Project 4: System Calls 1

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1 CMPT Preparatin Prject 4: System Calls 1 T cmplete this assignment, it is vital that yu have carefully cmpleted and understd the cntent in the fllwing guides which are psted n the curse website: Custm Kernel Guide: hw t dwnlad, build, and run a custm Linux kernel. Guide t Linux Sys-Calls: hw t create and test a simple Linux system call (sys-call). In this assignment, yu'll be cding in bth user space and kernel space. Since it takes a cuple f minutes t recmpile and re-run a new kernel, yu shuld cde carefully! Yu can wrk in a grup with up t tw students. Yu can als wrk alne, but yu get n extra credits fr this. 2. Array Statistics Sys-Call First, yu'll add a new system call that cmputes sme basic statistics n an array f data. In practice, it makes little sense t have this as a sys-call; hwever, it allws us t becme familiar with accessing memry between user and kernel space befre accessing ther kernel data structures. Requirements In the kernel's cs300/ flder, create a file named array_stats.h with the cntents: // Define the array_stats struct fr the array_stats sys-call. #ifndef _ARRAY_STATS_H_ #define _ARRAY_STATS_H_ struct array_stats{ lng min; lng max; lng sum; }; #endif Create a new sys-call named array_stats (functin sys_array_stats()): Implement it in yur kernel's cmpt00/ flder in a file name array_stats.c. Use #include "array_stats.h" Make it sys-call number 341 (in syscall_64.tbl). The sys-call's prttype is: asmlinkage lng sys_array_stats( struct array_stats *stats, lng data[], lng size); 1 Created/updated by Brian Fraser and Mhamed Hefeeda. Page 1/6

2 CMPT 300 stats: A pinter t ne array_stats structure allcated by the user-space applicatin. Structure will be written t by the sys-call t stre the minimum, maximum, and sum f all values in the array pinted t by data. data: An array f lng int values passed in by the user-space applicatin. size: The number f elements in data. Must be > 0. The array_stats sys-call must: Hints Set the three fields f the stats structure based n the data array. The values in data are signed (psitive r negative). Nthing special need be dne if the sum f all the data verflws/underflws a lng. Return 0 when successful. Returns -EINVAL if size <= 0. Returns -EFAULT if any prblems access stats r data. Yu must nt allcate r use a large amunt f kernel memry t cpy the entire input data array int. Use printk() calls in the kernel t print ut debug infrmatin. Yu may leave sme f these printk() messages in yur slutin as these messages are nt technically displayed by the user-space applicatin. The messages yu leave in shuld be helpful such as shwing parameters values r errrs it caught; they shuld nt be f the srt running line 17, r past lp 1. It is usually useful t printk() the parameters yu are given, and printk() any errr cnditins yu handle. Crrect memry access is the hardest part f writing this sys-call. The kernel cannt trust anything it is given by a user-level applicatin. Each pinter, fr example, culd be: a) perfectly fine, b) null, c) utside f the user prgram's memry space, r d) pinting t t small an area f memry. Since the kernel can read/write t any part f memry, it wuld be disastrus fr the kernel t trust a user-level pinter. Each read yu d using a pinter passed in as input (a user-level pinter) shuld be dne via the cpy_frm_user() macr. This macr safely cpies data frm a user-level pinter t a kernel-level variable: if there's a prblem reading r writing the memry, it stps and returns nn-zer withut crashing the kernel. First use this macr t cpy values int a lcal variables (which are in the kernel's memry space). Then, use these lcal variables in yur prgram. See the Linux Kernel Develpment (ch5, p75) fr mre n the macr. Hint: Create a lcal variable f type lng inside yur sys-call functin. Use cpy_frm_user() t cpy ne value at a time frm the user's data array int this variable. If a cpy fails (cpy_frm_user() returns nn-zer) then have yur syscall end immediately and return -EFAULT. Hint: Duble check that yu nly ever access the data array using cpy_frm_user()! Yu can directly access size because it is passed by value s there is n pssible prblem access memry. Page 2/6

3 CMPT 300 Likewise, when writing t a user-level pinter, use cpy_t_user() which checks the pinter is valid (nn-null), inside the user-prgram's memry space, and is writable (vs read-nly). Hint: Create a lcal variable f type struct array_stats inside yur sys-call functin. Cmpute the crrect values in this struct first, then at the very end use cpy_t_use() t cpy the cntents t user's pinter. The kernel is cmpiled with C90; yu must declare yur variables at the start f a blck (such as yur functin) vs in the middle f yur functin. A user-space test prgram is prvided t extensively test yur system call. Yur sys-call implementatin shuld pass all f these tests. It is likely that yur cde will be marked based n passing these (and perhaps ther) tests. Yu may want t run the tests ne at a time by cmmenting ut calls in main(). 3. Prcess Ancestr Sys-Call In this sectin, yu'll implement a sys-call which returns infrmatin abut the current prcess, plus its ancestrs (its parent prcess, it's grandparent prcess, and s n). Requirements In the kernel's cmpt300/ flder, create a file named prcess_ancestrs.h with the cntents: // Structure t hld values returned by prcess_ancestrs sys-call #ifndef _PROCESS_ANCESTORS_H #define _PROCESS_ANCESTORS_H #define ANCESTOR_NAME_LEN 16 struct prcess_inf { lng pid; /* Prcess ID */ char name[ancestor_name_len]; /* Prgram name f prcess */ lng state; /* Current prcess state */ lng uid; /* User ID f prcess wner */ lng nvcsw; /* # vluntary cntext switches */ lng nivcsw; /* # invluntary cntext switches */ lng num_children; /* # children prcess has */ lng num_siblings; /* # sibling prcess has */ }; #endif Create new sys-call named prcess_ancestrs (functin sys_prcess_ancestrs()): Implement it in yur kernel's cmpt300/ flder in a file name prcess_ancestrs.c. Use #include "prcess_ancestrs.h" Make its sys-call number 342 (in syscall_64.tbl). The sys-call's prttype is: Page 3/6

4 CMPT 300 Hints asmlinkage lng sys_prcess_ancestrs( struct prcess_inf inf_array[], lng size, lng *num_filled) inf_array[]: An array f prcess_inf structs that will be written t by the kernel as it fills in infrmatin frm the current prcess n up thrugh its ancestrs. size: The number f structs in inf_array[]. This is the maximum number f structs that the kernel will write int the array (starting with the current prcess as the first entry and wrking up frm there). The size may be larger r smaller than the actual number f ancestrs f the current prcess: larger means sme entries are left unused (see num_filled); smaller means infrmatin abut sme prcesses nt written int the array. num_filled: A pinter t a lng. T this lcatin the kernel will stre the number f structs (in inf_array[]) which are written by the kernel. May be less than size if the number f ancestrs fr the current prcess is less than size. The prcess_ancestrs sys-call must: Starting at the current prcess, fill the elements in inf_array[] with the crrect values. Ordering: the current prcess's infrmatin ges int inf_array[0]; the parent f the current prcess int inf_array[1]; grandparent int inf_array[2]; and s n. Extra structs in inf_array[] are left unmdified. Return 0 when successful. Returns -EINVAL if size <= 0. Returns -EFAULT if any prblems access inf_array r num_filled. Yu must nt allcate r use a large amunt f kernel memry t cpy/stre large arrays int. Yu must als create a user-space test prgram which calls yur sys-call and exercises its functinality. Yu must d at least sme testing n t shw it wrks crrectly, and that it generates crrect errr values in at least a few f the failure cnditins (bad pinters,...). Hint: Use asserts in yur test cde. We will have an extensive test suit t exercise yur slutin! Yu will make extensive use f the kernel's task_struct structures: each prcess has a task_struct. Yu can find the task_struct fr the current prcess using the macr current. Fr example, the PID fr the currently running prcess n this CPU can be fund with: lng pid = current->pid; Basic algrithm sketch: 1) Start frm current prcess and fill in fields fr inf_array[0]. 2) Mve t parent f this prcess (current->parent), and cpy its inf int inf_array[1]. 3) Repeat until the parent f the prcess yu are wrking n is itself (cur_task->parent == cur_task). Page 4/6

5 CMPT 300 The first task spawned by Linux is its wn parent, s hence its parent pinter pints t itself. This prcess has PID 0 and is the idle task (named swapper). I recmmend that yu first get the inf n the current prcess and print it t the screen (printk) t ensure yu have the crrect values. Then wrk n getting the data int the prcess_inf structs and handling ancestrs. Hints n the fields f prcess_inf: Quite a few f the values can be pulled directly ut f the task_struct structure. Lk fr fields with a matching name. task_struct is defined in include/linux/sched.h (in yur kernel flder). T include this in yur sys-call implementatin use: #include <linux/sched.h> Here is a gd nline site t navigate the kernel surce. The name f the prgram fr a prcess is stred in the task_struct.cmm field. The user ID fr the wner f a prcess can be fund inside the prcess's credentials (cred field). Inside cred, yu want t lk at the uid field. Fr cunting the number f children and siblings, yu'll want t start with the fllwing linked list ndes: task_struct.children, and task_struct.sibling. These are ndes in circular linked lists. Linux uses the struct list_head fr a nde because in a circular linked list, each nde can be thught f as the head f the list. Yu can fllw the next field f a nde in the list (a list_head) t get the nde (list_head) f the next element in the list. It is a circular linked list, s yu'll have t determine hw t cunt the number f elements (think f hw yu knw when t stp fllwing next pinters). Hint: Think f addresses. Nte that Linux has sme clever (cmplicated) ways f taking a nde in the list (which just has a next and prev field pinting t ther list_head structures) and accessing the full structure that cntains the nde. Fr example, given a list_head struct that is in a task_struct, the kernel includes macrs t give yu the full task_struct! Hwever, yu have (mercifully) been spared having t d this. If yu are interested, fr fun try printing ut the PID f each f the sibling prcesses. Safe memry access is critical. Apply all the suggestins frm the previus sys-call fr safe memry access. Yu can use the ps cmmand in yur QEMU virtual machine t display infrmatin n running prcesses and verify the sys-call utput. See ps's man page fr hw t select the infrmatin it displays. 4. Deliverables In CurSys, create a grup fr yur submissin. Even if yu wrked alne, yu need t be in a grup (f 1 in this case) t submit. Submit the fllwing t CurSys: 1. A patch f yur Linux kernel flder fr the files yu changed. Patch must be dne in accrdance with the Kernel Patch guide psted n the curse website. Must patch cleanly (either using Git r patch). Page 5/6

6 CMPT An archive file (zip r tar.gz) f yur kernel cde: - cmpt300/ flder - /makefile, syscall_64.tbl 3. An archive file (zip r tar.gz) f yur user mde flder cntaining yur test cde fr the syscalls. Must include a makefile with a target transfer t cpy the statically linked executables t QEMU. May include the prvided array_statistics test cde. Please remember that all submissins will autmatically be cmpared fr unexplainable similarities. Marking will be dne n a 64-bit system. If yu did yur develpment n a 32-bit system, yur cde must still cmpile and run perfectly n a 64 bit system. Specifically, duble check that yu have: 1. Created the necessary rws in arch/x86/syscall/syscall_64.tbl These will allw us t call yur functins, but the numbers must match exactly thse numbers listed in this assignment. (In 32-bit the numbers differ frm 64bit). 2. Yur user-level cde uses the crrect 64-bit sys-call numbers. Hint: Make yur user-level cde check the GCC defined cnstant _LP64 and autmatically pick the crrect sys-call number: #include <stdi.h> #include <unistd.h> #include <sys/syscall.h> #if _LP64 == 1 #define _CS300_TEST_ 340 #else #define _CS300_TEST_ 390 #endif int main(int argc, char *argv[]) { printf("\ndiving t kernel level\n\n"); int result = syscall(_cs300_test_, 12345); printf("\nrising t user level w/ result = %d\n\n", result); } return 0; Page 6/6