred *cred = current_cred();
	u32 perms;
	bool from_access;
	unsigned flags = mask & MAY_NOT_BLOCK;
	struct inode_security_struct *isec;
	u32 sid;
	struct av_decision avd;
	int rc, rc2;
	u32 audited, denied;

	from_access = mask & MAY_ACCESS;
	mask &= (MAY_READ|MAY_WRITE|MAY_EXEC|MAY_APPEND);

	/* No permission to check.  Existence test. */
	if (!mask)
		return 0;

	validate_creds(cred);

	if (unlikely(IS_PRIVATE(inode)))
		return 0;

	perms = file_mask_to_av(inode->i_mode, mask);

	sid = cred_sid(cred);
	isec = inode->i_security;

	rc = avc_has_perm_noaudit(sid, isec->sid, isec->sclass, perms, 0, &avd);
	audited = avc_audit_required(perms, &avd, rc,
				     from_access ? FILE__AUDIT_ACCESS : 0,
				     &denied);
	if (likely(!audited))
		return rc;

	rc2 = audit_inode_permission(inode, perms, audited, denied, rc, flags);
	if (rc2)
		return rc2;
	return rc;
}

/* Convert a Linux mode and permission mask to an access vector. */
static inline u32 file_mask_to_av(int mode, int mask)
{
	u32 av = 0;

	if (!S_ISDIR(mode)) {
		if (mask & MAY_EXEC)
			av |= FILE__EXECUTE;
		if (mask & MAY_READ)
			av |= FILE__READ;

		if (mask & MAY_APPEND)
			av |= FILE__APPEND;
		else if (mask & MAY_WRITE)
			av |= FILE__WRITE;

	} else {
		if (mask & MAY_EXEC)
			av |= DIR__SEARCH;
		if (mask & MAY_WRITE)
			av |= DIR__WRITE;
		if (mask & MAY_READ)
			av |= DIR__READ;
	}

	return av;
}

$ tcpdump -d -i lo0 tcp
(000) ld       [0]
(001) jeq      #0x1e000000      jt 2	jf 7
(002) ldb      [10]
(003) jeq      #0x6             jt 10	jf 4
(004) jeq      #0x2c            jt 5	jf 11
(005) ldb      [44]
(006) jeq      #0x6             jt 10	jf 11
(007) jeq      #0x2000000       jt 8	jf 11
(008) ldb      [13]
(009) jeq      #0x6             jt 10	jf 11
(010) ret      #262144
(011) ret      #0

int seccomp(unsigned int operation, unsigned int flags, void *args);

#include <errno.h>
#include <stddef.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <linux/audit.h>
#include <linux/filter.h>
#include <linux/seccomp.h>
#include <sys/prctl.h>

#define X32_SYSCALL_BIT 0x40000000
#define ARRAY_SIZE(arr) (sizeof(arr) / sizeof((arr)[0]))

static int
install_filter(int syscall_nr, int t_arch, int f_errno)
{
   unsigned int upper_nr_limit = 0xffffffff;

   /* Assume that AUDIT_ARCH_X86_64 means the normal x86-64 ABI
      (in the x32 ABI, all system calls have bit 30 set in the
      'nr' field, meaning the numbers are >= X32_SYSCALL_BIT) */
   if (t_arch == AUDIT_ARCH_X86_64)
       upper_nr_limit = X32_SYSCALL_BIT - 1;

   struct sock_filter filter[] = {
       /* [0] Load architecture from 'seccomp_data' buffer into
              accumulator */
       BPF_STMT(BPF_LD | BPF_W | BPF_ABS,
                (offsetof(struct seccomp_data, arch))),

       /* [1] Jump forward 5 instructions if architecture does not
              match 't_arch' */
       BPF_JUMP(BPF_JMP | BPF_JEQ | BPF_K, t_arch, 0, 5),

       /* [2] Load system call number from 'seccomp_data' buffer into
              accumulator */
       BPF_STMT(BPF_LD | BPF_W | BPF_ABS,
                (offsetof(struct seccomp_data, nr))),

       /* [3] Check ABI - only needed for x86-64 in deny-list use
              cases.  Use BPF_JGT instead of checking against the bit
              mask to avoid having to reload the syscall number. */
       BPF_JUMP(BPF_JMP | BPF_JGT | BPF_K, upper_nr_limit, 3, 0),

       /* [4] Jump forward 1 instruction if system call number
              does not match 'syscall_nr' */
       BPF_JUMP(BPF_JMP | BPF_JEQ | BPF_K, syscall_nr, 0, 1),

       /* [5] Matching architecture and system call: don't execute
          the system call, and return 'f_errno' in 'errno' */
       BPF_STMT(BPF_RET | BPF_K,
                SECCOMP_RET_ERRNO | (f_errno & SECCOMP_RET_DATA)),

       /* [6] Destination of system call number mismatch: allow other
              system calls */
       BPF_STMT(BPF_RET | BPF_K, SECCOMP_RET_ALLOW),

       /* [7] Destination of architecture mismatch: kill process */
       BPF_STMT(BPF_RET | BPF_K, SECCOMP_RET_KILL_PROCESS),
   };

   struct sock_fprog prog = {
       .len = ARRAY_SIZE(filter),
       .filter = filter,
   };

   if (seccomp(SECCOMP_SET_MODE_FILTER, 0, &prog)) {
       perror("seccomp");
       return 1;
   }

   return 0;
}

int
main(int argc, char **argv)
{
   if (argc < 5) {
       fprintf(stderr, "Usage: "
               "%s <syscall_nr> <arch> <errno> <prog> [<args>]\n"
               "Hint for <arch>: AUDIT_ARCH_I386: 0x%X\n"
               "                 AUDIT_ARCH_X86_64: 0x%X\n"
               "\n", argv[0], AUDIT_ARCH_I386, AUDIT_ARCH_X86_64);
       exit(EXIT_FAILURE);
   }

   if (prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0)) {
       perror("prctl");
       exit(EXIT_FAILURE);
   }

   if (install_filter(strtol(argv[1], NULL, 0),
                      strtol(argv[2], NULL, 0),
                      strtol(argv[3], NULL, 0)))
       exit(EXIT_FAILURE);

   execv(argv[4], &argv[4]);
   perror("execv");
   exit(EXIT_FAILURE);
}