April 2021 - kernelnewbies@kernelnewbies.org - Kernelnewbies Mailing Lists

Question about the linux switch_to macro parameters
by 武广阔 April 22, 2021

April 22, 2021

I read the source code in kernel(kernel/sched.c): (include/asm-i386/system.h) My question is that: switch_to is defined as a macro, so I thoutht it will expand "=a" (last) as "=a" (prev), if that the last parameter is just for easy to read, is that right?

1 0

[QUERY]drivers: pnp: isapnp:proc.c: assignment inside if statement
by anupamakpatil123 April 21, 2021

April 21, 2021

This is a code snippet from line 63 of drivers: pnp: isapnp: proc.c This is an error I came across while looking through some checkpatch errors. if (!(de = bus->procdir)) { sprintf(name, "%02x", bus->number); de = bus->procdir = proc_mkdir(name, isapnp_proc_bus_dir); if (!de) return -ENOMEM; } In the above code snippet is assignment inside the if statement necessary? Can the assignment be done outside the if statement as it is a good coding style practice? Is that change sufficient? I dont want to just shut checkpatch errors as I really want to improve the code. Thankyou for taking your time to go through this. Anupama K Patil

2 1

Is there documentation for what each script in linux/scripts does?
by Esme Xuan Lim April 20, 2021

April 20, 2021

Hey there, I was wondering since there’s loads of useful stuff in there it seems, but there’s little documented on what goodies there are or what they do Thanks Esme

3 2

Re: Compiling kernel-3.4.xxx with gcc-9.x. Need some help.
by Fawad Lateef April 18, 2021

April 18, 2021

Hi Arnd, On Mon, 29 Mar 2021 at 22:06, Arnd Bergmann <arnd(a)kernel.org> wrote: > > On Mon, Mar 29, 2021 at 9:23 PM Fawad Lateef <fawadlateef(a)gmail.com> wrote: > > > > On Mon, 29 Mar 2021 at 06:57, Greg KH <greg(a)kroah.com> wrote: > > > > > > On Sun, Mar 28, 2021 at 10:20:50PM +0200, Fawad Lateef wrote: > > > > Hi > > > > > > > > I am using an Olimex A20 SOM with NAND and due to some binary blob for > > > > NAND driver, I am stuck with the sunxi kernel 3.4.xxx version. (Repo > > > > here: https://github.com/linux-sunxi/linux-sunxi) > > > > > > Please work with the vendor that is forcing you to use this obsolete and > > > insecure kernel version. You are paying for that support, and they are > > > the only ones that can support you. > > > > > > > The problem is vendor Olimex now have eMMC based SOM which is > > supported by mainline kernel _but_ they still selling NAND SOM and > > only supporting 3.4 kernel (as this is the only latest version from > > sunxi with NAND support, after that sunxi is now moved away from NAND > > too). > > From a very quick look at the git history, I can tell that A20 NAND driver > support was added in linux-4.8. Have you actually tried a modern kernel? > I tried compiling and booting kernel v4.4 (from sunxi repo on github) but unable to make it boot. Stuck at "Starting kernel". By the way I am booting from RAM (using the sunxi usbboot/usb-otg option). You mentioned that it's supported from linux-4.8 (from your quick look at git history); can you tell me which driver/files? As I was able to see some sort of sunxi NAND driver even in v4.4 kernel (https://lxr.missinglinkelectronics.com/linux+v4.4/drivers/mtd/nand/sunxi_na…) When I tried to compare with the sunxi-3.4 kernel code, I found that the kernel has code for "nfc" but no reference to "nfd" (I don't know what they are referring to). The sunxi 3.4 kernel NAND driver at quick look seems quite different. (https://github.com/linux-sunxi/linux-sunxi/tree/sunxi-3.4/drivers/block/sun…) I will try the v4.8 or above kernel. > There is also a howto document at > https://linux-sunxi.org/Mainline_NAND_Howto > I somehow missed this earlier :( And the prerequisite says kernel 4.10 above and some extra patch set). Will look into it too. Thanks for the link > The olimex board specific dts files seem to be missing the entry for > the nand controller. If you have trouble figuring out how to enable that > from the howto above, Olimex should be able to prove a small patch > for it. > Will see. As above link have some details regarding this which might be helpful. > > > > I am currently using buildroot-2016 and gcc-5.5 for building the > > > > kernel and every other package needed. > > > > > > > > Now the requirement is to move to the latest version of gcc-9.x, so > > > > that we can have glibc++ provided by the gcc-9.1 toolchain. > > > > > > > > Main problem for moving to later versions of buildroot is the kernel > > > > 3.4 which we couldn't to work with gcc-6 a few years ago _but_ now the > > > > gcc-9.1 requirement is mandatory so now have to look into compiling > > > > linux-3.4 with gcc-9.1 or above. > > There is no need to compile user space and kernel with the same compiler. > So can I still use kernel-3.4 compiled with gcc-5.5, and boot full user-space with gcc-9.1? I was expecting it to be possible but might not work due to incompatibility? As I know that when I tried to compile buildroot-2019 (with latest version of openssl and others) it needs kernel headers and then I likely can't use 3.4 kernel. I will still give it a try. Thanks again > Arnd

4 5

HID Monitor Driver Advice
by mark＠bolhuis.dev April 16, 2021

April 16, 2021

Hi all My name is Mark and I'm currently writing a device driver for Eizo EV FlexScan monitors, so that I can control them over usb. This driver is built by reverse engineering the hid reports sent by the propriety and windows only software. It can be found here: https://github.com/markbolhuis/openeizo I have some questions about this that hopefully somebody can answer. 1) What would be the recommended way to interface this with userspace? My plan is to expose a device file for each unique setting that the monitor has such as brightness control, input selection and so on... However I'm not sure this is a suitable way of doing this since the monitors have many options that are very similar. For example contiguous numerical values only differ by their usage and value min and max, meaning that if I have a file per setting there will be a lot of repetition. There are already 40+ unique attributes/settings that may need a file. 2) To properly initialize the driver I will have to fetch a second report descriptor from the monitor. This isn't a standard hid report descriptor. It seems that Eizo are simply using the format instead of inventing their own. Is there a way for the hid subsystem to parse arbitrary report descriptors and expose them as a structure for generic use? I couldn't find much that would help with this. hid_parse_report @ inlcude/linux/hid.h:895 seems to be the closest to what I need but it's tied to a device. 3) I contacted Eizo and politely asked if they could provide an hid specification for the vendor defined usages however they weren't able to help. This forced me to reverse engineer. Is there anything that I should be aware of if I want to submit code that based on reverse engineering? I didn't reverse engineer the windows driver itself, just sniffed the packets with usbmon. 4) How would I find people who have compatible monitors to test it out and/or help reverse engineer? I only have one monitor so I'm writing against that, with no guarantee that it will work for others. Thank you for reading Mark Bolhuis

3 4

Why does dump_stack need asmlinkage?
by Lv Ying April 14, 2021

April 14, 2021

Hi all: asmlinkage is only defined in arch ia64 and x86_32: 1. arch/x86/include/asm/linkage.h: #ifdef CONFIG_X86_32 #define asmlinkage CPP_ASMLINKAGE __attribute__((regparm(0))) #endif /* CONFIG_X86_32 */ 2. arch/ia64/include/asm/linkage.h: #define asmlinkage CPP_ASMLINKAGE __attribute__((syscall_linkage)) 3. arch independent include/linux/linkage.h: #ifdef __cplusplus #define CPP_ASMLINKAGE extern "C" #else #define CPP_ASMLINKAGE #endif #ifndef asmlinkage #define asmlinkage CPP_ASMLINKAGE #endif As kernelnewbies FAQ(https://kernelnewbies.org/FAQ/asmlinkage) said, in short, asmlinkage tells your compiler to look on the CPU stack for the function parameters, instead of registers. So, there are two questions: 1. asmlinkage is only defined in x86_32 and ia64, what about other archs? How do they achieve the same effect as mentioned above? 2. "asmlinkage __visible void dump_stack(void)", dump_stack does not have any parameter, so why do it need asmlinkage? Thanks! Lv Ying

1 0

Linux kernel BSP
by Pankaj Vinodrao Joshi April 14, 2021

April 14, 2021

Hi, i am newbie to linux BSP and i want to get some hand on with BSP. Can someone kindly suggest what all the components of BSP also i need to port BSP from one arch to other what and all things i should consider to get it ported ?? Thanks

3 8

Notify special task kill using wait* functions
by John Wood April 11, 2021

April 11, 2021

Hi, I'm working in a LSM whose goal is to detect and mitigate fork brute force attacks against vulnerable userspace applications. The detection and mitigation works as expected by I'm stuck at this point. The mitigation method used is to kill all the offending tasks involved in the attack. To do so, I kill the tasks using: do_send_sig_info(SIGKILL, SEND_SIG_PRIV, p, PIDTYPE_PID); The question is: How can I notify to wait* functions that the task has been killed by the "Brute" LSM. For example, in the function wait_task_zombie, in the "out_info:" label, the code is the following: out_info: infop = wo->wo_info; if (infop) { if ((status & 0x7f) == 0) { infop->cause = CLD_EXITED; infop->status = status >> 8; } else { infop->cause = (status & 0x80) ? CLD_DUMPED : CLD_KILLED; infop->status = status & 0x7f; } infop->pid = pid; infop->uid = uid; } I think I need to modify this code to achieve the commented goal. But I don't know how to proceed. Is it possible to kill a task in a way that this code can detect ? Now, with this code, we know that a task has been killed and with what signal. But is it possible to know that the task has been killed by the "Brute" LSM. Using for example the 8 upper bits in the status that I think are unused when a process is killed or dumped. It is possible to use the do_send_sig_info passing to the struct kernel_siginfo the necessary information to be able to detect this scenario in the wait* functions? If yes, what info I need to pass? The final purpose of all of this is that using the waitid function from userspace (or waitpid function) we can know that a child task has been killed by the "Brute" LSM or not. I try to inform to userspace that a task has been killed due to the "Brute" mitigation. Sorry, but I'm stuck at this point. Any help would be greatly appreciated. Regards, John Wood

3 17

Controlling IOVA addresses generated by IOMMU
by Sadanand Warrier April 7, 2021

April 7, 2021

Folks Is there any kernel API or control mechanism for an IO-MMU that one could use to specify that a buffer earmarked for DMA would always have an IOVA greater than 4GB? Basically if I used pci_map_sg and passed a scatter list I would like the dma_address field in the scatterlist structure to be always greater than 4GB when it returned. Thanks S

1 0

How to free a page table page
by Wonkyo Choe April 6, 2021

April 6, 2021

Hello there, I'm trying to manage a custom page table list for my toy project. A basic process of the custom list is to allocate few pages (struct page) to the list from the buddy allocator and the list will give some pages when a user process needs page table pages. In contrast, when this process terminates, page table pages will return to the list. Finally, when the list is not needed, all pages will be delivered to the buddy allocator. This relationship can be described as: process - custom list - buddy allocator My problem is that a user process' page table pages (pud, pmd, pte) seem to free their page to the buddy allocator instead of to the custom list. (allocation works. I've checked it) I put my custom functions in release_pages / free_unref_page / free_unref_page_list, but somehow the functions do not work as I intended. (Actually the functions are not called so I may choose the wrong functions.) So, I was wondering whether I intercepted the right functions (three functions above). Also, I have a question about freeing a page-table page. 1. In x86, are all page-table pages released at the end of termination by using free_pgtables()? 2. In x86, a page-table page can be freed when there is no entry? In other words, does the Linux kernel release a page-table page on runtime? If so, what function would do this kind of task? I'm trying to find freeing functions, but I can't find them and I'm not sure when this kind of function is called. (I'm pretty sure that pte_free / pmd_free / pud_free functions are not for this case.) Any help would be greatly appreciated. Regards, Wonkyo

3 3