From: Kees Cook <keescook@chromium.org>
To: Luis Chamberlain <mcgrof@kernel.org>
Cc: tj@kernel.org, gregkh@linuxfoundation.org,
akpm@linux-foundation.org, minchan@kernel.org, jeyu@kernel.org,
shuah@kernel.org, bvanassche@acm.org, dan.j.williams@intel.com,
joe@perches.com, tglx@linutronix.de, rostedt@goodmis.org,
linux-spdx@vger.kernel.org, linux-doc@vger.kernel.org,
linux-block@vger.kernel.org, linux-fsdevel@vger.kernel.org,
linux-kselftest@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v8 06/12] kernel/module: add documentation for try_module_get()
Date: Tue, 5 Oct 2021 12:58:47 -0700 [thread overview]
Message-ID: <202110051252.790B3F2F0@keescook> (raw)
In-Reply-To: <20210927163805.808907-7-mcgrof@kernel.org>
On Mon, Sep 27, 2021 at 09:37:59AM -0700, Luis Chamberlain wrote:
> There is quite a bit of tribal knowledge around proper use of
> try_module_get() and that it must be used only in a context which
> can ensure the module won't be gone during the operation. Document
> this little bit of tribal knowledge.
>
> I'm extending this tribal knowledge with new developments which it
> seems some folks do not yet believe to be true: we can be sure a
> module will exist during the lifetime of a sysfs file operation.
> For proof, refer to test_sysfs test #32:
>
> ./tools/testing/selftests/sysfs/sysfs.sh -t 0032
>
> Without this being true, the write would fail or worse,
> a crash would happen, in this test. It does not.
>
> Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>
> ---
> include/linux/module.h | 34 ++++++++++++++++++++++++++++++++--
> 1 file changed, 32 insertions(+), 2 deletions(-)
>
> diff --git a/include/linux/module.h b/include/linux/module.h
> index c9f1200b2312..22eacd5e1e85 100644
> --- a/include/linux/module.h
> +++ b/include/linux/module.h
> @@ -609,10 +609,40 @@ void symbol_put_addr(void *addr);
> to handle the error case (which only happens with rmmod --wait). */
> extern void __module_get(struct module *module);
>
> -/* This is the Right Way to get a module: if it fails, it's being removed,
> - * so pretend it's not there. */
> +/**
> + * try_module_get() - yields to module removal and bumps refcnt otherwise
I find this hard to parse. How about:
"Take module refcount unless module is being removed"
> + * @module: the module we should check for
> + *
> + * This can be used to try to bump the reference count of a module, so to
> + * prevent module removal. The reference count of a module is not allowed
> + * to be incremented if the module is already being removed.
This I understand.
> + *
> + * Care must be taken to ensure the module cannot be removed during the call to
> + * try_module_get(). This can be done by having another entity other than the
> + * module itself increment the module reference count, or through some other
> + * means which guarantees the module could not be removed during an operation.
> + * An example of this later case is using try_module_get() in a sysfs file
> + * which the module created. The sysfs store / read file operations are
> + * gauranteed to exist through the use of kernfs's active reference (see
> + * kernfs_active()). If a sysfs file operation is being run, the module which
> + * created it must still exist as the module is in charge of removing the same
> + * sysfs file being read. Also, a sysfs / kernfs file removal cannot happen
> + * unless the same file is not active.
I can't understand this paragraph at all. "Care must be taken ..."? Why?
Shouldn't callers of try_module_get() be satisfied with the results? I
don't follow the example at all. It seems to just say "sysfs store/read
functions don't need try_module_get() because whatever opened the sysfs
file is already keeping the module referenced." ?
> + *
> + * One of the real values to try_module_get() is the module_is_live() check
> + * which ensures this the caller of try_module_get() can yield to userspace
> + * module removal requests and fail whatever it was about to process.
Please document the return value explicitly.
> + */
> extern bool try_module_get(struct module *module);
>
> +/**
> + * module_put() - release a reference count to a module
> + * @module: the module we should release a reference count for
> + *
> + * If you successfully bump a reference count to a module with try_module_get(),
> + * when you are finished you must call module_put() to release that reference
> + * count.
> + */
> extern void module_put(struct module *module);
>
> #else /*!CONFIG_MODULE_UNLOAD*/
> --
> 2.30.2
>
--
Kees Cook
next prev parent reply other threads:[~2021-10-05 19:58 UTC|newest]
Thread overview: 94+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-09-27 16:37 [PATCH v8 00/12] syfs: generic deadlock fix with module removal Luis Chamberlain
2021-09-27 16:37 ` [PATCH v8 01/12] LICENSES: Add the copyleft-next-0.3.1 license Luis Chamberlain
[not found] ` <202110050907.35FBD2A1@keescook>
[not found] ` <YWR2ZrtzChamY1y4@bombadil.infradead.org>
2021-10-11 17:57 ` Kees Cook
2021-09-27 16:37 ` [PATCH v8 02/12] testing: use the copyleft-next-0.3.1 SPDX tag Luis Chamberlain
2021-10-05 16:11 ` Kees Cook
2021-09-27 16:37 ` [PATCH v8 03/12] selftests: add tests_sysfs module Luis Chamberlain
2021-10-05 14:16 ` Greg KH
2021-10-05 16:57 ` Tim.Bird
2021-10-11 17:40 ` Luis Chamberlain
2021-10-11 17:38 ` Luis Chamberlain
2021-10-07 14:23 ` Miroslav Benes
2021-10-11 19:11 ` Luis Chamberlain
[not found] ` <202110050912.3DF681ED@keescook>
2021-10-11 19:03 ` Luis Chamberlain
2021-09-27 16:37 ` [PATCH v8 04/12] kernfs: add initial failure injection support Luis Chamberlain
2021-10-05 19:47 ` Kees Cook
2021-10-11 20:44 ` Luis Chamberlain
2021-09-27 16:37 ` [PATCH v8 05/12] test_sysfs: add support to use kernfs failure injection Luis Chamberlain
2021-10-05 19:51 ` Kees Cook
2021-10-11 20:56 ` Luis Chamberlain
2021-09-27 16:37 ` [PATCH v8 06/12] kernel/module: add documentation for try_module_get() Luis Chamberlain
2021-10-05 19:58 ` Kees Cook [this message]
2021-10-11 21:16 ` Luis Chamberlain
2021-09-27 16:38 ` [PATCH v8 07/12] fs/kernfs/symlink.c: replace S_IRWXUGO with 0777 on kernfs_create_link() Luis Chamberlain
2021-10-05 19:59 ` Kees Cook
2021-09-27 16:38 ` [PATCH v8 08/12] fs/sysfs/dir.c: replace S_IRWXU|S_IRUGO|S_IXUGO with 0755 sysfs_create_dir_ns() Luis Chamberlain
2021-10-05 16:05 ` Kees Cook
2021-09-27 16:38 ` [PATCH v8 09/12] sysfs: fix deadlock race with module removal Luis Chamberlain
2021-10-05 9:24 ` Ming Lei
2021-10-11 21:25 ` Luis Chamberlain
2021-10-12 0:20 ` Ming Lei
2021-10-12 21:18 ` Luis Chamberlain
2021-10-13 1:07 ` Ming Lei
2021-10-13 12:35 ` Luis Chamberlain
2021-10-13 15:04 ` Ming Lei
2021-10-13 21:16 ` Luis Chamberlain
2021-10-05 20:50 ` Kees Cook
2021-10-11 22:26 ` Luis Chamberlain
2021-10-13 12:41 ` Luis Chamberlain
2021-09-27 16:38 ` [PATCH v8 10/12] test_sysfs: enable deadlock tests by default Luis Chamberlain
2021-09-27 16:38 ` [PATCH v8 11/12] zram: fix crashes with cpu hotplug multistate Luis Chamberlain
2021-10-05 20:55 ` Kees Cook
2021-10-11 18:27 ` Luis Chamberlain
2021-10-14 1:55 ` Ming Lei
2021-10-14 2:11 ` Ming Lei
2021-10-14 20:24 ` Luis Chamberlain
2021-10-14 23:52 ` Ming Lei
2021-10-15 0:22 ` Luis Chamberlain
2021-10-15 8:36 ` Ming Lei
2021-10-15 8:52 ` Greg KH
2021-10-15 17:31 ` Luis Chamberlain
2021-10-16 11:28 ` Ming Lei
2021-10-18 19:32 ` Luis Chamberlain
2021-10-19 2:34 ` Ming Lei
2021-10-19 6:23 ` Miroslav Benes
2021-10-19 9:23 ` Ming Lei
2021-10-20 6:43 ` Miroslav Benes
2021-10-20 7:49 ` Ming Lei
2021-10-20 8:19 ` Miroslav Benes
2021-10-20 8:28 ` Greg KH
2021-10-25 9:58 ` Miroslav Benes
2021-10-20 10:09 ` Ming Lei
2021-10-26 8:48 ` Petr Mladek
2021-10-26 15:37 ` Ming Lei
2021-10-26 17:01 ` Luis Chamberlain
2021-10-27 11:57 ` Miroslav Benes
2021-10-27 14:27 ` Luis Chamberlain
2021-11-02 15:24 ` Petr Mladek
2021-11-02 16:25 ` Luis Chamberlain
2021-11-03 0:01 ` Ming Lei
2021-11-03 12:44 ` Luis Chamberlain
2021-10-27 11:42 ` Miroslav Benes
2021-11-02 14:15 ` Petr Mladek
2021-11-02 14:51 ` Petr Mladek
2021-11-02 15:17 ` Ming Lei
2021-11-02 14:56 ` Ming Lei
2021-10-19 15:28 ` Luis Chamberlain
2021-10-19 16:29 ` Ming Lei
2021-10-19 19:36 ` Luis Chamberlain
2021-10-20 1:15 ` Ming Lei
2021-10-20 15:48 ` Luis Chamberlain
2021-10-21 0:39 ` Ming Lei
2021-10-21 17:18 ` Luis Chamberlain
2021-10-22 0:05 ` Ming Lei
2021-10-19 15:50 ` Luis Chamberlain
2021-10-19 16:25 ` Greg KH
2021-10-19 16:30 ` Luis Chamberlain
2021-10-19 17:28 ` Greg KH
2021-10-19 19:46 ` Luis Chamberlain
2021-10-19 16:39 ` Ming Lei
2021-10-19 19:38 ` Luis Chamberlain
2021-10-20 0:55 ` Ming Lei
2021-09-27 16:38 ` [PATCH v8 12/12] zram: use ATTRIBUTE_GROUPS to fix sysfs deadlock module removal Luis Chamberlain
2021-10-05 20:57 ` Kees Cook
2021-10-11 18:28 ` Luis Chamberlain
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=202110051252.790B3F2F0@keescook \
--to=keescook@chromium.org \
--cc=akpm@linux-foundation.org \
--cc=bvanassche@acm.org \
--cc=dan.j.williams@intel.com \
--cc=gregkh@linuxfoundation.org \
--cc=jeyu@kernel.org \
--cc=joe@perches.com \
--cc=linux-block@vger.kernel.org \
--cc=linux-doc@vger.kernel.org \
--cc=linux-fsdevel@vger.kernel.org \
--cc=linux-kernel@vger.kernel.org \
--cc=linux-kselftest@vger.kernel.org \
--cc=linux-spdx@vger.kernel.org \
--cc=mcgrof@kernel.org \
--cc=minchan@kernel.org \
--cc=rostedt@goodmis.org \
--cc=shuah@kernel.org \
--cc=tglx@linutronix.de \
--cc=tj@kernel.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).