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SubjectRe: [PATCH] tty: n_hdlc: Use flexible-array member
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On 1/21/20 09:24, Gustavo A. R. Silva wrote:
>
>
> On 1/21/20 09:14, Gustavo A. R. Silva wrote:
>>
>>
>> On 1/21/20 09:00, Mikael Magnusson wrote:
>>> Gustavo Silva wrote:
>>>> On 1/20/20 23:54, Jiri Slaby wrote:
>>>>> On 21. 01. 20, 0:45, Gustavo A. R. Silva wrote:
>>>>>> diff --git a/drivers/tty/n_hdlc.c b/drivers/tty/n_hdlc.c
>>>>>> index 98361acd3053..b5499ca8757e 100644
>>>>>> --- a/drivers/tty/n_hdlc.c
>>>>>> +++ b/drivers/tty/n_hdlc.c
>>>>>> @@ -115,7 +115,7 @@
>>>>>> struct n_hdlc_buf {
>>>>>> struct list_head list_item;
>>>>>> int count;
>>>>>> - char buf[1];
>>>>>> + char buf[];
>>>>>> };
>>>>>>
>>>>>> #define N_HDLC_BUF_SIZE (sizeof(struct n_hdlc_buf) + maxframe)
>>>>>
>>>>> Have you checked, that you don't have to "+ 1" here now?
>>>>>
>>>>
>>>> Yep. That's not necessary.
>>>>
>>>> _In terms of memory allocation_, zero-length/one-element arrays and flexible-array
>>>> members work exactly the same way.
>>>
>>> This is not true, but maybe it's still not necessary in this particular code, I didn't examine it.
>>>
>>
>> I should have said _in terms of dynamic memory allocation_.
>>
>> Your example is correct:
>>
>> "... a one-element array always occupies at least as much space as a single object of the type."[1]
>>
>> But the above does not affect on the current code.
>>
>
> Oh wait! Yeah; I see the issue in #define N_HDLC_BUF_SIZE (sizeof(struct n_hdlc_buf) + maxframe) now...
>

This would be the new patch; and I'm making use the the struct_size helper
this time, to safely calculate the allocation size:

diff --git a/drivers/tty/n_hdlc.c b/drivers/tty/n_hdlc.c
index 98361acd3053..27b506bf03ce 100644
--- a/drivers/tty/n_hdlc.c
+++ b/drivers/tty/n_hdlc.c
@@ -115,11 +115,9 @@
struct n_hdlc_buf {
struct list_head list_item;
int count;
- char buf[1];
+ char buf[];
};

-#define N_HDLC_BUF_SIZE (sizeof(struct n_hdlc_buf) + maxframe)
-
struct n_hdlc_buf_list {
struct list_head list;
int count;
@@ -524,7 +522,8 @@ static void n_hdlc_tty_receive(struct tty_struct *tty, const __u8 *data,
/* no buffers in free list, attempt to allocate another rx buffer */
/* unless the maximum count has been reached */
if (n_hdlc->rx_buf_list.count < MAX_RX_BUF_COUNT)
- buf = kmalloc(N_HDLC_BUF_SIZE, GFP_ATOMIC);
+ buf = kmalloc(struct_size(buf, buf, maxframe),
+ GFP_ATOMIC);
}

if (!buf) {
@@ -853,7 +852,7 @@ static struct n_hdlc *n_hdlc_alloc(void)

/* allocate free rx buffer list */
for(i=0;i<DEFAULT_RX_BUF_COUNT;i++) {
- buf = kmalloc(N_HDLC_BUF_SIZE, GFP_KERNEL);
+ buf = kmalloc(struct_size(buf, buf, maxframe), GFP_KERNEL);
if (buf)
n_hdlc_buf_put(&n_hdlc->rx_free_buf_list,buf);
else if (debuglevel >= DEBUG_LEVEL_INFO)
@@ -862,7 +861,7 @@ static struct n_hdlc *n_hdlc_alloc(void)

/* allocate free tx buffer list */
for(i=0;i<DEFAULT_TX_BUF_COUNT;i++) {
- buf = kmalloc(N_HDLC_BUF_SIZE, GFP_KERNEL);
+ buf = kmalloc(struct_size(buf, buf, maxframe), GFP_KERNEL);
if (buf)
n_hdlc_buf_put(&n_hdlc->tx_free_buf_list,buf);
else if (debuglevel >= DEBUG_LEVEL_INFO)

And it seems that that extra "+ 1" is not needed. The frame size is always being
verified in multiple places:


/* verify frame size */
if (count > maxframe ) {
if (debuglevel & DEBUG_LEVEL_INFO)
printk (KERN_WARNING
"n_hdlc_tty_write: truncating user packet "
"from %lu to %d\n", (unsigned long) count,
maxframe );
count = maxframe;
}
^^^^^^
and _count_ is being limited to _maxframe_, before copying data into _buf_ :
/* copy received data to HDLC buffer */
memcpy(buf->buf,data,count);
buf->count=count;
So we might save some bytes, too.

I'll properly write and send v2, shortly.

Thanks
--
Gustavo

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