Linux System Calls

4. System Calls

http://www.win.tue.nl/~aeb/linux/lk/lk-4.html

4.1 System call numbers

System calls are identified by their numbers. The number ofthe callfoo is __NR_foo. For example,the number of _llseek used above is__NR__llseek,defined as 140 in /usr/include/asm-i386/unistd.h.Different architectures have different numbers.

Often, the kernel routine that handles the call foois called sys_foo. One finds the association betweennumbers and names in thesys_call_table, for example inarch/i386/kernel/entry.S.

Change

The world changes and system calls change.Since one must not break old binaries, the semantics associated toany given system call number must remain fully backwards compatible.

What happens in practice is one of two things: either one gets anew and improved system call with a new name and number, and thelibc routine that used to invoke the old call is changed to usethe new one, or the new call (with new number) gets the old name,and the old call gets "old" prefixed to its name.

For example, long ago user IDs had 16 bits, today they have 32.__NR_getuid is 24, and__NR_getuid32is 199, and the former belongs to the 16-bit version of the call,the latter to the 32-bit version.Looking at the associated kernel routines, we find that these aresys_getuid16 andsys_getuid, respectively.(Thus, sys_getuid does not have number__NR_getuid.)Looking at glibc, we find code somewhat like

int getuid32_available = UNKNOWN;uid_t getuid(void) {        if (getuid32_available == TRUE)                return INLINE_SYSCALL(getuid32, 0);        if (getuid32_available == UNKNOWN) {                uid_t res = INLINE_SYSCALL(getuid32, 0);                if (res == 0 || errno != ENOSYS) {                        getuid32_available = TRUE;                        return res;                }                getuid32_available = FALSE;        }        return INLINE_SYSCALL(getuid, 0);}

For an example where the name was moved and the old call gota name prefixed by "old", see__NR_oldolduname,__NR_olduname, __NR_uname, belonging tosys_olduname,sys_uname, sys_newuname,respectively.One also has __NR_oldstat, __NR_stat,__NR_stat64 belonging tosys_stat,sys_newstat, sys_stat64, respectively.And__NR_umount, __NR_umount2 belonging tosys_oldumount,sys_umount, respectively.And __NR_select, __NR__newselect belonging toold_select,sys_select, respectively.

These moving names are confusing - now you have been warned:the system call with number__NR_foo does not alwaysbelong to the kernel routine sys_foo().

4.2 The call

What happens? The assembler for a call with 0 parameters (on i386) is

#define _syscall0(type,name) \type name(void) \{ \long __res; \__asm__ volatile ("int $0x80" \        : "=a" (__res) \        : "0" (__NR_##name)); \__syscall_return(type,__res); \}

Thus, the basic ingredient is the assembler instruction INT 0x80.This causes a programmed exception and calls the kernelsystem_call routine. Some relevant code fragments:

/* include/asm-i386/hw_irq.h */#define SYSCALL_VECTOR              0x80/* arch/i386/kernel/traps.c */        set_system_gate(SYSCALL_VECTOR,&system_call);/* arch/i386/kernel/entry.S */#define GET_CURRENT(reg) \        movl $-8192, reg; \        andl %esp, reg#define SAVE_ALL \        cld; \        pushl %es; \        pushl %ds; \        pushl %eax; \        pushl %ebp; \        pushl %edi; \        pushl %esi; \        pushl %edx; \        pushl %ecx; \        pushl %ebx; \        movl $(__KERNEL_DS),%edx; \        movl %edx,%ds; \        movl %edx,%es;#define RESTORE_ALL     \        popl %ebx;      \        popl %ecx;      \        popl %edx;      \        popl %esi;      \        popl %edi;      \        popl %ebp;      \        popl %eax;      \1:      popl %ds;       \2:      popl %es;       \        addl $4,%esp;   \3:      iret;ENTRY(system_call)        pushl %eax                      # save orig_eax        SAVE_ALL        GET_CURRENT(%ebx)        testb $0x02,tsk_ptrace(%ebx)    # PT_TRACESYS        jne tracesys        cmpl $(NR_syscalls),%eax        jae badsys        call *SYMBOL_NAME(sys_call_table)(,%eax,4)        movl %eax,EAX(%esp)             # save the return valueENTRY(ret_from_sys_call)        cli                             # need_resched and signals atomic test        cmpl $0,need_resched(%ebx)        jne reschedule        cmpl $0,sigpending(%ebx)        jne signal_return        RESTORE_ALL

We transfer execution to system_call, save the originalvalue of the EAX register (it is the number of the system call),save all other registers, verify that we are not being traced(otherwise the tracer must be informed and entirely differentthings happen), make sure that the system call number is withinrange, and call the appropriate kernel routine from the tablesys_call_table. Upon return we check a few things andwhen all is well restore the registers and call IRET to returnfrom this INT.

(This was for the i386 architecture. All details differ on otherarchitectures, but the basic idea is the same: store the syscallnumber and the syscall parameters somewhere the kernel can find them,in registers, on the stack, or in a known place of memory,do something that causes a transfer to kernel code, etc.)

4.3 System call parameters

On i386, the parameters of a system call are transported viaregisters. The system call number goes into%eax,the first parameter in %ebx, the second in %ecx,the third in%edx, the fourth in %esi, the fifthin %edi, the sixth in%ebp.

Ancient history

Earlier versions of Linux could handle only four or five system callparameters, and therefore the system callsselect() (5 parameters)and mmap() (6 parameters) used to have a single parameterthat was a pointer to a parameter block in memory. Since Linux 1.3.0five parameters are supported (and the earlierselect withmemory block was renamed old_select), and since Linux 2.3.31six parameters are supported (and the earliermmap withmemory block was succeeded by the new mmap2).

4.4 Error return

Above we said: typically, the kernel returns a negative value toindicate an error. But this would mean that any system call onlycan return positive values. Since the negative error returns areof the form-ESOMETHING, and the error numbers have smallpositive values, there is only a small negative error range.Thus

#define __syscall_return(type, res) \do { \        if ((unsigned long)(res) >= (unsigned long)(-125)) { \                errno = -(res); \                res = -1; \        } \        return (type) (res); \} while (0)

Here the range [-125,-1] is reserved for errors (the constant 125is version and architecture dependent) and other values are OK.

What if a system call wants to return a small negative numberand it is not an error? The scheduling priority of a processis set bysetpriority() and read by getpriority(),and this value ranges from -20 (top priority) to 19 (lowest prioritybackground job). The library routines with these names use thesenumbers, but the system callgetpriority() returns20 - P instead of P, moving the output interval to positive numbers only.

Or, similarly, the subfunctions PEEK* of ptrace returnthe contents of a memory word in the traced process, and anyvalue is possible. However, the system call returns this value inthedata argument, and glibc does something like

        res = sys_ptrace(request, pid, addr, &data);        if (res >= 0) {                errno = 0;                res = data;        }        return res;

so that a user program has to do

        errno = 0;        res = ptrace(PTRACE_PEEKDATA, pid, addr, NULL);        if (res == -1 && errno != 0)                /* error */

4.5 Interrupted system calls

Above we saw in ret_from_sys_call the test onsigpending:if a signal arrived while we were executing kernel code, then justbefore returning from the system call we first call the user program'ssignal handler, and when this finishes return from the system call.

When a system call is slow and a signal arrives while it was blocked,waiting for something, the call is aborted and returns-EINTR,so that the library function will return -1 and set errnotoEINTR. Just before the system call returns, the user program'ssignal handler is called.

(So, what is "slow"? Mostly those calls that can block forever waitingfor external events; read and write to terminal devices, but notread and write to disk devices,wait, pause.)

This means that a system call can return an error while nothing waswrong. Usually one will want to redo the system call. That can beautomated by installing the signal handler using a call tosigaction with theSA_RESTART flag set.The effect is that upon an interrupt the system call is aborted,the user program's signal handler is called, and afterwardsthe system call is restarted from the beginning.

Why is this not the default? It was, for a while, but often it isnecessary to react to a signal while the reacting is not done bythe signal handler itself. It is difficult to do nontrivial thingsin a signal handler since the rest of the program is in an unknownstate, and most signal handlers just set a flag that is testedelsewhere.

A demo:

#include <stdio.h>#include <errno.h>#include <stdlib.h>#include <unistd.h>#include <signal.h>int got_interrupt;void intrup(int dummy) {        got_interrupt = 1;}void die(char *s) {        printf("%s\n", s);        exit(1);}int main() {        struct sigaction sa;        int n;        char c;        sa.sa_handler = intrup;        sigemptyset(&sa.sa_mask);        sa.sa_flags = 0;        if (sigaction(SIGINT, &sa, NULL))                die("sigaction-SIGINT");        sa.sa_flags = SA_RESTART;        if (sigaction(SIGQUIT, &sa, NULL))                die("sigaction-SIGQUIT");        got_interrupt = 0;        n = read(0, &c, 1);        if (n == -1 && errno == EINTR)                printf("read call was interrupted\n");        else if (got_interrupt)                printf("read call was restarted\n");        return 0;}

Here Ctrl-C will interrupt the read call, while after Ctrl-\the read call is restarted.

Partial success

There are other cases where a syscall has to be done in several steps.Instead of just calling the system callwrite() it may benecessary to do

ssize_t my_write(int fd, const void *buf, size_t count) {        ssize_t res;        while (count) {                res = write(fd, buf, count);                if (res < 0)                        return res;                buf += res;                count -= res;        }        return 0;}

even when writing to an ordinary disk file.Indeed, since 2.6.16 there is a limit MAX_RW_COUNT inread_write.cthat causes a maximal write size of INT_MAX & PAGE_CACHE_MASKwhich may be 2^31-1-4095 = 2147479552. This might violate POSIX.Usually a write only returns a short count when interrupted by a signal,or when the disk is full, or the max file size is reached.

4.6 Sysenter and the vsyscall page

It has been observedthat a 2 GHz Pentium 4 was much slower than an 850 MHz Pentium III oncertain tasks, and that this slowness is caused by the very large overheadof the traditionalint 0x80 interrupt on a Pentium 4.

Some models of the i386 family do have faster ways to enter the kernel.On Pentium II there is thesysenter instruction.Also AMD has a syscall instruction.It would be good if these could be used.

Something else is that in some applications gettimeofday()is a done very often, for example for timestamping all transactions.It would be nice if it could be implemented with very low overhead.

One way of obtaining a fast gettimeofday()is by writing the current time in a fixed place, on a page mappedinto the memory of all applications, and updating this location oneach clock interrupt. These applications could then read this fixedlocation with a single instruction - no system call required.

There might be other data that the kernel could make availablein a read-only way to the process, like perhaps the current process ID.Avsyscall is a "system" call that avoids crossingthe userspace-kernel boundary.

Linux is in the process of implementing such ideas.Since Linux 2.5.53 there is a fixed page, called the vsyscall page,filled by the kernel. At kernel initialization time the routinesysenter_setup() is called. It sets up a non-writable pageand writes code for the sysenter instruction if the CPUsupports that, and for the classicalint 0x80 otherwise.Thus, the C library can use the fastest type of system callby jumping to a fixed address in the vsyscall page.

This page was changed to have the structure of an ELF binary(calledlinux-vsyscall.so.1) in Linux 2.5.69.In Linux 2.5.74 the name was changed tolinux-gate.so.1.

Concerning gettimeofday(), a vsyscall version for the x86-64is already part of the vanilla kernel. Patches for i386 exist.(An example of the kind of timing differences: John Stultz reportson an experiment where he measures gettimeofday() andfinds 1.67 us for the int 0x80 way, 1.24 us for thesysenter way, and 0.88 us for the vsyscall.)

Some details

The kernel maps a page (0xffffe000-0xffffefff)in the memory of every process. (This is the next-to-last addressable page.The last is not mapped - maybe to avoid bugs related to wraparound.)We can read it:

/* get vsyscall page */#include <unistd.h>#include <string.h>int main() {        char *p = (char *) 0xffffe000;        char buf[4096];#if 0        write(1, p, 4096);        /* this gives EFAULT */#else        memcpy(buf, p, 4096);        write(1, buf, 4096);#endif        return 0;}

and if we do, find an ELF binary.

% ./get_vsyscall_page > syspage% file syspagesyspage: ELF 32-bit LSB shared object, Intel 80386, version 1 (SYSV), stripped% objdump -h syspagesyspage:     file format elf32-i386Sections:Idx Name          Size      VMA       LMA       File off  Algn  0 .hash         00000050  ffffe094  ffffe094  00000094  2**2                  CONTENTS, ALLOC, LOAD, READONLY, DATA  1 .dynsym       000000f0  ffffe0e4  ffffe0e4  000000e4  2**2                  CONTENTS, ALLOC, LOAD, READONLY, DATA  2 .dynstr       00000056  ffffe1d4  ffffe1d4  000001d4  2**0                  CONTENTS, ALLOC, LOAD, READONLY, DATA  3 .gnu.version  0000001e  ffffe22a  ffffe22a  0000022a  2**1                  CONTENTS, ALLOC, LOAD, READONLY, DATA  4 .gnu.version_d 00000038  ffffe248  ffffe248  00000248  2**2                  CONTENTS, ALLOC, LOAD, READONLY, DATA  5 .text         00000047  ffffe400  ffffe400  00000400  2**5                  CONTENTS, ALLOC, LOAD, READONLY, CODE  6 .eh_frame_hdr 00000024  ffffe448  ffffe448  00000448  2**2                  CONTENTS, ALLOC, LOAD, READONLY, DATA  7 .eh_frame     0000010c  ffffe46c  ffffe46c  0000046c  2**2                  CONTENTS, ALLOC, LOAD, READONLY, DATA  8 .dynamic      00000078  ffffe578  ffffe578  00000578  2**2                  CONTENTS, ALLOC, LOAD, DATA  9 .useless      0000000c  ffffe5f0  ffffe5f0  000005f0  2**2                  CONTENTS, ALLOC, LOAD, DATA% objdump -d syspagesyspage:     file format elf32-i386Disassembly of section .text:ffffe400 <.text>:ffffe400:       51                      push   %ecxffffe401:       52                      push   %edxffffe402:       55                      push   %ebpffffe403:       89 e5                   mov    %esp,%ebpffffe405:       0f 34                   sysenter ffffe407:       90                      nop    ffffe408:       90                      nop            ... more nops ...ffffe40d:       90                      nop    ffffe40e:       eb f3                   jmp    0xffffe403ffffe410:       5d                      pop    %ebpffffe411:       5a                      pop    %edxffffe412:       59                      pop    %ecxffffe413:       c3                      ret            ... zero bytes ...ffffe420:       58                      pop    %eaxffffe421:       b8 77 00 00 00          mov    $0x77,%eaxffffe426:       cd 80                   int    $0x80ffffe428:       90                      nop    ffffe429:       90                      nop            ... more nops ...ffffe43f:       90                      nop    ffffe440:       b8 ad 00 00 00          mov    $0xad,%eaxffffe445:       cd 80                   int    $0x80

The interesting addresses here are found via

% grep ffffe System.map ffffe000 A VSYSCALL_BASEffffe400 A __kernel_vsyscallffffe410 A SYSENTER_RETURNffffe420 A __kernel_sigreturnffffe440 A __kernel_rt_sigreturn%

So __kernel_vsyscall pushes a few registers and doesasysenter instruction. And SYSENTER_RETURNpops the registers again and returns. And__kernel_sigreturnand __kernel_rt_sigreturn do system calls 119 and 173,that is, sigreturn and rt_sigreturn, respectively.

What about the jump just before SYSENTER_RETURN?It is a trick to handle restarting of system calls with 6 parameters.As Linus said:I'm a disgusting pig, and proud of it to boot.

The code involved is most easily seen from aslightly earlier patch.

A tiny demo program.

#include <stdio.h>int pid;int main() {        __asm__(                "movl $20, %eax    \n"                "call 0xffffe400   \n"                "movl %eax, pid    \n"        );        printf("pid is %d\n", pid);        return 0;}

This does the getpid() system call (__NR_getpid is 20)usingcall 0xffffe400 instead of int 0x80.

Address space randomization

The layout of the vsyscall page changes, and the entry point varies.It can be found by inspection of the ELF headers of the page.

Since Linux 2.6.18 the page itself is mapped at a random address.The right entry point can now be found by searching the ELF auxiliary vector.

/* get vsyscall address and test - compile with -m32 on x86_64 */#include <stdio.h>#include <stdlib.h>#include <elf.h>static unsigned int getsys(char **envp) {        Elf32_auxv_t *auxv;        /* walk past all env pointers */        while (*envp++ != NULL)                ;        /* and find ELF auxiliary vectors (if this was an ELF binary) */        auxv = (Elf32_auxv_t *) envp;        for ( ; auxv->a_type != AT_NULL; auxv++)                if (auxv->a_type == AT_SYSINFO)                        return auxv->a_un.a_val;        fprintf(stderr, "no AT_SYSINFO auxv entry found\n");        exit(1);}unsigned int sys, pid;int main(int argc, char **argv, char **envp) {        sys = getsys(envp);        __asm__("               movl $20, %eax  \n"     /* getpid system call */"               call *sys       \n"     /* vsyscall */"               movl %eax, pid  \n"     /* get result */        );        printf("pid is %d\n", pid);        return 0;}

In the auxv vector one may find AT_SYSINFO data, which points atthe vsyscall entry address, and AT_SYSINFO_EHDR data, which pointsat the start of the vsyscall page.

Maybe in the very beginningcall *%gs:0x18 worked as replacement for the oldint $0x80. I have never seen a library version that actuallyused0x18.The 0x18 here is the offset of the sysinfo fieldin thestruct tcb_head at the start of the glibcTLS (thread-local storage) segment.It is0x10 on i386 and x86_64 (in 32-bit mode)in all sources I have examined.

Let us test, with getsys() as above.

#include <stdio.h>#include <stdlib.h>#include <unistd.h>#include <sys/syscall.h>#include <linux/unistd.h>#include <asm/ldt.h>#include <elf.h>...unsigned int sys, gs, *base;static void getgs() {        __asm__("movl %gs, gs\n");        if ((gs & 7) != 3) {                fprintf(stderr, "unexpected gs = 0x%x\n", gs);                exit(1);        }}static void getta(){        struct user_desc u;        int i;        u.entry_number = (gs >> 3);        if (syscall(__NR_get_thread_area, &u)) {                perror("get_thread_area");                exit(1);        }        base = (unsigned int *) u.base_addr;        for (i=0; i<100; i++)                if (base[i] == sys)                        goto gotit;        fprintf(stderr, "didn't find the sysinfo entry\n");        exit(1); gotit:        printf("Enter the kernel via  call *%%gs:0x%x .\n", 4*i);}int main(int argc, char **argv, char **envp) {        sys = getsys(envp); printf("sys = 0x%x\n", sys);        getgs();            printf("gs = 0x%x\n", gs);        getta();        return 0;}

And now, on x86_64:

% cc -m32 -Wall demo.c -o demo% ./demosys = 0x55573420gs = 0x63Enter the kernel via  call *%gs:0x10 .

and on i386:

% ./demosys = 0xffffe414gs = 0x33Enter the kernel via  call *%gs:0x10 .

And indeed this works:

% cat exit42.cint main() {__asm__("               movl $1, %eax  \n""               movl $42, %ebx \n""               call *%gs:0x10 \n");}% cc -m32 exit42.c -o x% ./x; echo $?42