June 2013 - kernelnewbies@kernelnewbies.org - Kernelnewbies Mailing Lists

linux segment
by Fan Yang June 20, 2013

June 20, 2013

Hi all: I print the cs ds and ss register in the user space, and it is same as the __USER_CS and __USER_DS which defined in kernel as 73 and 7b. In the kernel __KERNEL_CS and __KERNEL_DS defined as 60 and 68, but when I print this two value in my kernel module, I get 60 and 7b. Why ? It should be 60 and 68, shouldn't it?

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On creation of irq_desc and irq_data.
by mind entropy June 20, 2013

June 20, 2013

Hi, I am reading through mini2440 code. I am going through the init_irq (http://lxr.free-electrons.com/source/arch/arm/mach-s3c24xx/mach-mini2440.c#…) code. In it (http://lxr.free-electrons.com/source/arch/arm/mach-s3c24xx/irq.c#L446) there is irq_domain_add_legacy(..) which references irq_desc and irq_data. I am not sure where these descriptors are created? In the irq_domain_add_legacy there is a lookup for irq_data from the number. Could someone tell where these descriptors are created? Thanks.

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File System query
by Dibyayan Chakraborty June 18, 2013

June 18, 2013

Hi, I have the following question regarding the linux file system implementation. In linux/fs.h I saw the "struct super_operation" structure. I saw that two of the member function pointers are write_inode() and put_inode(). But In the ext2,ext3,ext4 implementation i saw that it uses the only write_inode() function. So, what is the reason behind keeping the other pointers ? -- With Regards Dibyayan Chakraborty

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tcp window concept
by Varun Sharma June 17, 2013

June 17, 2013

Hi, I am sending 5000 bytes of data in send system call. struct tcp_sock has max_window and snd_wnd two variable.Which are increase as packet sent out . whereas tcp header window variable value is fix for all packets.can anyone tell what is relationship b/w these windows ? and why max_window and snd_wnd size increase after packets sent out ?

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hibernation image size
by arden jay June 17, 2013

June 17, 2013

Hi, I'm trying to decrease hibernation image size. It's now, normal:21762, high: 8883 pages. Dump /proc/vmalloc, I can see those occupied high memory region. For example: 0xd99f8000-0xd99fd000 20480 init_mmdc_settings+0x3c/0x410 ioremap ... 0xd9a1e000-0xd9a3b000 118784 gckOS_AllocateMemory+0x44/0x60 pages=28 vmalloc For those ioremap region, pfn_valid check won't pass, so they shouldn't save in hibernation image, right? And I can see some vmalloc, I think those will be saved in hibernation image. To sum those vmalloc regions, I can't get the number calculated in count_highmem_pages. Can anyone show me how to dig out further? -- cheers, jay

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Writing device drivers
by shampavman June 17, 2013

June 17, 2013

Hi all, I've followed up LDD3 and started off with writing device drivers . As usual i've started with char drivers given in that book and also started to make minor modifications to learn it better and so on. Now i would like to know how to take it forward and start writing drivers for a profession. I know its too early to call myself a guy writing device drivers but how do i take if forward from here so that i can become one. Is the only solution to pick up a device which does not work on linux and get it working? Are there any other ways as well? Shampavman

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Sending an ICMP packet with ip_local_out
by Adel Qodmani June 17, 2013

June 17, 2013

Hello everyone, I am trying to send an ICMP message in a kernel module; typically I'd build the skb, set up the net_device, the ethernet header and then use dev_queue_xmit. But right now I want my packet to follow the IP routing rules set in the system, so I thought I'll use ip_local_out function and let it handle the routing and setting the rest of the parameters in the skb. Yet, my code causes the kernel to panic due to a NULL pointer. The code is as follows: /* This function assumes source and dest are in NETWORK byte order */ int sendICMPEcho(char *msg, unsigned int length, __be32 source, __be32 dest) { struct iphdr *iph; struct icmphdr *icmph; struct sk_buff *newPacket; unsigned char *data; unsigned int skbSize = length + sizeof(struct icmphdr) * 2 + sizeof(struct iphdr) * 2 + sizeof(struct ethhdr) * 2; /* Allocate the skb */ newPacket = alloc_skb(skbSize, GFP_ATOMIC); if(newPacket == NULL) return SEND_FAIL_MEMORY; /* Reserve the headers area */ skb_reserve(newPacket, sizeof(struct icmphdr) + sizeof(struct iphdr) + sizeof(struct ethhdr)); /* Extend the data area from 0 to the message length */ data = skb_put(newPacket, length); /* Copy the data from the message buffer to the newPacket */ memcpy(data, msg, length); /************** ICMP HEADER***************/ /* skb_push - pushing the icmp header in the packet data */ icmph = (struct icmphdr *) skb_push(newPacket, sizeof(struct icmphdr)); /*set ICMP header here */ icmph->type = ICMP_ECHO; icmph->code = 0; icmph->un.echo.id = 0; icmph->un.echo.sequence = htons(sendCounter); icmph->checksum= 0; icmph->checksum = in_cksum((unsigned short *)icmph, sizeof(struct icmphdr) + length); /************** END ICMP HEADER**************/ /************** IP HEADER ***************/ iph = (struct iphdr *) skb_push(newPacket, sizeof(struct iphdr)); /* set IP header here */ iph->ihl = 5;/* 5 * 32(bits) */ iph->version = 4; iph->tos = 0; /* The recommended value by IANA for ICMP request */ iph->tot_len = htons( sizeof(struct iphdr) + sizeof(struct icmphdr) + length); iph->id = 0; iph->frag_off = 0; /* No fragementation */ iph->ttl = 65; iph->protocol = IPPROTO_ICMP; iph->saddr = source; iph->daddr = dest; iph->check = 0; iph->check = in_cksum((unsigned short *)iph, sizeof(struct iphdr)); /************** END IP HEADER ***************/ // if(ip_local_out(newPacket) < 0) THIS CRASHES WITH A NULL POINTER // return SEND_FAIL_SEND; ++sendCounter; return SEND_SUCCESS; } /* end sendICMPEcho */ In my attempts when trying to solve this, I've tried to manually set the net_device in the skb and that didn't work either; I still had the same kernel panic. So I am sorry for the trouble, any hint where the error can be? Regards, Adel

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File system Query
by Dibyayan Chakraborty June 16, 2013

June 16, 2013

Hi, Please describe the functions of this function "mount_bdev()" defined in the header file "Linux/fs/super.c". I guess it somehow initiates a safe execution of the fill_super function passed as parameter but i want to know the actual steps performed by that function. -- With Regards Dibyayan Chakraborty

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UDP max packet size
by Pietro Paolini June 13, 2013

June 13, 2013

Hello all, I am not sure this newsletter is the right place to post that kind question then excuse me in advance is I am wrong and suggest me a good place where do that. I am trying to perform a simple but a little tricky task: I have to understand the maximum payload size of an UDP packet in order to avoid fragmentation of that. Some suggestions ? P.

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forum spam filtering
by Rik van Riel June 11, 2013

June 11, 2013

Over the past year, the forum has deteriorated to the point of being unusable. Today I installed (and fixed) a module that allows phorum to use spamassassin, and tweaked & trained spamassassin to deal with forum posts (no email headers, etc). Over the past few hours, I have seen spammers beat their heads against the spam filter, sometimes submitting the same message several times before giving up. I have also made a few test posts of my own to the forum. It looks like non-spam messages are let through. In short, the forum should be usable again. If something happens to break, please let me know and I will fix it. http://forum.kernelnewbies.org/ -- All rights reversed.

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