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To:     linux-fsdevel@vger.kernel.org,
        "viro@zeniv.linux.org.uk >> Al Viro" <viro@zeniv.linux.org.uk>
From:   "Pavel V. Panteleev" <panteleev_p@mcst.ru>
Subject: timestamp_truncate() incorrect truncate
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Date:   Fri, 7 Aug 2020 12:15:29 +0300
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Hello,

timestamp_truncate() truncates nsec to 0, if sec is 0 for nfs v3. There 
is a test:

$ cat ./t.c

#define _GNU_SOURCE

#include <fcntl.h>
#include <stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <unistd.h>

int
main (void)
{
   const char *fname = "./delme.log";
   int fd;
   struct stat sb;
   const struct timespec ts[2] = {{0, 199}, {0, 999}};

   unlink (fname);
   fd = open (fname, O_CREAT | O_TRUNC, 0660);

   if (futimens(fd, ts) != 0)
     {
       perror ("futimens()");
       return 1;
     }

   close (fd);

   stat (fname, &sb);

   printf ("atim: sec == %ld, nsec == %ld\n",
           (long) sb.st_atim.tv_sec,
           (long) sb.st_atim.tv_nsec);

   printf ("mtim: sec == %ld, nsec == %ld\n",
           (long) sb.st_mtim.tv_sec,
           (long) sb.st_mtim.tv_nsec);

   if (sb.st_atim.tv_sec != ts[0].tv_sec
       || sb.st_atim.tv_nsec != ts[0].tv_nsec
       || sb.st_mtim.tv_sec != ts[1].tv_sec
       || sb.st_mtim.tv_nsec != ts[1].tv_nsec)
     return 1;

   return 0;
}

(sec + nsec = nsec) is inside of interval, supported by nfs v3. Why nsec 
is truncated to 0?