On Fri, Apr 11, 2014 at 04:07:25PM +0100, Al Viro wrote: > On Fri, Apr 11, 2014 at 03:50:27PM +0100, David Howells wrote: > > Peter Zijlstra <pet...@infradead.org> wrote: > > > > > Al, David, any bright ideas on how to best fix this? > > > > Have the seq_xxx() code throw an error if current->in_execve is true. I > > can't > > think of any circumstance where execve() should be reading anything that > > uses > > seq_xxx(). > > *cringe* > > I don't like it. That really should be a responsiblity of specific ->show(); > "I'm going to take that mutex, bugger off if we are in execve()" makes a lot > more sense than having e.g. seq_read() care of that. IOW, I would very > much prefer the patch you've sent last week. > > And yes, it might leave lockdep false positives, but that's better dealt with > by annotating the sucker ("this guy has a separate lockdep class for its > ->lock"). E.g. by splitting proc_single_file_operations in two and having > the one used for those files do lockdep_set_class() in its ->open().
I've got annoyed by the lockdep warning. What about the patch below? >From 54d8c463e12f23c09d6a2dbf93a4dc9bcb493c67 Mon Sep 17 00:00:00 2001 From: "Kirill A. Shutemov" <kirill.shute...@linux.intel.com> Date: Thu, 31 Jul 2014 00:59:52 +0300 Subject: [PATCH] procfs: silence lockdep warning about read vs. exec seq_file Testcase: cat /proc/self/maps >/dev/null chmod +x /proc/self/net/packet exec /proc/self/net/packet It triggers lockdep warning: [ INFO: possible circular locking dependency detected ] 3.16.0-rc7-00064-g26bcd8b72563 #8 Not tainted ------------------------------------------------------- sh/157 is trying to acquire lock: (&p->lock){+.+.+.}, at: [<ffffffff8117f4f8>] seq_read+0x38/0x3e0 but task is already holding lock: (&sig->cred_guard_mutex){+.+.+.}, at: [<ffffffff81160018>] prepare_bprm_creds+0x28/0x90 which lock already depends on the new lock. the existing dependency chain (in reverse order) is: -> #1 (&sig->cred_guard_mutex){+.+.+.}: [<ffffffff8109a9c1>] __lock_acquire+0x531/0xde0 [<ffffffff8109b959>] lock_acquire+0x79/0xd0 [<ffffffff8173f838>] mutex_lock_killable_nested+0x68/0x460 [<ffffffff811c0d9f>] lock_trace+0x1f/0x60 [<ffffffff811c0ed7>] proc_pid_personality+0x17/0x60 [<ffffffff811be39b>] proc_single_show+0x4b/0x90 [<ffffffff8117f5a0>] seq_read+0xe0/0x3e0 [<ffffffff81158f1e>] vfs_read+0x8e/0x170 [<ffffffff81159be8>] SyS_read+0x48/0xc0 [<ffffffff81743712>] system_call_fastpath+0x16/0x1b -> #0 (&p->lock){+.+.+.}: [<ffffffff81097437>] validate_chain.isra.37+0xfe7/0x13b0 [<ffffffff8109a9c1>] __lock_acquire+0x531/0xde0 [<ffffffff8109b959>] lock_acquire+0x79/0xd0 [<ffffffff8173f09a>] mutex_lock_nested+0x6a/0x3d0 [<ffffffff8117f4f8>] seq_read+0x38/0x3e0 [<ffffffff811bd5f3>] proc_reg_read+0x43/0x70 [<ffffffff81158f1e>] vfs_read+0x8e/0x170 [<ffffffff8115ea13>] kernel_read+0x43/0x60 [<ffffffff8115ec65>] prepare_binprm+0xd5/0x170 [<ffffffff811605c8>] do_execve_common.isra.32+0x548/0x800 [<ffffffff81160893>] do_execve+0x13/0x20 [<ffffffff81160b70>] SyS_execve+0x20/0x30 [<ffffffff81743c89>] stub_execve+0x69/0xa0 other info that might help us debug this: Possible unsafe locking scenario: CPU0 CPU1 ---- ---- lock(&sig->cred_guard_mutex); lock(&p->lock); lock(&sig->cred_guard_mutex); lock(&p->lock); *** DEADLOCK *** 1 lock held by sh/157: #0: (&sig->cred_guard_mutex){+.+.+.}, at: [<ffffffff81160018>] prepare_bprm_creds+0x28/0x90 It's a false positive: seq files which take cred_guard_mutex are never executable. Let's use separate lock class for them. I don't know why we allow "chmod +x" on some proc files, notably net-related. Is it a bug? Also I suspect eb94cd96e05d fixes non-existing bug, like this one. Signed-off-by: Kirill A. Shutemov <kirill.shute...@linux.intel.com> --- fs/proc/base.c | 24 +++++++++++++++++++++++- fs/proc/task_mmu.c | 14 ++++++++++++++ fs/proc/task_nommu.c | 4 ++++ 3 files changed, 41 insertions(+), 1 deletion(-) diff --git a/fs/proc/base.c b/fs/proc/base.c index 2d696b0c93bf..c05b4a227acb 100644 --- a/fs/proc/base.c +++ b/fs/proc/base.c @@ -655,9 +655,31 @@ static int proc_single_show(struct seq_file *m, void *v) return ret; } +/* + * proc_pid_personality() and proc_pid_stack() take cred_guard_mutex via + * lock_trace() with seq_file->lock held. + * execve(2) calls vfs_read() with cred_guard_mutex held. + * + * So if you will try to execute a seq_file, lockdep will report a possible + * circular locking dependency. It's false-positive, since ONE() files are + * never executable. + * + * Let's set separate lock class for seq_file->lock of ONE() files. + */ +static struct lock_class_key proc_single_open_lock_class; + static int proc_single_open(struct inode *inode, struct file *filp) { - return single_open(filp, proc_single_show, inode); + struct seq_file *m; + int ret; + + ret = single_open(filp, proc_single_show, inode); + if (ret) + return ret; + + m = filp->private_data; + lockdep_set_class(&m->lock, &proc_single_open_lock_class); + return 0; } static const struct file_operations proc_single_file_operations = { diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c index cfa63ee92c96..536b9f9a9ff5 100644 --- a/fs/proc/task_mmu.c +++ b/fs/proc/task_mmu.c @@ -19,6 +19,18 @@ #include <asm/tlbflush.h> #include "internal.h" +/* + * m_start() takes cred_guard_mutex via mm_access() with seq_file->lock held. + * execve(2) calls vfs_read() with cred_guard_mutex held. + * + * So if you will try to execute a seq_file, lockdep will report a possible + * circular locking dependency. It's false positive, since m_start() users are + * never executable. + * + * Let's set separate class lock for seq_file->lock of m_start() users. + */ +static struct lock_class_key pid_maps_seq_file_lock; + void task_mem(struct seq_file *m, struct mm_struct *mm) { unsigned long data, text, lib, swap; @@ -242,6 +254,7 @@ static int do_maps_open(struct inode *inode, struct file *file, ret = seq_open(file, ops); if (!ret) { struct seq_file *m = file->private_data; + lockdep_set_class(&m->lock, &pid_maps_seq_file_lock); m->private = priv; } else { kfree(priv); @@ -1512,6 +1525,7 @@ static int numa_maps_open(struct inode *inode, struct file *file, ret = seq_open(file, ops); if (!ret) { struct seq_file *m = file->private_data; + lockdep_set_class(&m->lock, &pid_maps_seq_file_lock); m->private = priv; } else { kfree(priv); diff --git a/fs/proc/task_nommu.c b/fs/proc/task_nommu.c index 678455d2d683..35a799443990 100644 --- a/fs/proc/task_nommu.c +++ b/fs/proc/task_nommu.c @@ -9,6 +9,9 @@ #include <linux/seq_file.h> #include "internal.h" +/* See comment in task_mmu.c */ +static struct lock_class_key pid_maps_seq_file_lock; + /* * Logic: we've got two memory sums for each process, "shared", and * "non-shared". Shared memory may get counted more than once, for @@ -277,6 +280,7 @@ static int maps_open(struct inode *inode, struct file *file, ret = seq_open(file, ops); if (!ret) { struct seq_file *m = file->private_data; + lockdep_set_class(&m->lock, &pid_maps_seq_file_lock); m->private = priv; } else { kfree(priv); -- Kirill A. Shutemov -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/