[PATCH 5.15.y] signal: allow taks to temporarily block TIF_NOTIFY_SIGNAL

[Sasha Levin <sashal@kernel.org>]

From: Christian Brauner <brauner@kernel.org>

[ Upstream commit 22b7f491be8f93ff72de0a3fe02b8ee9e64d3f21 ]

TIF_NOTIFY_SIGNAL is used to kick a task in uninterruptible sleep to
return to userspace and run task work and then go back to sleep. This
mechanism works well but breaks e.g., coredumps. dump_interrupted() only
allows fatal signals to interrupt a coredump and the whole regular write
path going to actual filesystems is impervious to TIF_NOTIFY_SIGNAL as
well.

Add PF_NO_NOTIFY_SIGNAL and helpers to raise and restore it. This is the
same approach as memalloc_nofs_save(). signal_pending() will not report
a fake pending signal via TIF_NOTIFY_SIGNAL if inside a
PF_NO_NOTIFY_SIGNAL section.

No functional changes.

Link: https://patch.msgid.link/20260824-work-tif_notify_signal-v2-1-6609e42b3157@kernel.org
Reviewed-by: Oleg Nesterov <oleg@redhat.com>
Cc: stable@vger.kernel.org
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>
[ sashal: Assigned PF_NO_NOTIFY_SIGNAL to 0x01000000 instead of 0x00800000 to avoid colliding with PF_SWAPWRITE. ]
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 include/linux/sched.h        |  1 +
 include/linux/sched/signal.h | 27 +++++++++++++++++++++++++--
 2 files changed, 26 insertions(+), 2 deletions(-)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 561fb81675fa5..523d727d27a70 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1703,6 +1703,7 @@ extern struct pid *cad_pid;
 #define PF_KTHREAD		0x00200000	/* I am a kernel thread */
 #define PF_RANDOMIZE		0x00400000	/* Randomize virtual address space */
 #define PF_SWAPWRITE		0x00800000	/* Allowed to write to swap */
+#define PF_NO_NOTIFY_SIGNAL	0x01000000	/* see no_notify_signal_save() */
 #define PF_NO_SETAFFINITY	0x04000000	/* Userland is not allowed to meddle with cpus_mask */
 #define PF_MCE_EARLY		0x08000000      /* Early kill for mce process policy */
 #define PF_MEMALLOC_PIN		0x10000000	/* Allocation context constrained to zones which allow long term pinning. */
diff --git a/include/linux/sched/signal.h b/include/linux/sched/signal.h
index 324a3fd0f3c3e..19298445a6ce6 100644
--- a/include/linux/sched/signal.h
+++ b/include/linux/sched/signal.h
@@ -2,6 +2,7 @@
 #ifndef _LINUX_SCHED_SIGNAL_H
 #define _LINUX_SCHED_SIGNAL_H
 
+#include <linux/cleanup.h>
 #include <linux/rculist.h>
 #include <linux/signal.h>
 #include <linux/sched.h>
@@ -365,14 +366,36 @@ static inline int task_sigpending(struct task_struct *p)
 	return unlikely(test_tsk_thread_flag(p,TIF_SIGPENDING));
 }
 
+/* Prevent TIF_NOTIFY_SIGNAL from interrupting this task. */
+static inline unsigned int no_notify_signal_save(void)
+{
+	unsigned int flags = current->flags;
+
+	current->flags |= PF_NO_NOTIFY_SIGNAL;
+	return flags;
+}
+
+/* Restore the previous PF_NO_NOTIFY_SIGNAL state. */
+static inline void no_notify_signal_restore(unsigned int flags)
+{
+	current_restore_flags(flags, PF_NO_NOTIFY_SIGNAL);
+}
+
+DEFINE_LOCK_GUARD_0(no_notify_signal,
+		    _T->flags = no_notify_signal_save(),
+		    no_notify_signal_restore(_T->flags),
+		    unsigned int flags)
+
 static inline int signal_pending(struct task_struct *p)
 {
 	/*
 	 * TIF_NOTIFY_SIGNAL isn't really a signal, but it requires the same
 	 * behavior in terms of ensuring that we break out of wait loops
-	 * so that notify signal callbacks can be processed.
+	 * so that notify signal callbacks can be processed. Not for a task
+	 * that asked not to be interrupted by it, see no_notify_signal_save().
 	 */
-	if (unlikely(test_tsk_thread_flag(p, TIF_NOTIFY_SIGNAL)))
+	if (unlikely(test_tsk_thread_flag(p, TIF_NOTIFY_SIGNAL)) &&
+	    likely(!(READ_ONCE(p->flags) & PF_NO_NOTIFY_SIGNAL)))
 		return 1;
 	return task_sigpending(p);
 }
-- 
2.53.0