From: Al Viro <viro@zeniv.linux.org.uk>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>,
Marco Elver <elver@google.com>,
paulmck@kernel.org,
Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
Dmitry Vyukov <dvyukov@google.com>,
syzbot <syzbot+b7c3ba8cdc2f6cf83c21@syzkaller.appspotmail.com>,
linux-kernel@vger.kernel.org, syzkaller-bugs@googlegroups.com,
Nathan Chancellor <nathan@kernel.org>,
Arnd Bergmann <arnd@kernel.org>,
Jiri Slaby <jirislaby@kernel.org>
Subject: Re: [PATCH v3] tty: tty_io: remove hung_up_tty_fops
Date: Thu, 2 May 2024 19:14:44 +0100 [thread overview]
Message-ID: <20240502181444.GF2118490@ZenIV> (raw)
In-Reply-To: <CAHk-=wi_QBG68QshO1e-xK-jt0ZFsMpZczEJe4nQMu+U7q_7EQ@mail.gmail.com>
On Thu, May 02, 2024 at 10:29:52AM -0700, Linus Torvalds wrote:
> Yes, this is unusual. The *common* thing is to mark pointers as being
> volatile. But this really is something entirely different from that.
The common thing is to mark pointers are pointers to volatile;
calling them "volatile pointers" is common and incorrect, and the only
reason why that sloppy turn of phrase persists is that real "volatile
pointers" are rare...
Marco,
struct foo volatile *p;
declares p as a (non-volatile) pointer to volatile struct foo.
struct foo * volatile p;
declares p as volatile pointer to (non-volatile) struct foo.
The former is a statement about the objects whose addresses might
be stored in p; the latter is a statement about the object p itself.
Replace volatile with const and it becomes easier to experiment with:
char const *p;
char s[] = "barf";
char * const q = s;
...
p = "yuck"; - fine, p itself can be modified
*p = 'a'; - error *p can not be modified, it's an l-value of type const char
q = s + 1; - error, can't modify q
*q = 'a'; - fine, *q is l-value of type char
p = q; - fine, right-hand side of assignment loses the top
qualifier, so q (const pointer to char as l-value)
becomes a plain pointer to char, which can be
converted to pointer to const char, and stored in
p (l-value of type pointer to const char)
strlen(q); - almost the same story, except that it's passing
an argument rather than assignment; they act the
same way.
strcpy(q, "s"); - almost the same, except that here the type of
argument is pointer to char rather than pointer to
const char (strlen() promises not to modify the
string passed to it, strcpy() obviously doesn't)
strcpy(p, "s"); - error; pointer to char converts to a pointer
to const char, but not the other way round.
The situations where you want a const (or volatile) pointer (as opposed to
pointer to const or volatile object) are rare, but this is exactly what
you are asking for - you want to say that the value of 'f_op' member
in any struct file can change at any time. That value is an address of
some instance of struct file_operations and what you want to express is
the property of f_op member itself, not that of the objects whose addresses
might end up stored there.
So having a driver do
const struct file_operations *ops = file->f_op;
is fine - it's basically "take the value of 'file'; it will be an address
of some struct file instance. Fetch 'f_op' from that instance, without
any assumptions of the stability of that member. Use whatever value
you find there as initial value of 'ops'".
That's fine, and since nobody is going to change 'ops' itself behind your
back, you don't need any qualifiers on it. The type of 'ops' here is
"(unqualified) pointer to const struct file_operations".
next prev parent reply other threads:[~2024-05-02 18:14 UTC|newest]
Thread overview: 59+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-04-21 8:18 [syzbot] [kernel?] KCSAN: data-race in __fput / __tty_hangup (4) syzbot
2023-04-21 8:21 ` Dmitry Vyukov
2023-04-21 15:12 ` Tetsuo Handa
2023-04-21 16:02 ` Tetsuo Handa
2023-04-23 23:34 ` Al Viro
2023-04-23 23:55 ` Tetsuo Handa
2023-04-24 0:44 ` Al Viro
2023-04-24 1:09 ` Tetsuo Handa
2023-04-25 14:47 ` Tetsuo Handa
2023-04-25 16:03 ` Al Viro
2023-04-25 22:09 ` Tetsuo Handa
2023-04-26 11:05 ` [PATCH] tty: tty_io: remove hung_up_tty_fops Tetsuo Handa
2023-04-28 16:27 ` Nathan Chancellor
2023-04-28 16:41 ` Tetsuo Handa
2023-04-28 17:11 ` Al Viro
2023-04-29 10:43 ` Tetsuo Handa
2023-04-28 17:31 ` Greg Kroah-Hartman
2023-04-29 15:21 ` Guenter Roeck
2023-05-01 18:42 ` Geert Uytterhoeven
2023-05-14 1:02 ` [PATCH v2] " Tetsuo Handa
2023-05-30 10:44 ` Greg Kroah-Hartman
2023-05-30 11:57 ` Tetsuo Handa
2023-05-30 12:51 ` Greg Kroah-Hartman
2024-04-27 6:20 ` [PATCH v3] " Tetsuo Handa
2024-04-27 19:02 ` Linus Torvalds
2024-04-28 10:19 ` Tetsuo Handa
2024-04-28 18:50 ` Linus Torvalds
2024-04-29 13:55 ` Marco Elver
2024-04-29 15:38 ` Linus Torvalds
2024-05-01 18:45 ` Paul E. McKenney
2024-05-01 18:56 ` Linus Torvalds
2024-05-01 19:02 ` Paul E. McKenney
2024-05-01 20:14 ` Marco Elver
2024-05-01 21:06 ` Linus Torvalds
2024-05-01 21:20 ` Linus Torvalds
2024-05-01 21:49 ` Paul E. McKenney
2024-05-01 22:32 ` Paul E. McKenney
2024-05-02 16:37 ` Boqun Feng
2024-05-03 23:59 ` Paul E. McKenney
2024-05-04 0:14 ` Linus Torvalds
2024-05-04 5:08 ` Paul E. McKenney
2024-05-04 17:50 ` Linus Torvalds
2024-05-04 18:18 ` Paul E. McKenney
2024-05-04 19:11 ` Linus Torvalds
2024-05-04 19:25 ` Linus Torvalds
2024-05-04 22:17 ` Paul E. McKenney
2024-05-04 22:04 ` Paul E. McKenney
2024-05-02 14:14 ` Marco Elver
2024-05-02 16:42 ` Tetsuo Handa
2024-05-02 17:20 ` Marco Elver
2024-05-02 17:29 ` Linus Torvalds
2024-05-02 18:14 ` Al Viro [this message]
2024-05-02 19:29 ` Marco Elver
2024-05-02 23:54 ` Tetsuo Handa
2024-05-03 1:12 ` Linus Torvalds
2023-04-23 13:28 ` [syzbot] [kernel?] KCSAN: data-race in __fput / __tty_hangup (4) Tetsuo Handa
2023-04-23 14:00 ` Greg Kroah-Hartman
2023-04-23 14:03 ` Greg Kroah-Hartman
2023-04-23 14:17 ` Tetsuo Handa
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