lsfd: (bugfix) do not reuse stat(2) buffer for files with identical names

lsfd: (bugfix) do not reuse stat(2) buffer for files with identical names

To reduce the number of stat(2) calls, lsfd reused the buffer returned
from stat(2) when file descriptors opened files with the same name.

If file descriptors open different files that happen to have the same
name, lsfd may report incorrect results. In such cases, the stat(2)
buffer must not be reused.

The program a.out is run with a file name "D/a".  It opens the file
twice during its execution with an interval.  Between the two open()
calls, a different filesystem is mounted on "D".

<the source code of ./a.out>

  #include <fcntl.h>
  #include <err.h>
  #include <string.h>
  #include <stdio.h>
  #include <stdlib.h>
  #include <unistd.h>

  static void usage(const char *prog, int eval, FILE *fp)
  {
    fputs("Usage :\n", fp);
    fprintf(fp, " %s FILE\n", prog);
    exit(eval);
  }

  int main(int argc, char **argv)
  {
    const char *fname;
    int fd0, fd1;

    if (argc < 2)
      errx(2, "too few arguements");
    if (strcmp(argv[1], "-h") == 0 ||
	strcmp(argv[1], "--h") == 0)
      usage(argv[0], 0, stdout);
    if (argc > 2)
      errx(2, "too many arguements");

    fname = argv[1];

    printf("pid: %d\n", getpid());
    fd0 = open(fname, O_RDONLY);
    if (fd0 < 0)
      err(1, "error in open \"%s\" in the first time", fname);

    fputs("[press RETURN to go to the next step] ", stderr);
    getchar();

    fd1 = open(fname, O_RDONLY);
    if (fd1 < 0)
      err(1, "error in open \"%s\" in the second time", fname);

    fputs("[press RETURN to exit] ", stderr);
    getchar();

    return 0;
  }

<PREPARATION>
  $ mkdir D
  $ touch D/a
  $ dd if=/dev/zero of=img.xfs count=1 bs=400MB
  1+0 records in
  1+0 records out
  400000000 bytes (400 MB, 381 MiB) copied, 0.427125 s, 936 MB/s
  $ mkfs.xfs img.xfs
  meta-data=img.xfs                isize=512    agcount=4, agsize=24414 blks
	   =                       sectsz=512   attr=2, projid32bit=1
	   =                       crc=1        finobt=1, sparse=1, rmapbt=1
	   =                       reflink=1    bigtime=1 inobtcount=1 nrext64=1
	   =                       exchange=0   metadir=0
  data     =                       bsize=4096   blocks=97656, imaxpct=25
	   =                       sunit=0      swidth=0 blks
  naming   =version 2              bsize=4096   ascii-ci=0, ftype=1, parent=0
  log      =internal log           bsize=4096   blocks=16384, version=2
	   =                       sectsz=512   sunit=0 blks, lazy-count=1
  realtime =none                   extsz=4096   blocks=0, rtextents=0
	   =                       rgcount=0    rgsize=0 extents
	   =                       zoned=0      start=0 reserved=0
  $ sudo mount img.xfs D
  $ sudo touch D/a
  $ sudo umount D

Let's see the bug.

<TERMNAL-1>
  $ ./a.out D/a
  pid: 770257
  [press RETURN to go to the next step]

<TERMNAL-2>
  $ sudo mount img.xfs D

<TERMNAL-1>
  (press RETURN)
  [press RETURN to exit]

<TERMNAL-2>
  $ ./lsfd-orignal -p 770257 -Q 'FD > 2'
  COMMAND    PID   USER ASSOC  XMODE TYPE SOURCE MNTID    INODE NAME
  a.out   770257 yamato     3 r-----  REG   dm-3    95 30947324 /home/yamato/D/a
  a.out   770257 yamato     4 r-----  REG   dm-3  1631 30947324 /home/yamato/D/a

  $ stat D/a
    File: D/a
    Size: 0               Blocks: 0          IO Block: 4096   regular empty file
  Device: 7,10    Inode: 131         Links: 1
  ...

Although D/a has inode number 131, lsfd-original reports 30947324.
After removing the code that reuses the stat(2) buffer, lsfd-new reports:

<TERMNAL-2>
  $ ./lsfd-new -p 770257 -Q 'FD > 2'
  COMMAND    PID   USER ASSOC  XMODE TYPE SOURCE MNTID    INODE NAME
  a.out   770257 yamato     3 r-----  REG   dm-3    95 30947324 /home/yamato/D/a
  a.out   770257 yamato     4 r-----  REG loop10  1631      131 /home/yamato/D/a

Signed-off-by: Masatake YAMATO <yamato@redhat.com>