[PATCH v7 4/4] treewide: Switch memcpy() users of 'task->comm' to a more safer implementation

[Bhupesh <bhupesh@igalia.com>]

As Linus mentioned in [1], currently we have several memcpy() use-cases
which use 'current->comm' to copy the task name over to local copies.
For an example:

 ...
 char comm[TASK_COMM_LEN];
 memcpy(comm, current->comm, TASK_COMM_LEN);
 ...

These should be rather calling a wrappper like "get_task_array()",
which is implemented as:

   static __always_inline void
       __cstr_array_copy(char *dst,
            const char *src, __kernel_size_t size)
   {
        memcpy(dst, src, size);
        dst[size] = 0;
   }

   #define get_task_array(dst,src) \
      __cstr_array_copy(dst, src, __must_be_array(dst))

The relevant 'memcpy()' users were identified using the following search
pattern:
 $ git grep 'memcpy.*->comm\>'

[1]. https://lore.kernel.org/all/CAHk-=wi5c=_-FBGo_88CowJd_F-Gi6Ud9d=TALm65ReN7YjrMw@mail.gmail.com/

Signed-off-by: Bhupesh <bhupesh@igalia.com>
---
 include/linux/coredump.h                      |  2 +-
 include/linux/sched.h                         | 32 +++++++++++++++++++
 include/linux/tracepoint.h                    |  4 +--
 include/trace/events/block.h                  | 10 +++---
 include/trace/events/oom.h                    |  2 +-
 include/trace/events/osnoise.h                |  2 +-
 include/trace/events/sched.h                  | 13 ++++----
 include/trace/events/signal.h                 |  2 +-
 include/trace/events/task.h                   |  4 +--
 tools/bpf/bpftool/pids.c                      |  6 ++--
 .../bpf/test_kmods/bpf_testmod-events.h       |  2 +-
 11 files changed, 54 insertions(+), 25 deletions(-)

diff --git a/include/linux/coredump.h b/include/linux/coredump.h
index 68861da4cf7c..bcee0afc5eaf 100644
--- a/include/linux/coredump.h
+++ b/include/linux/coredump.h
@@ -54,7 +54,7 @@ extern void vfs_coredump(const kernel_siginfo_t *siginfo);
 	do {	\
 		char comm[TASK_COMM_LEN];	\
 		/* This will always be NUL terminated. */ \
-		memcpy(comm, current->comm, sizeof(comm)); \
+		get_task_array(comm, current->comm); \
 		printk_ratelimited(Level "coredump: %d(%*pE): " Format "\n",	\
 			task_tgid_vnr(current), (int)strlen(comm), comm, ##__VA_ARGS__);	\
 	} while (0)	\
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 97ea2ac2a97a..6602ec132297 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1960,12 +1960,44 @@ extern void wake_up_new_task(struct task_struct *tsk);
 
 extern void kick_process(struct task_struct *tsk);
 
+/*
+ * - Why not use task_lock()?
+ *   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 logic inside __set_task_comm() should ensure that the task comm is
+ *   always NUL-terminated and zero-padded. Therefore the race condition between
+ *   reader and writer is not an issue.
+ */
+
 extern void __set_task_comm(struct task_struct *tsk, const char *from, bool exec);
 #define set_task_comm(tsk, from) ({			\
 	BUILD_BUG_ON(sizeof(from) < TASK_COMM_LEN);	\
 	__set_task_comm(tsk, from, false);		\
 })
 
+/*
+ * 'get_task_array' can be 'data-racy' in the destination and
+ * should not be used for cases where a 'stable NUL at the end'
+ * is needed. Its better to use strscpy and friends for such
+ * use-cases.
+ *
+ * It is suited mainly for a 'just copy comm to a constant-sized
+ * array' case - especially in performance sensitive use-cases,
+ * like tracing.
+ */
+
+static __always_inline void
+	__cstr_array_copy(char *dst, const char *src,
+			  __kernel_size_t size)
+{
+	memcpy(dst, src, size);
+	dst[size] = 0;
+}
+
+#define get_task_array(dst, src) \
+	__cstr_array_copy(dst, src, __must_be_array(dst))
+
 static __always_inline void scheduler_ipi(void)
 {
 	/*
diff --git a/include/linux/tracepoint.h b/include/linux/tracepoint.h
index 826ce3f8e1f8..40e04cb660ce 100644
--- a/include/linux/tracepoint.h
+++ b/include/linux/tracepoint.h
@@ -570,10 +570,10 @@ static inline struct tracepoint *tracepoint_ptr_deref(tracepoint_ptr_t *p)
  *	*
  *
  *	TP_fast_assign(
- *		memcpy(__entry->next_comm, next->comm, TASK_COMM_LEN);
+ *		get_task_array(__entry->next_comm, next->comm);
  *		__entry->prev_pid	= prev->pid;
  *		__entry->prev_prio	= prev->prio;
- *		memcpy(__entry->prev_comm, prev->comm, TASK_COMM_LEN);
+ *		get_task_array(__entry->prev_comm, prev->comm);
  *		__entry->next_pid	= next->pid;
  *		__entry->next_prio	= next->prio;
  *	),
diff --git a/include/trace/events/block.h b/include/trace/events/block.h
index 6aa79e2d799c..de1fe35333fc 100644
--- a/include/trace/events/block.h
+++ b/include/trace/events/block.h
@@ -213,7 +213,7 @@ DECLARE_EVENT_CLASS(block_rq,
 
 		blk_fill_rwbs(__entry->rwbs, rq->cmd_flags);
 		__get_str(cmd)[0] = '\0';
-		memcpy(__entry->comm, current->comm, TASK_COMM_LEN);
+		get_task_array(__entry->comm, current->comm);
 	),
 
 	TP_printk("%d,%d %s %u (%s) %llu + %u %s,%u,%u [%s]",
@@ -351,7 +351,7 @@ DECLARE_EVENT_CLASS(block_bio,
 		__entry->sector		= bio->bi_iter.bi_sector;
 		__entry->nr_sector	= bio_sectors(bio);
 		blk_fill_rwbs(__entry->rwbs, bio->bi_opf);
-		memcpy(__entry->comm, current->comm, TASK_COMM_LEN);
+		get_task_array(__entry->comm, current->comm);
 	),
 
 	TP_printk("%d,%d %s %llu + %u [%s]",
@@ -434,7 +434,7 @@ TRACE_EVENT(block_plug,
 	),
 
 	TP_fast_assign(
-		memcpy(__entry->comm, current->comm, TASK_COMM_LEN);
+		get_task_array(__entry->comm, current->comm);
 	),
 
 	TP_printk("[%s]", __entry->comm)
@@ -453,7 +453,7 @@ DECLARE_EVENT_CLASS(block_unplug,
 
 	TP_fast_assign(
 		__entry->nr_rq = depth;
-		memcpy(__entry->comm, current->comm, TASK_COMM_LEN);
+		get_task_array(__entry->comm, current->comm);
 	),
 
 	TP_printk("[%s] %d", __entry->comm, __entry->nr_rq)
@@ -504,7 +504,7 @@ TRACE_EVENT(block_split,
 		__entry->sector		= bio->bi_iter.bi_sector;
 		__entry->new_sector	= new_sector;
 		blk_fill_rwbs(__entry->rwbs, bio->bi_opf);
-		memcpy(__entry->comm, current->comm, TASK_COMM_LEN);
+		get_task_array(__entry->comm, current->comm);
 	),
 
 	TP_printk("%d,%d %s %llu / %llu [%s]",
diff --git a/include/trace/events/oom.h b/include/trace/events/oom.h
index 9f0a5d1482c4..31e5b7295188 100644
--- a/include/trace/events/oom.h
+++ b/include/trace/events/oom.h
@@ -23,7 +23,7 @@ TRACE_EVENT(oom_score_adj_update,
 
 	TP_fast_assign(
 		__entry->pid = task->pid;
-		memcpy(__entry->comm, task->comm, TASK_COMM_LEN);
+		get_task_array(__entry->comm, task->comm);
 		__entry->oom_score_adj = task->signal->oom_score_adj;
 	),
 
diff --git a/include/trace/events/osnoise.h b/include/trace/events/osnoise.h
index 3f4273623801..f67f8b5eca75 100644
--- a/include/trace/events/osnoise.h
+++ b/include/trace/events/osnoise.h
@@ -116,7 +116,7 @@ TRACE_EVENT(thread_noise,
 	),
 
 	TP_fast_assign(
-		memcpy(__entry->comm, t->comm, TASK_COMM_LEN);
+		get_task_array(__entry->comm, t->comm);
 		__entry->pid = t->pid;
 		__entry->start = start;
 		__entry->duration = duration;
diff --git a/include/trace/events/sched.h b/include/trace/events/sched.h
index 7b2645b50e78..66fe808f2654 100644
--- a/include/trace/events/sched.h
+++ b/include/trace/events/sched.h
@@ -152,7 +152,7 @@ DECLARE_EVENT_CLASS(sched_wakeup_template,
 	),
 
 	TP_fast_assign(
-		memcpy(__entry->comm, p->comm, TASK_COMM_LEN);
+		get_task_array(__entry->comm, p->comm);
 		__entry->pid		= p->pid;
 		__entry->prio		= p->prio; /* XXX SCHED_DEADLINE */
 		__entry->target_cpu	= task_cpu(p);
@@ -237,11 +237,11 @@ TRACE_EVENT(sched_switch,
 	),
 
 	TP_fast_assign(
-		memcpy(__entry->prev_comm, prev->comm, TASK_COMM_LEN);
+		get_task_array(__entry->prev_comm, prev->comm);
 		__entry->prev_pid	= prev->pid;
 		__entry->prev_prio	= prev->prio;
 		__entry->prev_state	= __trace_sched_switch_state(preempt, prev_state, prev);
-		memcpy(__entry->next_comm, next->comm, TASK_COMM_LEN);
+		get_task_array(__entry->next_comm, next->comm);
 		__entry->next_pid	= next->pid;
 		__entry->next_prio	= next->prio;
 		/* XXX SCHED_DEADLINE */
@@ -346,7 +346,7 @@ TRACE_EVENT(sched_process_exit,
 	),
 
 	TP_fast_assign(
-		memcpy(__entry->comm, p->comm, TASK_COMM_LEN);
+		get_task_array(__entry->comm, p->comm);
 		__entry->pid		= p->pid;
 		__entry->prio		= p->prio; /* XXX SCHED_DEADLINE */
 		__entry->group_dead	= group_dead;
@@ -787,14 +787,13 @@ TRACE_EVENT(sched_skip_cpuset_numa,
 	),
 
 	TP_fast_assign(
-		memcpy(__entry->comm, tsk->comm, TASK_COMM_LEN);
+		get_task_array(__entry->comm, tsk->comm);
 		__entry->pid		 = task_pid_nr(tsk);
 		__entry->tgid		 = task_tgid_nr(tsk);
 		__entry->ngid		 = task_numa_group_id(tsk);
 		BUILD_BUG_ON(sizeof(nodemask_t) != \
 			     BITS_TO_LONGS(MAX_NUMNODES) * sizeof(long));
-		memcpy(__entry->mem_allowed, mem_allowed_ptr->bits,
-		       sizeof(__entry->mem_allowed));
+		get_task_array(__entry->mem_allowed, mem_allowed_ptr->bits);
 	),
 
 	TP_printk("comm=%s pid=%d tgid=%d ngid=%d mem_nodes_allowed=%*pbl",
diff --git a/include/trace/events/signal.h b/include/trace/events/signal.h
index 1db7e4b07c01..0681dc5ab1de 100644
--- a/include/trace/events/signal.h
+++ b/include/trace/events/signal.h
@@ -67,7 +67,7 @@ TRACE_EVENT(signal_generate,
 	TP_fast_assign(
 		__entry->sig	= sig;
 		TP_STORE_SIGINFO(__entry, info);
-		memcpy(__entry->comm, task->comm, TASK_COMM_LEN);
+		get_task_array(__entry->comm, task->comm);
 		__entry->pid	= task->pid;
 		__entry->group	= group;
 		__entry->result	= result;
diff --git a/include/trace/events/task.h b/include/trace/events/task.h
index af535b053033..9553946943a6 100644
--- a/include/trace/events/task.h
+++ b/include/trace/events/task.h
@@ -21,7 +21,7 @@ TRACE_EVENT(task_newtask,
 
 	TP_fast_assign(
 		__entry->pid = task->pid;
-		memcpy(__entry->comm, task->comm, TASK_COMM_LEN);
+		get_task_array(__entry->comm, task->comm);
 		__entry->clone_flags = clone_flags;
 		__entry->oom_score_adj = task->signal->oom_score_adj;
 	),
@@ -44,7 +44,7 @@ TRACE_EVENT(task_rename,
 	),
 
 	TP_fast_assign(
-		memcpy(entry->oldcomm, task->comm, TASK_COMM_LEN);
+		get_task_array(entry->oldcomm, task->comm);
 		strscpy(entry->newcomm, comm, TASK_COMM_LEN);
 		__entry->oom_score_adj = task->signal->oom_score_adj;
 	),
diff --git a/tools/bpf/bpftool/pids.c b/tools/bpf/bpftool/pids.c
index 23f488cf1740..a5d339cb8ca3 100644
--- a/tools/bpf/bpftool/pids.c
+++ b/tools/bpf/bpftool/pids.c
@@ -53,8 +53,7 @@ static void add_ref(struct hashmap *map, struct pid_iter_entry *e)
 		refs->refs = tmp;
 		ref = &refs->refs[refs->ref_cnt];
 		ref->pid = e->pid;
-		memcpy(ref->comm, e->comm, sizeof(ref->comm));
-		ref->comm[sizeof(ref->comm) - 1] = '\0';
+		get_task_array(ref->comm, e->comm);
 		refs->ref_cnt++;
 
 		return;
@@ -77,8 +76,7 @@ static void add_ref(struct hashmap *map, struct pid_iter_entry *e)
 	}
 	ref = &refs->refs[0];
 	ref->pid = e->pid;
-	memcpy(ref->comm, e->comm, sizeof(ref->comm));
-	ref->comm[sizeof(ref->comm) - 1] = '\0';
+	get_task_array(ref->comm, e->comm);
 	refs->ref_cnt = 1;
 	refs->has_bpf_cookie = e->has_bpf_cookie;
 	refs->bpf_cookie = e->bpf_cookie;
diff --git a/tools/testing/selftests/bpf/test_kmods/bpf_testmod-events.h b/tools/testing/selftests/bpf/test_kmods/bpf_testmod-events.h
index aeef86b3da74..81880748550f 100644
--- a/tools/testing/selftests/bpf/test_kmods/bpf_testmod-events.h
+++ b/tools/testing/selftests/bpf/test_kmods/bpf_testmod-events.h
@@ -20,7 +20,7 @@ TRACE_EVENT(bpf_testmod_test_read,
 	),
 	TP_fast_assign(
 		__entry->pid = task->pid;
-		memcpy(__entry->comm, task->comm, TASK_COMM_LEN);
+		get_task_array(__entry->comm, task->comm);
 		__entry->off = ctx->off;
 		__entry->len = ctx->len;
 	),
-- 
2.38.1