On Tue, Sep 18, 2012 at 01:49:52PM -0700, Linus Torvalds wrote: > On Tue, Sep 18, 2012 at 1:37 PM, Konrad Rzeszutek Wilk > <[email protected]> wrote: > > > > 30 words. ~360 + 29 spaces + NULL = 390? > > Just allocate the max then. That's tiny. > > And it's actually just 330: max ten characters for an unsigned 32-bit number.
Linus, Could you take a look at these two patches to see if I missed anything? Thank you. >From 0806b133b5b28081adf23d0d04a99636ed3b861b Mon Sep 17 00:00:00 2001 From: Konrad Rzeszutek Wilk <[email protected]> Date: Wed, 19 Sep 2012 11:23:01 -0400 Subject: [PATCH 1/2] debugfs: Add lock for u32_array_read Dave Jones spotted that the u32_array_read was doing something funny: ============================================================================= 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 .. snip.. 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 Linus tracked it down and found out that "debugfs is racy for that case [read calls in parallel on the debugfs]. At least the file->private_data accesses are, for the case of that "u32_array" case. In fact it is racy in ... the whole "file->private_data" access .. 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." He suggested putting a lock which this patch does. The consequence of this is that it will trigger more spinlock usage, as this particular debugfs is used to provide a histogram of spinlock contention. But memory corruption is a worst offender then that. Reported-by: Dave Jones <[email protected]> Suggested-by: Linus Torvalds <[email protected]> --- fs/debugfs/file.c | 7 ++++++- 1 files changed, 6 insertions(+), 1 deletions(-) diff --git a/fs/debugfs/file.c b/fs/debugfs/file.c index 2340f69..c6d9088 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); @@ -594,8 +596,10 @@ static ssize_t u32_array_read(struct file *file, char __user *buf, size_t len, if (file->private_data) size = strlen(file->private_data); - return simple_read_from_buffer(buf, len, ppos, + size = simple_read_from_buffer(buf, len, ppos, file->private_data, size); + mutex_unlock(&data->lock); + return size; } static int u32_array_release(struct inode *inode, struct file *file) @@ -643,6 +647,7 @@ 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); } -- 1.7.7.6 >From c3937cb7144a6ead80e6aabee89420645945a926 Mon Sep 17 00:00:00 2001 From: Konrad Rzeszutek Wilk <[email protected]> Date: Wed, 19 Sep 2012 14:04:22 -0400 Subject: [PATCH 2/2] debugfs: Pre-allocate u32 array to thwart a race. Linus spotted via code inspection "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." This patch fixes this by pre-allocating the buffer to the maximum size during debugfs initialization by the driver. We print %u values, so the math is pretty straightforward: 10 bytes for the maximum that %u can use (4294967295) + spaces for the number of elements and \n\0. We also add an extra byte to compensate for the data->len == size check which we would hit if all of the array entries were of their maximum size (-1U). If we end up using exactly up to the size we allocated (this includes the extra byte), then we allocate a new buffer twice the size. And if we fail again, we print a warning as snprintf is doing something silly. Reported-by: Linus Torvalds <[email protected]> Signed-off-by: Konrad Rzeszutek Wilk <[email protected]> --- fs/debugfs/file.c | 66 ++++++++++++++++++++++------------------------------ 1 files changed, 28 insertions(+), 38 deletions(-) diff --git a/fs/debugfs/file.c b/fs/debugfs/file.c index c6d9088..4c59200 100644 --- a/fs/debugfs/file.c +++ b/fs/debugfs/file.c @@ -524,12 +524,13 @@ EXPORT_SYMBOL_GPL(debugfs_create_blob); struct array_data { void *array; u32 elements; + ssize_t len; + char *buf; struct mutex lock; }; static int u32_array_open(struct inode *inode, struct file *file) { - file->private_data = NULL; return nonseekable_open(inode, file); } @@ -560,20 +561,6 @@ static size_t format_array(char *buf, size_t bufsize, const char *fmt, return ret; } -static char *format_array_alloc(const char *fmt, u32 *array, - u32 array_size) -{ - size_t len = format_array(NULL, 0, fmt, array, array_size); - char *ret; - - ret = kmalloc(len, GFP_KERNEL); - if (ret == NULL) - return NULL; - - format_array(ret, len, fmt, array, array_size); - return ret; -} - static ssize_t u32_array_read(struct file *file, char __user *buf, size_t len, loff_t *ppos) { @@ -582,37 +569,33 @@ static ssize_t u32_array_read(struct file *file, char __user *buf, size_t len, size_t size; mutex_lock(&data->lock); - if (*ppos == 0) { - if (file->private_data) { - kfree(file->private_data); - file->private_data = NULL; - } - - file->private_data = format_array_alloc("%u", data->array, - data->elements); + if (*ppos == 0) + size = format_array(data->buf, data->len - 1 /* for \0 */, + "%u", data->array, data->elements); + else + size = strlen(data->buf); + + if (size == data->len) { + char *p = krealloc(data->buf, data->len * 2, GFP_KERNEL); + if (ZERO_OR_NULL_PTR(p)) + goto out; + data->buf = p; + data->len *= 2; + size = format_array(data->buf, data->len - 1 /* for \0 */, + "%u", data->array, data->elements); + /* It keeps on growing! Early pre-allocation MUST be wrong. */ + WARN_ON(size == data->len); } - - size = 0; - if (file->private_data) - size = strlen(file->private_data); - +out: size = simple_read_from_buffer(buf, len, ppos, - file->private_data, size); + data->buf, size); mutex_unlock(&data->lock); return size; } -static int u32_array_release(struct inode *inode, struct file *file) -{ - kfree(file->private_data); - - return 0; -} - static const struct file_operations u32_array_fops = { .owner = THIS_MODULE, .open = u32_array_open, - .release = u32_array_release, .read = u32_array_read, .llseek = no_llseek, }; @@ -648,7 +631,14 @@ struct dentry *debugfs_create_u32_array(const char *name, umode_t mode, data->array = array; data->elements = elements; mutex_init(&data->lock); - + data->len = elements * 10 /* max value for %u */ + + elements - 1 /* spaces */ + 2 /* \n\0 */ + + 1 /* to thwart the size == data->len check. */; + data->buf = kmalloc(data->len, GFP_KERNEL); + if (!data->buf) { + kfree(data); + return ERR_PTR(-ENOMEM); + } return debugfs_create_file(name, mode, parent, data, &u32_array_fops); } EXPORT_SYMBOL_GPL(debugfs_create_u32_array); -- 1.7.7.6 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
