linux-kernel - Re: [PATCH] tracefs: avoid setting i

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Message-Id: <A9B746AC-DF36-4ED3-BBA3-2EEC1BA166C8@gmail.com>
Date:   Thu, 17 Aug 2023 19:47:34 -0400
From:   Sishuai Gong <sishuai.system@...il.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     mhiramat@...nel.org, linux-kernel@...r.kernel.org,
        linux-trace-kernel@...r.kernel.org
Subject: Re: [PATCH] tracefs: avoid setting i_mode to a temp value


> On Aug 16, 2023, at 3:52 PM, Steven Rostedt <rostedt@...dmis.org> wrote:
> 
> On Thu, 10 Aug 2023 20:59:26 -0400
> Sishuai Gong <sishuai.system@...il.com> wrote:
> 
>> Right now inode->i_mode is updated twice to reach the desired value
>> in tracefs_apply_options(). Because there is no lock protecting the two
>> writes, other threads might read the intermediate value of inode->i_mode.
>> 
>> Thread-1 Thread-2
>> // tracefs_apply_options() //e.g., acl_permission_check
>> inode->i_mode &= ~S_IALLUGO;
>> unsigned int mode = inode->i_mode;
>> inode->i_mode |= opts->mode;
>> 
>> I think there is no need to introduce a lock but it is better to
>> only update inode->i_mode ONCE, so the readers will either see the old
>> or latest value, rather than an intermediate/temporary value.
>> 
>> Signed-off-by: Sishuai Gong <sishuai.system@...il.com>
>> ---
>> fs/tracefs/inode.c | 3 +--
>> 1 file changed, 1 insertion(+), 2 deletions(-)
>> 
>> diff --git a/fs/tracefs/inode.c b/fs/tracefs/inode.c
>> index 57ac8aa4a724..dca84ebb62fa 100644
>> --- a/fs/tracefs/inode.c
>> +++ b/fs/tracefs/inode.c
>> @@ -297,8 +297,7 @@ static int tracefs_apply_options(struct super_block *sb, bool remount)
>>  */
>> 
>> if (!remount || opts->opts & BIT(Opt_mode)) {
>> - inode->i_mode &= ~S_IALLUGO;
>> - inode->i_mode |= opts->mode;
>> + inode->i_mode = (inode->i_mode & ~S_IALLUGO) | opts->mode;
> 
> You do realize that the compiler could decide to keep the original logic
> even with this change? If it was crucial that the compiler did not, you
> would need to have:
> 
> if (!remount || opts->opts & BIT(Opt_mode)) {
> umode_t tmp = READ_ONCE(inode->i_mode);
> 
> tmp &= ~S_IALLUGO
> tmp |= opts->mode;
> WRITE_ONCE(inode->i_mode, tmp);
> } 
> 
You are right. This will prevent the compiler from emitting two writes.
I will incorporate your suggestion in the new version.

> And if you notice the !remount flag, this is only preformed when the file
> system is actually mounted. Are the files visible before then?
> 
> Can you produce this race?
This data race was detected when I was testing the kernel (e.g., fuzzing)
but I did not make the attempt to reproduce it.

> 
> -- Steve
> 
> 
> 
>> }
>> 
>> if (!remount || opts->opts & BIT(Opt_uid))