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Message-Id: <20240613023044.45873-2-laoar.shao@gmail.com>
Date: Thu, 13 Jun 2024 10:30:35 +0800
From: Yafang Shao <laoar.shao@...il.com>
To: torvalds@...ux-foundation.org
Cc: ebiederm@...ssion.com,
alexei.starovoitov@...il.com,
rostedt@...dmis.org,
linux-mm@...ck.org,
linux-fsdevel@...r.kernel.org,
linux-trace-kernel@...r.kernel.org,
audit@...r.kernel.org,
linux-security-module@...r.kernel.org,
selinux@...r.kernel.org,
bpf@...r.kernel.org,
netdev@...r.kernel.org,
dri-devel@...ts.freedesktop.org,
Yafang Shao <laoar.shao@...il.com>,
Alexander Viro <viro@...iv.linux.org.uk>,
Christian Brauner <brauner@...nel.org>,
Jan Kara <jack@...e.cz>,
Kees Cook <keescook@...omium.org>,
Matus Jokay <matus.jokay@...ba.sk>
Subject: [PATCH v2 01/10] fs/exec: Drop task_lock() inside __get_task_comm()
Quoted from Linus [0]:
Since user space can randomly change their names anyway, using locking
was always wrong for readers (for writers it probably does make sense
to have some lock - although practically speaking nobody cares there
either, but at least for a writer some kind of race could have
long-term mixed results
Suggested-by: Linus Torvalds <torvalds@...ux-foundation.org>
Link: https://lore.kernel.org/all/CAHk-=wivfrF0_zvf+oj6==Sh=-npJooP8chLPEfaFV0oNYTTBA@mail.gmail.com [0]
Signed-off-by: Yafang Shao <laoar.shao@...il.com>
Cc: Alexander Viro <viro@...iv.linux.org.uk>
Cc: Christian Brauner <brauner@...nel.org>
Cc: Jan Kara <jack@...e.cz>
Cc: Eric Biederman <ebiederm@...ssion.com>
Cc: Kees Cook <keescook@...omium.org>
Cc: Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc: Matus Jokay <matus.jokay@...ba.sk>
---
fs/exec.c | 10 ++++++++--
include/linux/sched.h | 4 ++--
2 files changed, 10 insertions(+), 4 deletions(-)
diff --git a/fs/exec.c b/fs/exec.c
index 40073142288f..fa6b61c79df8 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1238,12 +1238,18 @@ static int unshare_sighand(struct task_struct *me)
return 0;
}
+/*
+ * User space can randomly change their names anyway, so locking for readers
+ * doesn't make sense. For writers, locking is probably necessary, as a race
+ * condition could lead to long-term mixed results.
+ * The strscpy_pad() in __set_task_comm() can ensure that the task comm is
+ * always NUL-terminated. Therefore the race condition between reader and writer
+ * is not an issue.
+ */
char *__get_task_comm(char *buf, size_t buf_size, struct task_struct *tsk)
{
- task_lock(tsk);
/* Always NUL terminated and zero-padded */
strscpy_pad(buf, tsk->comm, buf_size);
- task_unlock(tsk);
return buf;
}
EXPORT_SYMBOL_GPL(__get_task_comm);
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 61591ac6eab6..95888d1da49e 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1086,9 +1086,9 @@ struct task_struct {
/*
* executable name, excluding path.
*
- * - normally initialized setup_new_exec()
+ * - normally initialized begin_new_exec()
* - access it with [gs]et_task_comm()
- * - lock it with task_lock()
+ * - lock it with task_lock() for writing
*/
char comm[TASK_COMM_LEN];
--
2.39.1
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