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Commit 4e229d7

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Solutions to 15-4 and 15-5
1 parent 7ba5f9f commit 4e229d7

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4 files changed

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4 files changed

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‎15_File_Attributes/EX_15_5.md‎

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Exercise 15-5
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=============
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The man page for [umask(2)](https://man7.org/linux/man-pages/man2/umask.2.html)
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says:
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> It is impossible to use umask() to fetch a process's umask without at the same
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> time changing it. A second call to umask() would then be needed to restore the
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> umask. **The nonatomicity of these two steps provides the potential for races
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> in multi-threaded programs.**
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>
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> Since Linux 4.7, the umask of any process can be viewed via the Umask field of
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> `/proc/[pid]/status`. Inspecting this field in `/proc/self/status` allows a
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> process to retrieve its umask without at the same time changing it.
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Example
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-------
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```
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douglas@coogee:~$ umask
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0002
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douglas@coogee:~$ grep -i umask: /proc/self/status
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Umask: 0002
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```

‎15_File_Attributes/Makefile‎

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@@ -8,5 +8,9 @@ all : t_stat my_access
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t_stat: t_stat.c
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gcc -o t_stat t_stat.c file_perms.c
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my_access: my_access.c
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chown test_user my_access
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chmod u+s my_access
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%: %.c
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gcc -o $@ $^

‎15_File_Attributes/README.md‎

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1-
Chapter 15: File atAttributes
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===========================
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# Chapter 15: File Attributes
32

4-
Exercise 15-1
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-------------
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## Table of Contents
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* [Exercise 15-1](#exercise-15-1)
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* [Exercise 15-1 (a)](#exercise-15-1-a)
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* [Exercise 15-1 (b)](#exercise-15-1-b)
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* [Exercise 15-1 (c)](#exercise-15-1-c)
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* [Exercise 15-2](#exercise-15-2)
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* [Exercise 15-3](#exercise-15-3)
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* [Exercise 15-4](#exercise-15-4)
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* [Exercise 15-5](#exercise-15-5)
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## Exercise 15-1
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### Exercise 15-1 (a)
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My answer is in [Ex15_1A.md](Ex15_1A.md)
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### Exercise 15-1 (b)
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My answer is in [Ex15_1B.md](Ex15_1B.md)
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### Exercise 15-1 (c)
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1. Run `make my_access`
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1. Run `./ex15_1c.sh >results.md`
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1. Results are in [Ex15_1C.md](Ex15_1C.md)
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## Exercise 15-2
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My answer is in [Ex15_2.md](Ex15_2.md)
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## Exercise 15-3
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1. Run `make t_stat`
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1. Run `touch test`
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1. Run `./t_stat test`
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Sample output is:
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```
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File type: regular file
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Device containing i-node: major=253 minor=1
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I-node number: 51773935
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Mode: 100400 (r--------)
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Number of (hard) links: 1
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Ownership: UID=1000 GID=1000
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File size: 0 bytes
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Optimal I/O block size: 4096 bytes
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512B blocks allocated: 0
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Last file access: Thu Apr 1 08:26:36.752583228 2021
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Last file modification: Sun Mar 21 21:42:26.305140934 2021
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Last status change: Thu Apr 1 21:23:46.819378661 2021
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```
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## Exercise 15-4
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1. Run `make my_access`
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1. Run `sudo chown test_user my_access`
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1. Run `sudo chmod u+sx my_access`
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1. Run `touch test`
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1. Run `chmod 400 test`
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1. Run `./my_access -f test`
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1. Run `./my_access -r test`
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1. Run `./my_access -w test`
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Sample output is:
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```
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$ ./my_access -f test
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Access for uid to 'test' using mask 0 is 0 (Success)
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seteuid(32767) failed: Operation not permitted
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$ ./my_access -r test
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Access for uid to 'test' using mask 4 is 0 (Success)
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seteuid(32766) failed: Operation not permitted
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$ ./my_access -w test
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Access for uid to 'test' using mask 2 is -1 (Permission denied)
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seteuid(32764) failed: Operation not permitted
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```
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## Exercise 15-5
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My answer is in [EX_15_5.md](EX_15_5.md)

‎15_File_Attributes/my_access.c‎

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* non-root users, the check uses an empty set of capabilities.
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******************************************************************************/
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#include <errno.h>
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#include <stdlib.h>
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#include <stdio.h>
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#include <string.h>
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#include <sys/types.h>
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#include <sys/stat.h>
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#include <unistd.h>
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if (msg != NULL) fprintf(stderr, "%s\n", msg);
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fprintf(stderr, "my_access -h\n");
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fprintf(stderr, "my_access -f file\n");
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fprintf(stderr, "my_access [-r] [-w] [-x] file\n");
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fprintf(stderr, "my_access [-r] [-w] [-x] [-u uid] file\n");
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fprintf(stderr, "\t-h\tPrint this help message\n");
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fprintf(stderr, "\t-f\tTest for existence of file\n");
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fprintf(stderr, "\t-r\tTest whether effective user can read file\n");
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fprintf(stderr, "\t-w\tTest whether effective user can write file\n");
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fprintf(stderr, "\t-x\tTest whether effective user can execute file\n");
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fprintf(stderr, "\n\t-u\tEffective User ID to use\n");
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exit(rc);
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}
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int my_access(const char *pathname, int mode)
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int euid_access(const char *pathname, int mode, uid_tuid)
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{
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int save_errno = 0;
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int res = 0;
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struct stat statbuf;
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res = access(pathname, mode);
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if (res == 0) return 0;
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save_errno = errno;
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if (stat(pathname, &statbuf) == -1) return -1;
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if (mode == F_OK) return 0; /* File exists */
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if ((uid == statbuf.st_uid) &&
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((statbuf.st_mode & S_IRWXU) == mode << 8))
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{
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return 0; /* owner has required permissions */
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}
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errno = EACCES;
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return -1;
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}
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int
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main(int argc, char *argv[])
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{
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int fname; /* Location of filename argument in argv[] */
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uid_t euid; /* effective user-id */
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int opt;
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int f_mode = 0; /* F_OK maps to zero (0) */
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int rwx_mode = 0;
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char *invalid_opt_msg_fmt = "'%c' is an invalid option";
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char *invalid_opt_msg;
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char *file_missing_msg = "file name is missing";
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char *conflict_args_msg = "specify either -f or a combination of -r -w -x";
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char *uid_msg = "Access for uid to '%s' using mask %d is %d (%m)\n";
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char *euid_msg = "Access for euid to '%s' using mask %d is %d (%m)\n";
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invalid_opt_msg = malloc(strlen(invalid_opt_msg_fmt));
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while ((opt = getopt(argc, argv, ":hfrwx")) != -1)
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while ((opt = getopt(argc, argv, ":hfru:wx")) != -1)
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{
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switch (opt)
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{
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case 'r': rwx_mode |= R_OK; break;
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case 'w': rwx_mode |= W_OK; break;
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case 'x': rwx_mode |= X_OK; break;
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case 'u': euid = atoi(optarg); break;
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case '?':
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sprintf(invalid_opt_msg, invalid_opt_msg_fmt, (char)optopt);
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print_help(invalid_opt_msg, EXIT_FAILURE);
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if (rwx_mode != 0) print_help(conflict_args_msg, EXIT_FAILURE);
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}
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int mode = (f_mode) ? F_OK : rwx_mode;
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int res = 0;
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res = access(argv[fname], mode);
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printf(uid_msg, argv[fname], mode, res);
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if (seteuid(euid) != 0)
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{
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fprintf(stderr, "seteuid(%d) failed: %m\n", euid);
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exit(EXIT_FAILURE);
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}
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res = euid_access(argv[fname], mode, euid);
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printf(euid_msg, argv[fname], mode, res);
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exit(EXIT_SUCCESS);
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}

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