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From: Segher Boessenkool <segher@kernel.crashing.org>
To: Alexander Potapenko <glider@google.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Alexei Starovoitov <ast@kernel.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Andrey Konovalov <andreyknvl@google.com>,
	Andy Lutomirski <luto@kernel.org>, Arnd Bergmann <arnd@arndb.de>,
	Borislav Petkov <bp@alien8.de>, Christoph Hellwig <hch@lst.de>,
	Christoph Lameter <cl@linux.com>,
	David Rientjes <rientjes@google.com>,
	Dmitry Vyukov <dvyukov@google.com>,
	Eric Dumazet <edumazet@google.com>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Herbert Xu <herbert@gondor.apana.org.au>,
	Ilya Leoshkevich <iii@linux.ibm.com>,
	Ingo Molnar <mingo@redhat.com>, Jens Axboe <axboe@kernel.dk>,
	Joonsoo Kim <iamjoonsoo.kim@lge.com>,
	Kees Cook <keescook@chromium.org>, Marco Elver <elver@google.com>,
	Mark Rutland <mark.rutland@arm.com>,
	Matthew Wilcox <willy@infradead.org>,
	"Michael S. Tsirkin" <mst@redhat.com>,
	Pekka Enberg <penberg@kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Petr Mladek <pmladek@suse.com>,
	Steven Rostedt <rostedt@goodmis.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Vasily Gorbik <gor@linux.ibm.com>,
	Vegard Nossum <vegard.nossum@oracle.com>,
	Vlastimil Babka <vbabka@suse.cz>,
	kasan-dev <kasan-dev@googlegroups.com>,
	Linux-MM <linux-mm@kvack.org>,
	linux-arch <linux-arch@vger.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Evgenii Stepanov <eugenis@google.com>,
	Nathan Chancellor <nathan@kernel.org>,
	Nick Desaulniers <ndesaulniers@google.com>,
	Vitaly Buka <vitalybuka@google.com>,
	linux-toolchains <linux-toolchains@vger.kernel.org>
Subject: Re: [PATCH v4 43/45] namei: initialize parameters passed to step_into()
Date: Mon, 4 Jul 2022 13:23:10 -0500	[thread overview]
Message-ID: <20220704182310.GQ25951@gate.crashing.org> (raw)
In-Reply-To: <CAG_fn=VTihJSzQ106WPaQNxwTuuB8iPQpZR4306v8KmXxQT_GQ@mail.gmail.com>

On Mon, Jul 04, 2022 at 05:49:13PM +0200, Alexander Potapenko wrote:
> One of the reasons to do so is standard compliance - passing an
> uninitialized value to a function is UB in C11, as Segher pointed out
> here: https://lore.kernel.org/linux-toolchains/20220614214039.GA25951@gate.crashing.org/
> The compilers may not be smart enough to take advantage of this _yet_,
> but I wouldn't underestimate their ability to evolve (especially that
> of Clang).

GCC doesn't currently detect this UB, and doesn't even warn or error for
this, although that shouldn't be hard to do: it is all completely local.
An error is warranted here, and you won't get UB ever either then.

> I also believe it's fragile to rely on the callee to ignore certain
> parameters: it may be doing so today, but if someone changes
> step_into() tomorrow we may miss it.

There isn't any choice usually, this is C, do you want varargs?  :-)

But yes, you always should only pass "safe" values; callers should do
their part, and not assume the callee will do in the future as it does
now.  Defensive programming is mostly about defending your own sanity!


Segher

  parent reply	other threads:[~2022-07-04 18:39 UTC|newest]

Thread overview: 33+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
     [not found] <20220701142310.2188015-1-glider@google.com>
2022-07-01 14:23 ` [PATCH v4 43/45] namei: initialize parameters passed to step_into() Alexander Potapenko
2022-07-02 17:23   ` Linus Torvalds
2022-07-03  3:59     ` Al Viro
2022-07-04  2:52     ` Al Viro
2022-07-04  8:20       ` Alexander Potapenko
2022-07-04 13:44         ` Al Viro
2022-07-04 13:55           ` Al Viro
2022-07-04 15:49           ` Alexander Potapenko
2022-07-04 16:03             ` Greg Kroah-Hartman
2022-07-04 16:33               ` Alexander Potapenko
2022-07-04 18:23             ` Segher Boessenkool [this message]
2022-07-04 16:00           ` Al Viro
2022-07-04 16:47             ` Alexander Potapenko
2022-07-04 17:36       ` Linus Torvalds
2022-07-04 19:02         ` Al Viro
2022-07-04 19:16           ` Linus Torvalds
2022-07-04 19:55             ` Al Viro
2022-07-04 20:24               ` Linus Torvalds
2022-07-04 20:46                 ` Al Viro
2022-07-04 20:51                   ` Linus Torvalds
2022-07-04 21:04                     ` Al Viro
2022-07-04 23:13                       ` [PATCH 1/7] __follow_mount_rcu(): verify that mount_lock remains unchanged Al Viro
2022-07-04 23:14                         ` [PATCH 2/7] follow_dotdot{,_rcu}(): change calling conventions Al Viro
2022-07-04 23:14                         ` [PATCH 3/7] namei: stash the sampled ->d_seq into nameidata Al Viro
2022-07-04 23:15                         ` [PATCH 4/7] step_into(): lose inode argument Al Viro
2022-07-04 23:15                         ` [PATCH 5/7] follow_dotdot{,_rcu}(): don't bother with inode Al Viro
2022-07-04 23:16                         ` [PATCH 6/7] lookup_fast(): " Al Viro
2022-07-04 23:17                         ` [PATCH 7/7] step_into(): move fetching ->d_inode past handle_mounts() Al Viro
2022-07-04 23:19                         ` [PATCH 1/7] __follow_mount_rcu(): verify that mount_lock remains unchanged Al Viro
2022-07-05  0:06                           ` Linus Torvalds
2022-07-05  3:48                             ` Al Viro
2022-07-04 20:47                 ` [PATCH v4 43/45] namei: initialize parameters passed to step_into() Linus Torvalds
2022-08-08 16:37   ` Alexander Potapenko

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