linux-kernel - [PATCH] bcachefs: no console_lock in bch2_print_string_as

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Message-ID: <20240710130335.765885-1-daniel.vetter@ffwll.ch>
Date: Wed, 10 Jul 2024 15:03:35 +0200
From: Daniel Vetter <daniel.vetter@...ll.ch>
To: DRI Development <dri-devel@...ts.freedesktop.org>,
	LKML <linux-kernel@...r.kernel.org>
Cc: Intel Graphics Development <intel-gfx@...ts.freedesktop.org>,
	Daniel Vetter <daniel.vetter@...ll.ch>,
	syzbot+6cebc1af246fe020a2f0@...kaller.appspotmail.com,
	Daniel Vetter <daniel.vetter@...el.com>,
	stable@...r.kernel.org,
	Kent Overstreet <kent.overstreet@...ux.dev>,
	Brian Foster <bfoster@...hat.com>,
	linux-bcachefs@...r.kernel.org,
	Petr Mladek <pmladek@...e.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	John Ogness <john.ogness@...utronix.de>,
	Sergey Senozhatsky <senozhatsky@...omium.org>
Subject: [PATCH] bcachefs: no console_lock in bch2_print_string_as_lines

console_lock is the outermost subsystem lock for a lot of subsystems,
which means get/put_user must nest within. Which means it cannot be
acquired somewhere deeply nested in other locks, and most definitely
not while holding fs locks potentially needed to resolve faults.

console_trylock is the best we can do here. But John pointed out on a
previous version that this is futile:

"Using the console lock here at all is wrong. The console lock does not
prevent other CPUs from calling printk() and inserting lines in between.

"There is no way to guarantee a contiguous ringbuffer block using
multiple printk() calls.

"The console_lock usage should be removed."

https://lore.kernel.org/lkml/87frsh33xp.fsf@jogness.linutronix.de/

Do that.

Reported-by: syzbot+6cebc1af246fe020a2f0@...kaller.appspotmail.com
References: https://lore.kernel.org/dri-devel/00000000000026c1ff061cd0de12@google.com/
Signed-off-by: Daniel Vetter <daniel.vetter@...el.com>
Fixes: a8f354284304 ("bcachefs: bch2_print_string_as_lines()")
Cc: <stable@...r.kernel.org> # v6.7+
Cc: Kent Overstreet <kent.overstreet@...ux.dev>
Cc: Brian Foster <bfoster@...hat.com>
Cc: linux-bcachefs@...r.kernel.org
Cc: Petr Mladek <pmladek@...e.com>
Cc: Steven Rostedt <rostedt@...dmis.org>
Cc: John Ogness <john.ogness@...utronix.de>
Cc: Sergey Senozhatsky <senozhatsky@...omium.org>
Signed-off-by: Daniel Vetter <daniel.vetter@...ll.ch>
--
v2: Dont trylock, drop console_lock entirely
---
 fs/bcachefs/util.c | 3 ---
 1 file changed, 3 deletions(-)

diff --git a/fs/bcachefs/util.c b/fs/bcachefs/util.c
index de331dec2a99..dc891563d502 100644
--- a/fs/bcachefs/util.c
+++ b/fs/bcachefs/util.c
@@ -8,7 +8,6 @@
 
 #include <linux/bio.h>
 #include <linux/blkdev.h>
-#include <linux/console.h>
 #include <linux/ctype.h>
 #include <linux/debugfs.h>
 #include <linux/freezer.h>
@@ -261,7 +260,6 @@ void bch2_print_string_as_lines(const char *prefix, const char *lines)
 		return;
 	}
 
-	console_lock();
 	while (1) {
 		p = strchrnul(lines, '\n');
 		printk("%s%.*s\n", prefix, (int) (p - lines), lines);
@@ -269,7 +267,6 @@ void bch2_print_string_as_lines(const char *prefix, const char *lines)
 			break;
 		lines = p + 1;
 	}
-	console_unlock();
 }
 
 int bch2_save_backtrace(bch_stacktrace *stack, struct task_struct *task, unsigned skipnr,
-- 
2.45.2