System calls are identified by their numbers. The number of
the call foo 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 foo
is called sys_foo. One finds the association between
numbers and names in the sys_call_table, for example in
arch/i386/kernel/entry.S.
The world changes and system calls change. Since one must not break old binaries, the semantics associated to any given system call number must remain fully backwards compatible.
What happens in practice is one of two things: either one gets a new and improved system call with a new name and number, and the libc routine that used to invoke the old call is changed to use the 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_getuid32
is 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 are
sys_getuid16 and sys_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 got
a name prefixed by "old", see __NR_oldolduname,
__NR_olduname, __NR_uname, belonging to
sys_olduname, sys_uname, sys_newuname,
respectively.
One also has __NR_oldstat, __NR_stat,
__NR_stat64 belonging to sys_stat,
sys_newstat, sys_stat64, respectively.
And __NR_umount, __NR_umount2 belonging to
sys_oldumount, sys_umount, respectively.
And __NR_select, __NR__newselect belonging to
old_select, sys_select, respectively.
These moving names are confusing - now you have been warned:
the system call with number __NR_foo does not always
belong to the kernel routine sys_foo().
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 kernel
system_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 value
ENTRY(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 original
value 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 different
things happen), make sure that the system call number is within
range, and call the appropriate kernel routine from the table
sys_call_table. Upon return we check a few things and
when all is well restore the registers and call IRET to return
from this INT.
(This was for the i386 architecture. All details differ on other architectures, but the basic idea is the same: store the syscall number 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.)
On i386, the parameters of a system call are transported via
registers. 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 fifth
in %edi, the sixth in %ebp.
Earlier versions of Linux could handle only four or five system call
parameters, and therefore the system calls select() (5 parameters)
and mmap() (6 parameters) used to have a single parameter
that was a pointer to a parameter block in memory. Since Linux 1.3.0
five parameters are supported (and the earlier select with
memory block was renamed old_select), and since Linux 2.3.31
six parameters are supported (and the earlier mmap with
memory block was succeeded by the new mmap2).
Above we said: typically, the kernel returns a negative value to
indicate an error. But this would mean that any system call only
can return positive values. Since the negative error returns are
of the form -ESOMETHING, and the error numbers have small
positive 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 125
is version and architecture dependent) and other values are OK.
What if a system call wants to return a small negative number
and it is not an error? The scheduling priority of a process
is set by setpriority() and read by getpriority(),
and this value ranges from -20 (top priority) to 19 (lowest priority
background job). The library routines with these names use these
numbers, but the system call getpriority() returns
20 - P instead of P, moving the output interval to positive numbers only.
Or, similarly, the subfunctions PEEK* of ptrace return
the contents of a memory word in the traced process, and any
value is possible. However, the system call returns this value in
the data 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 */
Above we saw in ret_from_sys_call the test on sigpending:
if a signal arrived while we were executing kernel code, then just
before returning from the system call we first call the user program's
signal 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 errno
to EINTR. Just before the system call returns, the user program's
signal handler is called.
(So, what is "slow"? Mostly those calls that can block forever waiting
for external events; read and write to terminal devices, but not
read and write to disk devices, wait, pause.)
This means that a system call can return an error while nothing was
wrong. Usually one will want to redo the system call. That can be
automated by installing the signal handler using a call to
sigaction with the SA_RESTART flag set.
The effect is that upon an interrupt the system call is aborted,
the user program's signal handler is called, and afterwards
the system call is restarted from the beginning.
Why is this not the default? It was, for a while, but often it is necessary to react to a signal while the reacting is not done by the signal handler itself. It is difficult to do nontrivial things in a signal handler since the rest of the program is in an unknown state, and most signal handlers just set a flag that is tested elsewhere.
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.
There are other cases where a syscall has to be done in several steps.
Instead of just calling the system call write() it may be
necessary 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 in read_write.c
that causes a maximal write size of INT_MAX & PAGE_CACHE_MASK
which 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.
It has been
observed
that a 2 GHz Pentium 4 was much slower than an 850 MHz Pentium III on
certain tasks, and that this slowness is caused by the very large overhead
of the traditional int 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 the sysenter 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 mapped
into the memory of all applications, and updating this location on
each clock interrupt. These applications could then read this fixed
location with a single instruction - no system call required.
There might be other data that the kernel could make available in a read-only way to the process, like perhaps the current process ID. A vsyscall is a "system" call that avoids crossing the 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 routine
sysenter_setup() is called. It sets up a non-writable page
and writes code for the sysenter instruction if the CPU
supports that, and for the classical int 0x80 otherwise.
Thus, the C library can use the fastest type of system call
by jumping to a fixed address in the vsyscall page.
This page was changed to have the structure of an ELF binary
(called linux-vsyscall.so.1) in Linux 2.5.69.
In Linux 2.5.74 the name was changed to linux-gate.so.1.
Concerning gettimeofday(), a vsyscall version for the x86-64
is already part of the vanilla kernel. Patches for i386 exist.
(An example of the kind of timing differences: John Stultz reports
on an experiment where he measures gettimeofday() and
finds 1.67 us for the int 0x80 way, 1.24 us for the
sysenter way, and 0.88 us for the vsyscall.)
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 syspage
syspage: ELF 32-bit LSB shared object, Intel 80386, version 1 (SYSV), stripped
% objdump -h syspage
syspage: file format elf32-i386
Sections:
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 syspage
syspage: file format elf32-i386
Disassembly of section .text:
ffffe400 <.text>:
ffffe400: 51 push %ecx
ffffe401: 52 push %edx
ffffe402: 55 push %ebp
ffffe403: 89 e5 mov %esp,%ebp
ffffe405: 0f 34 sysenter
ffffe407: 90 nop
ffffe408: 90 nop
... more nops ...
ffffe40d: 90 nop
ffffe40e: eb f3 jmp 0xffffe403
ffffe410: 5d pop %ebp
ffffe411: 5a pop %edx
ffffe412: 59 pop %ecx
ffffe413: c3 ret
... zero bytes ...
ffffe420: 58 pop %eax
ffffe421: b8 77 00 00 00 mov $0x77,%eax
ffffe426: cd 80 int $0x80
ffffe428: 90 nop
ffffe429: 90 nop
... more nops ...
ffffe43f: 90 nop
ffffe440: b8 ad 00 00 00 mov $0xad,%eax
ffffe445: cd 80 int $0x80
The interesting addresses here are found via
% grep ffffe System.map ffffe000 A VSYSCALL_BASE ffffe400 A __kernel_vsyscall ffffe410 A SYSENTER_RETURN ffffe420 A __kernel_sigreturn ffffe440 A __kernel_rt_sigreturn %
So __kernel_vsyscall pushes a few registers and does
a sysenter instruction. And SYSENTER_RETURN
pops the registers again and returns. And __kernel_sigreturn
and __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 a slightly 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)
using call 0xffffe400 instead of int 0x80.
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 at
the vsyscall entry address, and AT_SYSINFO_EHDR data, which points
at the start of the vsyscall page.
Maybe
in the very beginning
call *%gs:0x18 worked as replacement for the old
int $0x80. I have never seen a library version that actually
used 0x18.
The 0x18 here is the offset of the sysinfo field
in the struct tcb_head at the start of the glibc
TLS (thread-local storage) segment.
It is 0x10 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 % ./demo sys = 0x55573420 gs = 0x63 Enter the kernel via call *%gs:0x10 .and on i386:
% ./demo sys = 0xffffe414 gs = 0x33 Enter the kernel via call *%gs:0x10 .
And indeed this works:
% cat exit42.c
int main() {
__asm__(
" movl $1, %eax \n"
" movl $42, %ebx \n"
" call *%gs:0x10 \n"
);
}
% cc -m32 exit42.c -o x
% ./x; echo $?
42