Messages in this thread Patch in this message | | | Date | Tue, 18 Sep 2012 15:23:38 -0400 | From | Konrad Rzeszutek Wilk <> | Subject | Re: 3.6rc6 slab corruption. |
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On Tue, Sep 18, 2012 at 11:38:44AM -0700, Linus Torvalds wrote: > Quoting the entire email, since I added Greg to the list of people (as > the documented maintainer of debugfs) along with what I think are the > guilty parties. > > Dave, is trinity perhaps doing read calls on the same file in parallel? > > Because it looks to me like debugfs is racy for that case. At least > the file->private_data accesses are, for the case of that "u32_array" > case. > > In fact it is racy in two different ways: > > - the whole "file->private_data" access is racy (and this is, I > think, the bug you are hitting) > > If you have multiple readers on the same file, the whole > > if (file->private_data) { > kfree(file->private_data); > file->private_data = NULL; > } > > file->private_data = format_array_alloc("%u", data->array, > data->elements); > > thing is just a disaster waiting to happen. > > It should be easyish to fix by just adding a lock around those things.
Like this:
diff --git a/fs/debugfs/file.c b/fs/debugfs/file.c index 2340f69..309b235 100644 --- a/fs/debugfs/file.c +++ b/fs/debugfs/file.c @@ -524,6 +524,7 @@ EXPORT_SYMBOL_GPL(debugfs_create_blob); struct array_data { void *array; u32 elements; + struct mutex lock; }; static int u32_array_open(struct inode *inode, struct file *file) @@ -580,6 +581,7 @@ static ssize_t u32_array_read(struct file *file, char __user *buf, size_t len, struct array_data *data = inode->i_private; size_t size; + mutex_lock(&data->lock); if (*ppos == 0) { if (file->private_data) { kfree(file->private_data); @@ -594,6 +596,7 @@ static ssize_t u32_array_read(struct file *file, char __user *buf, size_t len, if (file->private_data) size = strlen(file->private_data); + mutex_unlock(&data->lock); return simple_read_from_buffer(buf, len, ppos, file->private_data, size); } @@ -643,6 +646,7 @@ struct dentry *debugfs_create_u32_array(const char *name, umode_t mode, data->array = array; data->elements = elements; + mutex_init(&data->lock); return debugfs_create_file(name, mode, parent, data, &u32_array_fops); } > The other bug is slightly subtler and probably harder to hit (but also > harder to fix): > > - the whole format_array_alloc() code is one buggy piece of sh*t, > since afaik there is nothing that guarantees that the values cannot > change. So the notion of "let's format the output once to know how big > it is, and then a second time to actually print things into the array > we just allocated based on the first time" is pure and utter garbage, > afaik.
Yikes. The fix could be to allocate a buffer large enough for the maximum that %u could take * array_size and not bother with the first pass. > > Anyway, this is all fairly recent, and came in through the Xen tree. > See commit 9fe2a7015393 ("debugfs: Add support to print u32 array in > debugfs"). > > Guys?
Let me include Jeremy on this. He was the original author of it and perhaps he has some patch stashed away were he re-implemented it..
> > Linus > > On Tue, Sep 18, 2012 at 7:35 AM, Dave Jones <davej@redhat.com> wrote: > > I was chasing a networking bug, and had trinity reduced to just making read & setsockopt calls, > > and let that run overnight. I woke up to 800mb of traces from a different bug.. > > > > The traces look mostly like this.. > > > > > > ============================================================================= > > BUG kmalloc-64 (Not tainted): Redzone overwritten > > ----------------------------------------------------------------------------- > > > > INFO: 0xffff88001f4b4970-0xffff88001f4b4977. First byte 0xbb instead of 0xcc > > INFO: Allocated in u32_array_read+0xd1/0x110 age=0 cpu=6 pid=32767 > > __slab_alloc+0x516/0x5a5 > > __kmalloc+0x213/0x2c0 > > u32_array_read+0xd1/0x110 > > vfs_read+0xac/0x180 > > sys_read+0x4d/0x90 > > system_call_fastpath+0x1a/0x1f > > INFO: Freed in u32_array_read+0x99/0x110 age=0 cpu=0 pid=32749 > > __slab_free+0x3f/0x3bf > > kfree+0x2d5/0x310 > > u32_array_read+0x99/0x110 > > vfs_read+0xac/0x180 > > sys_read+0x4d/0x90 > > system_call_fastpath+0x1a/0x1f > > INFO: Slab 0xffffea00007d2d00 objects=41 used=14 fp=0xffff88001f4b7410 flags=0x10000000004081 > > INFO: Object 0xffff88001f4b4930 @offset=2352 fp=0xffff88001f4b7410 > > > > Bytes b4 ffff88001f4b4920: 1b 20 1c 00 01 00 00 00 5a 5a 5a 5a 5a 5a 5a 5a . ......ZZZZZZZZ > > Object ffff88001f4b4930: 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b kkkkkkkkkkkkkkkk > > Object ffff88001f4b4940: 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b kkkkkkkkkkkkkkkk > > Object ffff88001f4b4950: 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b kkkkkkkkkkkkkkkk > > Object ffff88001f4b4960: 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b a5 kkkkkkkkkkkkkkk. > > Redzone ffff88001f4b4970: bb bb bb bb bb bb bb bb ........ > > Padding ffff88001f4b4ab0: 5a 5a 5a 5a 5a 5a 5a 5a ZZZZZZZZ > > Pid: 32756, comm: trinity-child52 Not tainted 3.6.0-rc6+ #44 > > Call Trace: > > [<ffffffff811c10ad>] ? print_section+0x3d/0x40 > > [<ffffffff811c23fe>] print_trailer+0xfe/0x160 > > [<ffffffff811c2592>] check_bytes_and_report+0xe2/0x120 > > [<ffffffff81023b79>] ? native_sched_clock+0x19/0x80 > > [<ffffffff811c2b5b>] check_object+0x18b/0x250 > > [<ffffffff8169b7d7>] free_debug_processing+0xc0/0x1fd > > [<ffffffff812d2e29>] ? u32_array_read+0x99/0x110 > > [<ffffffff8169ba5c>] __slab_free+0x3f/0x3bf > > [<ffffffff81365a1c>] ? debug_check_no_obj_freed+0x16c/0x210 > > [<ffffffff810db04f>] ? lock_release_holdtime.part.26+0xf/0x180 > > [<ffffffff812d2e29>] ? u32_array_read+0x99/0x110 > > [<ffffffff811c5725>] kfree+0x2d5/0x310 > > [<ffffffff812d2e29>] u32_array_read+0x99/0x110 > > [<ffffffff811df88c>] vfs_read+0xac/0x180 > > [<ffffffff811df9ad>] sys_read+0x4d/0x90 > > [<ffffffff816aea2d>] system_call_fastpath+0x1a/0x1f > > FIX kmalloc-64: Restoring 0xffff88001f4b4970-0xffff88001f4b4977=0xcc > > ============================================================================= > > > > Which looks like we read some file (probably something in sysfs/procfs) that corrupted some internal state. > > Any ideas on what I could do to narrow this down ? > > > > The full traces are at http://www.codemonkey.org.uk/junk/slab-corrupt.txt > > They vary a little later, but it looks like it's probably all the same problem to me. > > Sometimes it flip-flops between "First byte 0xbb instead of 0xcc" and "First byte 0xcc instead of 0xbb" > > > > > > The one outlier being this weird message.. > > > > Sep 18 02:00:13 bitcrush kernel: [36617.487681] hrtimer: interrupt took 242337 ns > > > > Which is weird, but probably unrelated. > > > > Dave > >
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