[PATCH v2 4/7] 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.

Suggested-by: Kees Cook <redacted>
Signed-off-by: Yafang Shao <redacted>
Reviewed-by: David Hildenbrand <redacted>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Arnaldo Carvalho de Melo <redacted>
Cc: Andrii Nakryiko <redacted>
Cc: Michal Miroslaw <mirq-linux@rere.qmqm.pl>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: David Hildenbrand <redacted>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Kees Cook <redacted>
Cc: Petr Mladek <pmladek@suse.com>
---
 fs/binfmt_elf.c                | 2 +-
 include/linux/elfcore-compat.h | 5 +++++
 include/linux/elfcore.h        | 5 +++++
 3 files changed, 11 insertions(+), 1 deletion(-)

diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c
index f8c7f26f1fbb..b9a33cc34d6b 100644
--- a/fs/binfmt_elf.c
+++ b/fs/binfmt_elf.c

@@ -1585,7 +1585,7 @@ static int fill_psinfo(struct elf_prpsinfo *psinfo, struct task_struct *p,
 	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;
 }

diff --git a/include/linux/elfcore-compat.h b/include/linux/elfcore-compat.h
index e272c3d452ce..54feb64e9b5d 100644
--- a/include/linux/elfcore-compat.h
+++ b/include/linux/elfcore-compat.h

@@ -43,6 +43,11 @@ struct compat_elf_prpsinfo
 	__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];
 };

diff --git a/include/linux/elfcore.h b/include/linux/elfcore.h
index 957ebec35aad..746e081879a5 100644
--- a/include/linux/elfcore.h
+++ b/include/linux/elfcore.h

@@ -65,6 +65,11 @@ struct elf_prpsinfo
 	__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 */
 };

-- 
2.17.1