Re: [PATCH RESEND v5] proc: Allow pid_revalidate() during LOOKUP_RCU

[Stephen Brennan]

Stephen Brennan <stephen.s.brennan@xxxxxxxxxx> writes:
> The pid_revalidate() function drops from RCU into REF lookup mode. When
> many threads are resolving paths within /proc in parallel, this can
> result in heavy spinlock contention on d_lockref as each thread tries to
> grab a reference to the /proc dentry (and drop it shortly thereafter).
>
> Investigation indicates that it is not necessary to drop RCU in
> pid_revalidate(), as no RCU data is modified and the function never
> sleeps. So, remove the LOOKUP_RCU check.
>
> Signed-off-by: Stephen Brennan <stephen.s.brennan@xxxxxxxxxx>
> ---
>
> Resending this in the hopes of Al picking this up, or else more feedback about
> how to test for RCU-unsafe code in procfs.

I just wanted to bump this again - I'm very open to suggestions of new
workloads, configuration options, or selinux configurations which could
help me verify this is fully safe.

Thanks,
Stephen

> When running running ~128 parallel instances of "TZ=/etc/localtime ps -fe
> >/dev/null" on a 128CPU machine, the %sys utilization reaches 97%, and perf
> shows the following code path as being responsible for heavy contention on
> the d_lockref spinlock:
>
>       walk_component()
>         lookup_fast()
>           d_revalidate()
>             pid_revalidate() // returns -ECHILD
>           unlazy_child()
>             lockref_get_not_dead(&nd->path.dentry->d_lockref) <-- contention
>
> By applying this patch, %sys utilization falls to around 85% under the same
> workload, and the number of ps processes executed per unit time increases by
> 3x-4x. Although this particular workload is a bit contrived, we have seen some
> monitoring scripts which produced similarly high %sys time due to this
> contention.
>
> As a result this patch, several procfs methods which were only called in
> ref-walk mode could now be called from RCU mode. To ensure that this patch
> is safe, I audited all the inode get_link and permission() implementations,
> as well as dentry d_revalidate() implementations, in fs/proc. These methods
> are called in the following ways:
>
> * get_link() receives a NULL dentry pointer when called in RCU mode.
> * permission() receives MAY_NOT_BLOCK in the mode parameter when called
>   from RCU.
> * d_revalidate() receives LOOKUP_RCU in flags.
>
> There were generally three groups I found. Group (1) are functions which
> contain a check at the top of the function and return -ECHILD, and so
> appear to be trivially RCU safe (although this is by dropping out of RCU
> completely). Group (2) are functions which have no explicit check, but
> on my audit, I was confident that there were no sleeping function calls,
> and thus were RCU safe as is. However, I would appreciate any additional
> review if possible. Group (3) are functions which call security hooks, but
> which ought to be safe (especially after Al's commits: 23d8f5b684fc ("make
> dump_common_audit_data() safe to be called from RCU pathwalk") and 2
> previous).
>
> Group (1):
>  proc_ns_get_link()
>  proc_pid_get_link()
>  map_files_d_revalidate()
>  proc_misc_d_revalidate()
>  tid_fd_revalidate()
>
> Group (2):
>  proc_get_link()
>  proc_self_get_link()
>  proc_thread_self_get_link()
>  proc_fd_permission()
>
> Group (3):
>  pid_revalidate()            -- addressed by my patch,
>                                 calls security_task_to_inode()
>  proc_pid_permission()       -- calls security_ptrace_access_check()
>  proc_map_files_get_link()   -- calls security_capable()
>
> I've tested this patch by enabling CONFIG_PROVE_RCU to warn on sleeping during
> RCU, and running heavy procfs-related workloads (like the PS one described
> above). I would love more input on selinux/audit rules to explore to attempt to
> catch any other potential issues.
>
> Changes in v5:
> - Al's commits are now in linux-next, resolving proc_pid_permission() issue.
> - Add NULL check after d_inode_rcu(dentry), because inode may become NULL if
>   we do not hold a reference.
> Changes in v4:
> - Simplify by unconditionally calling pid_update_inode() from pid_revalidate,
>   and removing the LOOKUP_RCU check.
> Changes in v3:
> - Rather than call pid_update_inode() with flags, create
>   proc_inode_needs_update() to determine whether the call can be skipped.
> - Restore the call to the security hook
> Changes in v2:
> - Remove get_pid_task_rcu_user() and get_proc_task_rcu(), since they were
>   unnecessary.
> - Remove the call to security_task_to_inode().
>
>  fs/proc/base.c | 18 ++++++++++--------
>  1 file changed, 10 insertions(+), 8 deletions(-)
>
> diff --git a/fs/proc/base.c b/fs/proc/base.c
> index ebea9501afb8..3e105bd05801 100644
> --- a/fs/proc/base.c
> +++ b/fs/proc/base.c
> @@ -1830,19 +1830,21 @@ static int pid_revalidate(struct dentry *dentry, unsigned int flags)
>  {
>  	struct inode *inode;
>  	struct task_struct *task;
> +	int ret = 0;
>  
> -	if (flags & LOOKUP_RCU)
> -		return -ECHILD;
> -
> -	inode = d_inode(dentry);
> -	task = get_proc_task(inode);
> +	rcu_read_lock();
> +	inode = d_inode_rcu(dentry);
> +	if (!inode)
> +		goto out;
> +	task = pid_task(proc_pid(inode), PIDTYPE_PID);
>  
>  	if (task) {
>  		pid_update_inode(task, inode);
> -		put_task_struct(task);
> -		return 1;
> +		ret = 1;
>  	}
> -	return 0;
> +out:
> +	rcu_read_unlock();
> +	return ret;
>  }
>  
>  static inline bool proc_inode_is_dead(struct inode *inode)
> -- 
> 2.27.0