January 2023 - kernelnewbies@kernelnewbies.org

Re: POSIX.1b book/documentation
by Alexander Kapshuk Jan. 20, 2023

Jan. 20, 2023

On Fri, Jan 20, 2023 at 9:20 AM Axel M. <axel.moittier(a)pm.me> wrote: > > Hi all, > > I'm looking for a book that explains the content of POSIX.1 standard > (especially POSIX.1b). > Do you know anything about this ? > Thanks for the help. > > -- > Thanks, > Axel M. > > > _______________________________________________ > Kernelnewbies mailing list > Kernelnewbies(a)kernelnewbies.org > https://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies Is this, https://pubs.opengroup.org/onlinepubs/9699919799/, what you're looking for?

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linked list within a kernel probe
by chrishell Jan. 12, 2023

Jan. 12, 2023

Hello out there, I have an issue with a linked list which holds pointers to memory pages for a kprobe. The linked list get initialised but I cannot use it within the kprobe. I use main or head pointer to hold everything in one place: struct kprobe_head { struct list_head *p_mem_cache; struct kprobe_fsops *pfops; struct kprobe *pProbe; }; and the pointer to access everything: static struct kprobe_head *pHead; then I have the structure which this list is made of: struct kprobe_mem_cache { void *p_page; __u32 size; __u8 dirty; struct list_head list_element; }; the list looks like: ____________________ _________________ | |<-| |<-| pHead->p_mem_cache->| kprobe_mem_cache |->| krobe_mem_cache |->| ... -------------------- ------------------- the p_page pointer is the pointer to the alloacted page. Therefore each element of that linked list holds a pointer to allocated memory segment. in the __init function I do, among other things, allocate some memory for that structure, thought a static struct would be also sufficient. /* create the head storage record */ pHead = (struct kprobe_head*) kzalloc(sizeof(struct kprobe_head), GFP_KERNEL); if(pHead == NULL) return -ENOMEM; then I create the linked list with this function: static __u8 _kprobe_setup_cache_elements( struct kprobe_head *pHead, __u16 elements, __u32 size) { unsigned int count = 0; __u8 rc = 0; struct list_head local_head; INIT_LIST_HEAD(&local_head); if(pHead != NULL) { pr_err("create list with %p\n", pHead); //INIT_LIST_HEAD(pHead->p_mem_cache); for(count=0; count<elements; count++) { struct kprobe_mem_cache *new = kzalloc(sizeof(struct kprobe_mem_cache), GFP_KERNEL); if(NULL == new) { pr_err("error: kzalloc issue"); return -ENOMEM; } /* allocate memory for one page */ new->p_page = vmalloc(size); if(NULL == new->p_page) { pr_err("error: vmalloc issue"); return -ENOMEM; } else { pr_err("List Element %d added Size: %u addr: %p \n", count, size, new->p_page); } new->size = size; new->dirty = 0; list_add_tail(&new->list_element, &local_head); } pHead->p_mem_cache = &local_head; } return rc; } right after that, still in the __init function I traverse trough this linked list, just to see if its work. if(pHead->p_mem_cache != NULL) { pr_err("within loop %p %p\n", pHead, pHead->p_mem_cache); list_for_each(local_head, pHead->p_mem_cache) { local_page = list_entry(local_head, struct kprobe_mem_cache, list_element); if(local_page->size) pr_err("address: %u \n", local_page->size ); } } And here it works. The problem now is that this is a kprobe kernel module and I defined a kprobe function as pre_handler function called submit_bio_pre which is evoked whenever the block layer function submit_bio is called by the kernel. Within that function however the access to that linked list failed int submit_bio_pre(struct kprobe *p_submit_bio, struct pt_regs *regs) { int rc = 0; struct bio *bio = NULL; static unsigned int len = 0; static unsigned int counter; struct kprobe_mem_cache *tmp = NULL; struct kprobe_mem_cache *local_page = NULL; struct list_head *local_head = NULL; bio = (struct bio*) regs_get_kernel_argument(regs, 0); if(pHead != NULL) pr_err("#### pHead is initialized %p #####\n", pHead); if(pHead->p_mem_cache != NULL) pr_err("#### pHead->p_mem_cache is initialized %p #######\n", pHead->p_mem_cache); if(pHead->p_mem_cache != NULL && counter == 0) { spin_lock(&sl); pr_err("within loop submit_bio_pre \n"); list_for_each(local_head, pHead->p_mem_cache) { local_page = list_entry(local_head, struct kprobe_mem_cache, list_element); if(local_page->size) pr_err("address bio: %u \n", local_page->size ); } spin_unlock(&sl); counter++; } As it seems is the head pointer valid and has the same address as in the init function. Also the head pointer to the linked list is a valid one. But traversing through the linked list is not possible any more. As you can see I added a spin_lock to that kprobe function, albeit I only read from the linked list. This function does something completely different normally anyway. When I load this module, the initialisation of the linked list works, also the following walk through the list, but within the kprobe function sometimes I can see the first and second pr_err. But after that the kernel breaks: [ 191.460196] Unable to handle kernel paging request at virtual address 000323bfa8c17bf5 [ 191.669571] Call trace: [ 191.672021] submit_bio_pre+0xcc/0x150 [ 191.676641] kprobe_breakpoint_handler+0x100/0x190 [ 191.681445] call_break_hook+0x68/0x80 [ 191.685201] brk_handler+0x1c/0x60 So can anybody tell me, what is the reason that the linked list doesn't work within the kprobe? Thank you in advance BR Christian

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Determining the reason for reserved RAM
by Mikhail Krylov Jan. 8, 2023

Jan. 8, 2023

Hello, I was doing some research figuring out what kernel reserves memory for, and found myself in a sort of dead-end. I found out that /proc/kpageflags bit 32 is set for reserved physical pages in include/linux/kernel-page-flags.c, wrote a simple decoder of that file, and on an old 32-bit x86 machine (it's the simplest there, since it has less memory and fewest amount of memory holes) those are reserved: 0x00000000-0x0000ffff - I assume those addresses are used for interrupt vectors and whatnot 0x0009b000-0x000fffff - Video RAM/ROM for VGA/EGA/CGA and whatnot 0x01c00000-0x01c1ffff - no idea, it also has KPF_UNCACHED flag as well 0x1d000000-0x1dd73fff - Kernel code, rodata, rwdata, bss 0x35b4a000-0x36b55fff - no idea. I also found out that region #5 is getting bigger or smaller depending on the machine's memory size. On this 1.5G machine it's a bit more than 16M. On a 32G machine it's more than 500M size, feels a bit wasteful IMO. So my question is, could you tell me where to look for what those regions are and why those regions exist? I've checked /proc/iomem and it doesn't say it is reserved or anything, just "System RAM". Thanks in advance, Mikhail.

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RE: How can I reserve contiguous mem alloc region?
by Chan Kim Jan. 3, 2023

Jan. 3, 2023

Hi, all, I was able to get cma-reserved region by adding CONFIG_DMA_CMA=y. This is the result. Memory: 233708K/1048576K available (3200K kernel code, 386K rwdata, 808K rodata, 7808K init, 105K bss, 28436K reserved, 786432K cma-reserved) Thanks! Chan Kim >-----Original Message----- >From: Chan Kim <ckim(a)etri.re.kr> >Sent: Tuesday, January 3, 2023 2:36 PM >To: kernelnewbies(a)kernelnewbies.org >Subject: How can I reserve contiguous mem alloc region? > >Hello kernel experts and users, > >I wanted to test cma-allocator in linux (5.15.68). >So I added linux,cma node in the /reserved-memory node like this. > > reserved-memory { > #address-cells = <2>; > #size-cells = <2>; > ranges; > > axpu_reserved_mem: axpursvd@90000000 { > no-map; > reg = <0x0 0xc0000000 0x0 0x8000000>; > }; > > linux,cma { > compatible = "shared-dma-pool"; > reusable; > size = <0 0x30000000>; > alloc-ranges = <0 0x90000000 0 0x30000000>; > linux,cma-default; > }; > }; > >BTW, this test was done on qemu arm64 machine and there is only 1GB ram >(from 0x80000000 ~ 0xbfffffff) in the virtual machine, and notice I'm >assigning 3/4 of the ram to cma region and 1/8 to a device driver (just for >test). > >When I boot the machine, I see this message during the boot. > > Memory: 1020140K/1048576K available (3200K kernel code, 386K rwdata, >808K rodata, 7808K init, 106K bss, 28436K reserved, 0K cma-reserved) > >Why is the CMA-reserved area is 0KB? >This is some config variables I added for CMA test. > > CONFIG_MEMORY_ISOLATION=y > CONFIG_CONTIG_ALLOC=y > ONFIG_CMA=y > CONFIG_CMA_DEBUG=y > CONFIG_CMA_DEBUGFS=y > CONFIG_CMA_SYSFS=y > CONFIG_CMA_AREAS=7 > # CONFIG_DMA_CMA is not set > >I tried adding 'cma=768MB' in the boot args or changed CONFIG_CMA_AREAS to 1 >but it is the same. >What am I missing?? > >Thank you. > >Chan Kim

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How can I reserve contiguous mem alloc region?
by Chan Kim Jan. 3, 2023

Jan. 3, 2023

Hello kernel experts and users, I wanted to test cma-allocator in linux (5.15.68). So I added linux,cma node in the /reserved-memory node like this. reserved-memory { #address-cells = <2>; #size-cells = <2>; ranges; axpu_reserved_mem: axpursvd@90000000 { no-map; reg = <0x0 0xc0000000 0x0 0x8000000>; }; linux,cma { compatible = "shared-dma-pool"; reusable; size = <0 0x30000000>; alloc-ranges = <0 0x90000000 0 0x30000000>; linux,cma-default; }; }; BTW, this test was done on qemu arm64 machine and there is only 1GB ram (from 0x80000000 ~ 0xbfffffff) in the virtual machine, and notice I'm assigning 3/4 of the ram to cma region and 1/8 to a device driver (just for test). When I boot the machine, I see this message during the boot. Memory: 1020140K/1048576K available (3200K kernel code, 386K rwdata, 808K rodata, 7808K init, 106K bss, 28436K reserved, 0K cma-reserved) Why is the CMA-reserved area is 0KB? This is some config variables I added for CMA test. CONFIG_MEMORY_ISOLATION=y CONFIG_CONTIG_ALLOC=y ONFIG_CMA=y CONFIG_CMA_DEBUG=y CONFIG_CMA_DEBUGFS=y CONFIG_CMA_SYSFS=y CONFIG_CMA_AREAS=7 # CONFIG_DMA_CMA is not set I tried adding 'cma=768MB' in the boot args or changed CONFIG_CMA_AREAS to 1 but it is the same. What am I missing?? Thank you. Chan Kim

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How is the size of init ram disk determined when initramfs is used?
by Chan Kim Jan. 2, 2023

Jan. 2, 2023

Hello all, I'm using initramfs.cpio.gz for initial file system image. I embed it in the kernel Image file. After linux boots, when I'm in the shell, I can create files. But I'm curious how much the file system can grow. I remember somewhere reading that the initramdisk is made and the initramfs archive is extracted in the ramdisk. Then how is the ramdisk size determined? During the build usig Kconfig variable, or does it dynamically grow? Thanks in advance. Chan Kim

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