Messages in this thread Patch in this message | | | From | "Gustavo A. R. Silva" <> | Subject | Re: [PATCH] tty: n_hdlc: Use flexible-array member | Date | Tue, 21 Jan 2020 10:03:09 -0600 |
| |
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
| |