fs: proc: lockdep spew and questions

Sasha Levin
2013-12-08T20:58:06Z

Hi all,

I've been looking at permissions in procfs, and noticed that directories are usually added with
execute permission.

As far as I know, there's nothing executable by default inside procfs, and on top of that, many of
the files there don't deal well with being executed so it's easy to cause lockdep spews (one below).

Is there a reason execute bit is set for directories?

sh-4.1# chmod +x /proc/10/net/igmp
sh-4.1# /proc/10/net/igmp
/proc/10/net/igmp: line 1: Idx: command not found
sh-4.1# cat /proc/10/maps
[ 64.535651]
[ 64.535898] ======================================================
[ 64.536674] [ INFO: possible circular locking dependency detected ]
[ 64.537288] 3.13.0-rc2-next-20131206-sasha-00005-g8be2375-dirty #4052 Not tainted
[ 64.538211] -------------------------------------------------------
[ 64.538831] cat/7896 is trying to acquire lock:
[ 64.539307] (&sig->cred_guard_mutex){+.+.+.}, at: [<ffffffff8112e684>] mm_access+0x34/0xb0
[ 64.540349]
[ 64.540349] but task is already holding lock:
[ 64.540906] (&p->lock){+.+.+.}, at: [<ffffffff813000df>] seq_read+0x3f/0x430
[ 64.541877]
[ 64.541877] which lock already depends on the new lock.
[ 64.541877]
[ 64.542449]
[ 64.542449] the existing dependency chain (in reverse order) is:
[ 64.542449]
-> #1 (&p->lock){+.+.+.}:
[ 64.542449] [<ffffffff81194833>] validate_chain+0x6c3/0x7b0
[ 64.542449] [<ffffffff81194dcd>] __lock_acquire+0x4ad/0x580
[ 64.542449] [<ffffffff81195022>] lock_acquire+0x182/0x1d0
[ 64.542449] [<ffffffff8439825f>] mutex_lock_nested+0x6f/0x520
[ 64.542449] [<ffffffff813000df>] seq_read+0x3f/0x430
[ 64.542449] [<ffffffff8134e62f>] proc_reg_read+0x5f/0x80
[ 64.542449] [<ffffffff812d7177>] vfs_read+0xc7/0x180
[ 64.542449] [<ffffffff812dce66>] kernel_read+0x46/0x60
[ 64.542449] [<ffffffff812dcfd3>] prepare_binprm+0x153/0x170
[ 64.542449] [<ffffffff812de2d5>] do_execve_common+0x1a5/0x360
[ 64.542449] [<ffffffff812de557>] do_execve+0x37/0x40
[ 64.542449] [<ffffffff812de5a6>] SyS_execve+0x46/0x70
[ 64.542449] [<ffffffff843a6c19>] stub_execve+0x69/0xa0
[ 64.542449]
-> #0 (&sig->cred_guard_mutex){+.+.+.}:
[ 64.542449] [<ffffffff81193d5f>] check_prev_add+0x13f/0x550
[ 64.542449] [<ffffffff81194833>] validate_chain+0x6c3/0x7b0
[ 64.542449] [<ffffffff81194dcd>] __lock_acquire+0x4ad/0x580
[ 64.542449] [<ffffffff81195022>] lock_acquire+0x182/0x1d0
[ 64.542449] [<ffffffff8439967f>] mutex_lock_killable_nested+0x6f/0x620
[ 64.542449] [<ffffffff8112e684>] mm_access+0x34/0xb0
[ 64.542449] [<ffffffff8134bf49>] m_start+0x79/0x1d0
[ 64.542449] [<ffffffff81300228>] seq_read+0x188/0x430
[ 64.542449] [<ffffffff812d7177>] vfs_read+0xc7/0x180
[ 64.542449] [<ffffffff812d7332>] SyS_read+0x62/0xa0
[ 64.542449] [<ffffffff843a6810>] tracesys+0xdd/0xe2
[ 64.542449]
[ 64.542449] other info that might help us debug this:
[ 64.542449]
[ 64.542449] Possible unsafe locking scenario:
[ 64.542449]
[ 64.542449] CPU0 CPU1
[ 64.542449] ---- ----
[ 64.542449] lock(&p->lock);
[ 64.542449] lock(&sig->cred_guard_mutex);
[ 64.542449] lock(&p->lock);
[ 64.542449] lock(&sig->cred_guard_mutex);
[ 64.542449]
[ 64.542449] *** DEADLOCK ***
[ 64.542449]
[ 64.542449] 1 lock held by cat/7896:
[ 64.542449] #0: (&p->lock){+.+.+.}, at: [<ffffffff813000df>] seq_read+0x3f/0x430
[ 64.542449]
[ 64.542449] stack backtrace:
[ 64.542449] CPU: 0 PID: 7896 Comm: cat Not tainted
3.13.0-rc2-next-20131206-sasha-00005-g8be2375-dirty #4052
[ 64.542449] 0000000000000000 ffff880fbc169af8 ffffffff843956f7 0000000000000000
[ 64.542449] 0000000000000000 ffff880fbc169b48 ffffffff81191909 ffff880fbc169b68
[ 64.542449] ffff880fbc169b68 ffff880fbc169b48 ffff880fb89fbbd8 ffff880fb89fbc10
[ 64.542449] Call Trace:
[ 64.542449] [<ffffffff843956f7>] dump_stack+0x52/0x7f
[ 64.542449] [<ffffffff81191909>] print_circular_bug+0x129/0x160
[ 64.542449] [<ffffffff81193d5f>] check_prev_add+0x13f/0x550
[ 64.542449] [<ffffffff812aec6f>] ? deactivate_slab+0x8cf/0x920
[ 64.542449] [<ffffffff81194833>] validate_chain+0x6c3/0x7b0
[ 64.542449] [<ffffffff811755b8>] ? sched_clock_cpu+0x108/0x120
[ 64.542449] [<ffffffff81194dcd>] __lock_acquire+0x4ad/0x580
[ 64.542449] [<ffffffff81195022>] lock_acquire+0x182/0x1d0
[ 64.542449] [<ffffffff8112e684>] ? mm_access+0x34/0xb0
[ 64.542449] [<ffffffff8439967f>] mutex_lock_killable_nested+0x6f/0x620
[ 64.542449] [<ffffffff8112e684>] ? mm_access+0x34/0xb0
[ 64.542449] [<ffffffff81159480>] ? get_pid_task+0xd0/0x100
[ 64.542449] [<ffffffff8112e684>] ? mm_access+0x34/0xb0
[ 64.542449] [<ffffffff8112e684>] mm_access+0x34/0xb0
[ 64.542449] [<ffffffff811593b0>] ? alloc_pid+0x260/0x260
[ 64.542449] [<ffffffff8134bf49>] m_start+0x79/0x1d0
[ 64.542449] [<ffffffff81176376>] ? vtime_account_user+0x96/0xb0
[ 64.542449] [<ffffffff81300228>] seq_read+0x188/0x430
[ 64.542449] [<ffffffff812d7177>] vfs_read+0xc7/0x180
[ 64.542449] [<ffffffff812d7332>] SyS_read+0x62/0xa0
[ 64.542449] [<ffffffff843a6810>] tracesys+0xdd/0xe2