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Subject[PATCH] 'select' failure or signal should not update timeout
[This follows up a thread in the emacs-pretesters mailing list about a
problem with Emacs, 'select', SA_RESTART, and interrupts.]

> Date: Wed, 17 Jul 2002 08:30:57 -0600 (MDT)
> From: Richard Stallman <rms@gnu.org>
>
> Is it true today that restarting a `select' call after a signal (with
> SA_RESTART) alters the contents of the user's timeout parameter?

I looked into this a bit more, and it turns out to be a problem in the
Linux kernel. POSIX 1003.1-2001
<http://www.opengroup.org/onlinepubs/007904975/functions/select.html>
says that 'select' may modify its timeout argument only "upon
successful completion". However, the Linux kernel sometimes modifies
the timeout argument even when 'select' fails or is interrupted.

Here is a program that illustrates the problem. At the end of this
message I enclose a proposed patch to the Linux kernel to fix the
problem.

/* Conformance test for POSIX 1003.1-2001's requirement that select()
must not update its timeout argument unless it succeeds. */

#include <signal.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/time.h>
#include <unistd.h>

static void
moan (char const *string)
{
perror (string);
exit (1);
}

struct timeval timeout;
struct timeval timeout_when_in_handler;
struct sigaction act;

void
handle_sigalrm (int sig)
{
timeout_when_in_handler = timeout;
}

enum { TIMEOUT_SECONDS = 5 };

int
main (int argc, char **argv)
{
act.sa_handler = handle_sigalrm;
act.sa_flags = SA_RESTART;
if (sigaction (SIGALRM, &act, 0) != 0)
moan ("sigaction");
timeout.tv_sec = TIMEOUT_SECONDS;
timeout_when_in_handler = timeout;
alarm (TIMEOUT_SECONDS / 2);
if (select (0, 0, 0, 0, &timeout) != 0)
{
if (timeout.tv_sec != TIMEOUT_SECONDS)
{
perror ("select");
fprintf (stderr, "select failed, but timeout was updated to %ld.%.9ld seconds\n",
(long) timeout.tv_sec, timeout.tv_usec);
}
}

if (timeout_when_in_handler.tv_sec != TIMEOUT_SECONDS)
fprintf (stderr, "timeout was updated to %ld.%.9ld seconds while signal handler was active\n",
(long) timeout_when_in_handler.tv_sec,
timeout_when_in_handler.tv_usec);

return 0;
}


Here is a proposed patch to Linux kernel 2.5.26. The patch also
applies to Linux 2.4.18, though you have to ignore the patches to
files that do not exist in 2.4.18. I haven't tested this patch, but
it's pretty straightforward.

diff -prU6 2.5.26/arch/ia64/ia32/sys_ia32.c 2.5.26-select/arch/ia64/ia32/sys_ia32.c
--- 2.5.26/arch/ia64/ia32/sys_ia32.c Tue Jul 16 16:49:35 2002
+++ 2.5.26-select/arch/ia64/ia32/sys_ia32.c Fri Jul 19 02:17:08 2002
@@ -1058,32 +1058,32 @@ sys32_select (int n, fd_set *inp, fd_set
zero_fd_set(n, fds.res_in);
zero_fd_set(n, fds.res_out);
zero_fd_set(n, fds.res_ex);

ret = do_select(n, &fds, &timeout);

+ if (ret < 0)
+ goto out;
+ if (!ret) {
+ ret = -ERESTARTNOHAND;
+ if (signal_pending(current))
+ goto out;
+ ret = 0;
+ }
+
if (tvp32 && !(current->personality & STICKY_TIMEOUTS)) {
time_t sec = 0, usec = 0;
if (timeout) {
sec = timeout / HZ;
usec = timeout % HZ;
usec *= (1000000/HZ);
}
if (put_user(sec, &tvp32->tv_sec) || put_user(usec, &tvp32->tv_usec)) {
ret = -EFAULT;
goto out;
}
- }
-
- if (ret < 0)
- goto out;
- if (!ret) {
- ret = -ERESTARTNOHAND;
- if (signal_pending(current))
- goto out;
- ret = 0;
}

set_fd_set(n, inp, fds.res_in);
set_fd_set(n, outp, fds.res_out);
set_fd_set(n, exp, fds.res_ex);

diff -prU6 2.5.26/arch/mips64/kernel/linux32.c 2.5.26-select/arch/mips64/kernel/linux32.c
--- 2.5.26/arch/mips64/kernel/linux32.c Tue Jul 16 16:49:25 2002
+++ 2.5.26-select/arch/mips64/kernel/linux32.c Fri Jul 19 02:18:19 2002
@@ -1170,30 +1170,30 @@ asmlinkage int sys32_select(int n, u32 *
zero_fd_set(n, fds.res_in);
zero_fd_set(n, fds.res_out);
zero_fd_set(n, fds.res_ex);

ret = do_select(n, &fds, &timeout);

+ if (ret < 0)
+ goto out;
+ if (!ret) {
+ ret = -ERESTARTNOHAND;
+ if (signal_pending(current))
+ goto out;
+ ret = 0;
+ }
+
if (tvp && !(current->personality & STICKY_TIMEOUTS)) {
time_t sec = 0, usec = 0;
if (timeout) {
sec = timeout / HZ;
usec = timeout % HZ;
usec *= (1000000/HZ);
}
put_user(sec, &tvp->tv_sec);
put_user(usec, &tvp->tv_usec);
- }
-
- if (ret < 0)
- goto out;
- if (!ret) {
- ret = -ERESTARTNOHAND;
- if (signal_pending(current))
- goto out;
- ret = 0;
}

set_fd_set32(nn, inp, fds.res_in);
set_fd_set32(nn, outp, fds.res_out);
set_fd_set32(nn, exp, fds.res_ex);

diff -prU6 2.5.26/arch/ppc64/kernel/sys_ppc32.c 2.5.26-select/arch/ppc64/kernel/sys_ppc32.c
--- 2.5.26/arch/ppc64/kernel/sys_ppc32.c Tue Jul 16 16:49:36 2002
+++ 2.5.26-select/arch/ppc64/kernel/sys_ppc32.c Fri Jul 19 02:17:58 2002
@@ -762,30 +762,30 @@ asmlinkage long sys32_select(int n, u32
zero_fd_set(n, fds.res_in);
zero_fd_set(n, fds.res_out);
zero_fd_set(n, fds.res_ex);

ret = do_select(n, &fds, &timeout);

+ if (ret < 0)
+ goto out;
+ if (!ret) {
+ ret = -ERESTARTNOHAND;
+ if (signal_pending(current))
+ goto out;
+ ret = 0;
+ }
+
if (tvp && !(current->personality & STICKY_TIMEOUTS)) {
time_t sec = 0, usec = 0;
if (timeout) {
sec = timeout / HZ;
usec = timeout % HZ;
usec *= (1000000/HZ);
}
put_user(sec, &tvp->tv_sec);
put_user(usec, &tvp->tv_usec);
- }
-
- if (ret < 0)
- goto out;
- if (!ret) {
- ret = -ERESTARTNOHAND;
- if (signal_pending(current))
- goto out;
- ret = 0;
}

set_fd_set32(nn, inp, fds.res_in);
set_fd_set32(nn, outp, fds.res_out);
set_fd_set32(nn, exp, fds.res_ex);

diff -prU6 2.5.26/arch/s390x/kernel/linux32.c 2.5.26-select/arch/s390x/kernel/linux32.c
--- 2.5.26/arch/s390x/kernel/linux32.c Tue Jul 16 16:49:27 2002
+++ 2.5.26-select/arch/s390x/kernel/linux32.c Fri Jul 19 02:18:27 2002
@@ -1420,30 +1420,30 @@ asmlinkage int sys32_select(int n, u32 *
zero_fd_set(n, fds.res_in);
zero_fd_set(n, fds.res_out);
zero_fd_set(n, fds.res_ex);

ret = do_select(n, &fds, &timeout);

+ if (ret < 0)
+ goto out;
+ if (!ret) {
+ ret = -ERESTARTNOHAND;
+ if (signal_pending(current))
+ goto out;
+ ret = 0;
+ }
+
if (tvp && !(current->personality & STICKY_TIMEOUTS)) {
int sec = 0, usec = 0;
if (timeout) {
sec = timeout / HZ;
usec = timeout % HZ;
usec *= (1000000/HZ);
}
put_user(sec, &tvp->tv_sec);
put_user(usec, &tvp->tv_usec);
- }
-
- if (ret < 0)
- goto out;
- if (!ret) {
- ret = -ERESTARTNOHAND;
- if (signal_pending(current))
- goto out;
- ret = 0;
}

set_fd_set32(nn, inp, fds.res_in);
set_fd_set32(nn, outp, fds.res_out);
set_fd_set32(nn, exp, fds.res_ex);

diff -prU6 2.5.26/arch/sparc64/kernel/sys_sparc32.c 2.5.26-select/arch/sparc64/kernel/sys_sparc32.c
--- 2.5.26/arch/sparc64/kernel/sys_sparc32.c Tue Jul 16 16:49:35 2002
+++ 2.5.26-select/arch/sparc64/kernel/sys_sparc32.c Fri Jul 19 02:16:59 2002
@@ -1387,30 +1387,30 @@ asmlinkage int sys32_select(int n, u32 *
zero_fd_set(n, fds.res_in);
zero_fd_set(n, fds.res_out);
zero_fd_set(n, fds.res_ex);

ret = do_select(n, &fds, &timeout);

+ if (ret < 0)
+ goto out;
+ if (!ret) {
+ ret = -ERESTARTNOHAND;
+ if (signal_pending(current))
+ goto out;
+ ret = 0;
+ }
+
if (tvp && !(current->personality & STICKY_TIMEOUTS)) {
time_t sec = 0, usec = 0;
if (timeout) {
sec = timeout / HZ;
usec = timeout % HZ;
usec *= (1000000/HZ);
}
put_user(sec, &tvp->tv_sec);
put_user(usec, &tvp->tv_usec);
- }
-
- if (ret < 0)
- goto out;
- if (!ret) {
- ret = -ERESTARTNOHAND;
- if (signal_pending(current))
- goto out;
- ret = 0;
}

set_fd_set32(nn, inp, fds.res_in);
set_fd_set32(nn, outp, fds.res_out);
set_fd_set32(nn, exp, fds.res_ex);

diff -prU6 2.5.26/arch/x86_64/ia32/sys_ia32.c 2.5.26-select/arch/x86_64/ia32/sys_ia32.c
--- 2.5.26/arch/x86_64/ia32/sys_ia32.c Tue Jul 16 16:49:32 2002
+++ 2.5.26-select/arch/x86_64/ia32/sys_ia32.c Fri Jul 19 02:18:07 2002
@@ -878,30 +878,30 @@ sys32_select(int n, fd_set *inp, fd_set
zero_fd_set(n, fds.res_in);
zero_fd_set(n, fds.res_out);
zero_fd_set(n, fds.res_ex);

ret = do_select(n, &fds, &timeout);

+ if (ret < 0)
+ goto out;
+ if (!ret) {
+ ret = -ERESTARTNOHAND;
+ if (signal_pending(current))
+ goto out;
+ ret = 0;
+ }
+
if (tvp32 && !(current->personality & STICKY_TIMEOUTS)) {
time_t sec = 0, usec = 0;
if (timeout) {
sec = timeout / HZ;
usec = timeout % HZ;
usec *= (1000000/HZ);
}
put_user(sec, (int *)&tvp32->tv_sec);
put_user(usec, (int *)&tvp32->tv_usec);
- }
-
- if (ret < 0)
- goto out;
- if (!ret) {
- ret = -ERESTARTNOHAND;
- if (signal_pending(current))
- goto out;
- ret = 0;
}

set_fd_set(n, inp, fds.res_in);
set_fd_set(n, outp, fds.res_out);
set_fd_set(n, exp, fds.res_ex);

diff -prU6 2.5.26/fs/select.c 2.5.26-select/fs/select.c
--- 2.5.26/fs/select.c Tue Jul 16 16:49:24 2002
+++ 2.5.26-select/fs/select.c Fri Jul 19 02:18:41 2002
@@ -316,30 +316,30 @@ sys_select(int n, fd_set *inp, fd_set *o
zero_fd_set(n, fds.res_in);
zero_fd_set(n, fds.res_out);
zero_fd_set(n, fds.res_ex);

ret = do_select(n, &fds, &timeout);

+ if (ret < 0)
+ goto out;
+ if (!ret) {
+ ret = -ERESTARTNOHAND;
+ if (signal_pending(current))
+ goto out;
+ ret = 0;
+ }
+
if (tvp && !(current->personality & STICKY_TIMEOUTS)) {
time_t sec = 0, usec = 0;
if (timeout) {
sec = timeout / HZ;
usec = timeout % HZ;
usec *= (1000000/HZ);
}
put_user(sec, &tvp->tv_sec);
put_user(usec, &tvp->tv_usec);
- }
-
- if (ret < 0)
- goto out;
- if (!ret) {
- ret = -ERESTARTNOHAND;
- if (signal_pending(current))
- goto out;
- ret = 0;
}

set_fd_set(n, inp, fds.res_in);
set_fd_set(n, outp, fds.res_out);
set_fd_set(n, exp, fds.res_ex);

-
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