Re: [PATCH] select: fix sys_select to not leak ERESTARTNOHAND touserspace

[John Blackwood]

Hi Neil,

We've been having problems with this select patch change.

Specifically -- previously, when a ptrace attach was done to a task
blocked in a select() call and that task had a timeout value,
the task would restart the select() call with an updated timeout value.

With this patch in place, the task now instead returns EINTR.

A test that shows this issue is provided below.

We also confirmed that attaching to a program sitting
in select() with gdb makes the select get an EINTR, so this
behavior also shows up in gdb.

Thank you for your considerations in this matter.

------------------- -------------------
#include <stdio.h>
#include <sys/time.h>
#include <sys/types.h>
#include <unistd.h>
#include <errno.h>
#include <stdlib.h>
#include <string.h>
#include <ctype.h>
#include <time.h>
#include <signal.h>
#include <sys/wait.h>
#include <sys/ptrace.h>

int
main(int argc, char ** argv) {
   pid_t kid;
   if ((kid = fork()) == 0) {
      int ms_wait = 2000;
      int rval;
      struct timeval timeout;

      timeout.tv_sec = ms_wait / 1000;
      timeout.tv_usec = (ms_wait % 1000) * 1000;
      rval = select(0, NULL, NULL, NULL, &timeout);
      if (rval == -1) {
         int errcode = errno;
         printf("Hey! Why did my select error off with errno %d (%s)?\n",
                errcode, strerror(errcode));
         fflush(stdout);
      } else {
         printf("select call completed, return value: %d\n", rval);
      }
      exit(0);
   } else if (kid == (pid_t)-1) {
      perror("fork");
   } else {
      int ms_wait = 500;
      int rval;
      struct timeval timeout;

      /* Wait a bit to make sure kid has a chance to get into its
       * select call
       */
      timeout.tv_sec = ms_wait / 1000;
      timeout.tv_usec = (ms_wait % 1000) * 1000;
      rval = select(0, NULL, NULL, NULL, &timeout);

      /* Now attach to the kid, then continue him.
       */
      if (ptrace(PTRACE_ATTACH, kid, (void *)0, (void *)0) != 0) {
         perror("ptrace");
      }
      if (waitpid(kid, &rval, 0) != kid) {
         perror("waitpid");
      }
      if (ptrace(PTRACE_CONT, kid, (void *)0, (void *)0) != 0) {
         perror("ptrace");
      }
      if (waitpid(kid, &rval, 0) != kid) {
         perror("waitpid");
      }
   }
   return 0;
}
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