fs/binfmt_elf: replace open-coded string copy with get_task_comm

It is better to use get_task_comm() instead of the open coded string
copy as we do in other places.

struct elf_prpsinfo is used to dump the task information in userspace
coredump or kernel vmcore. Below is the verification of vmcore,

crash> ps
PID PPID CPU TASK ST %MEM VSZ RSS COMM
0 0 0 ffffffff9d21a940 RU 0.0 0 0 [swapper/0]
> 0 0 1 ffffa09e40f85e80 RU 0.0 0 0 [swapper/1]
> 0 0 2 ffffa09e40f81f80 RU 0.0 0 0 [swapper/2]
> 0 0 3 ffffa09e40f83f00 RU 0.0 0 0 [swapper/3]
> 0 0 4 ffffa09e40f80000 RU 0.0 0 0 [swapper/4]
> 0 0 5 ffffa09e40f89f80 RU 0.0 0 0 [swapper/5]
0 0 6 ffffa09e40f8bf00 RU 0.0 0 0 [swapper/6]
> 0 0 7 ffffa09e40f88000 RU 0.0 0 0 [swapper/7]
> 0 0 8 ffffa09e40f8de80 RU 0.0 0 0 [swapper/8]
> 0 0 9 ffffa09e40f95e80 RU 0.0 0 0 [swapper/9]
> 0 0 10 ffffa09e40f91f80 RU 0.0 0 0 [swapper/10]
> 0 0 11 ffffa09e40f93f00 RU 0.0 0 0 [swapper/11]
> 0 0 12 ffffa09e40f90000 RU 0.0 0 0 [swapper/12]
> 0 0 13 ffffa09e40f9bf00 RU 0.0 0 0 [swapper/13]
> 0 0 14 ffffa09e40f98000 RU 0.0 0 0 [swapper/14]
> 0 0 15 ffffa09e40f9de80 RU 0.0 0 0 [swapper/15]

It works well as expected.

Some comments are added to explain why we use the hard-coded 16.

Link: https://lkml.kernel.org/r/[e-mail]
Suggested-by: Kees Cook <[e-mail]>
Signed-off-by: Yafang Shao <[e-mail]>
Reviewed-by: David Hildenbrand <[e-mail]>
Cc: Mathieu Desnoyers <[e-mail]>
Cc: Arnaldo Carvalho de Melo <[e-mail]>
Cc: Andrii Nakryiko <[e-mail]>
Cc: Michal Miroslaw <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Cc: Steven Rostedt <[e-mail]>
Cc: Matthew Wilcox <[e-mail]>
Cc: David Hildenbrand <[e-mail]>
Cc: Al Viro <[e-mail]>
Cc: Kees Cook <[e-mail]>
Cc: Petr Mladek <[e-mail]>
Cc: Alexei Starovoitov <[e-mail]>
Cc: Andrii Nakryiko <[e-mail]>
Cc: Dennis Dalessandro <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

fs/binfmt_elf.c

···
 	SET_UID(psinfo->pr_uid, from_kuid_munged(cred->user_ns, cred->uid));
 	SET_GID(psinfo->pr_gid, from_kgid_munged(cred->user_ns, cred->gid));
 	rcu_read_unlock();
-	strncpy(psinfo->pr_fname, p->comm, sizeof(psinfo->pr_fname));
+	get_task_comm(psinfo->pr_fname, p);
 
 	return 0;
 }

include/linux/elfcore-compat.h

···
 	__compat_uid_t			pr_uid;
 	__compat_gid_t			pr_gid;
 	compat_pid_t			pr_pid, pr_ppid, pr_pgrp, pr_sid;
+	/*
+	 * The hard-coded 16 is derived from TASK_COMM_LEN, but it can't be
+	 * changed as it is exposed to userspace. We'd better make it hard-coded
+	 * here.
+	 */
 	char				pr_fname[16];
 	char				pr_psargs[ELF_PRARGSZ];
 };

include/linux/elfcore.h

···
 	__kernel_gid_t	pr_gid;
 	pid_t	pr_pid, pr_ppid, pr_pgrp, pr_sid;
 	/* Lots missing */
+	/*
+	 * The hard-coded 16 is derived from TASK_COMM_LEN, but it can't be
+	 * changed as it is exposed to userspace. We'd better make it hard-coded
+	 * here.
+	 */
 	char	pr_fname[16];	/* filename of executable */
 	char	pr_psargs[ELF_PRARGSZ];	/* initial part of arg list */
 };