stat results are faked incorrectly

[bmwiedemann]

I am working on reproducible builds for openSUSE and there I would like to use libfaketime to make software behave exactly as if date+time were

• in the future
• deterministic (this is why I use the i1 param)

e.g. I would like to use faketime -f '@2020-01-01 01:01:01 i1'

But one problem with stat is visible when you compare

cmd='echo $SECONDS;touch /tmp/x ; stat /tmp/x ; echo $SECONDS; sleep 1 ; stat /tmp/x ; echo $SECONDS;echo $SECONDS'
bash -c "$cmd"
faketime -f '+0y i1' bash -c "$cmd"

That produces

> bash -c "$cmd"
0
  File: /tmp/x
  Size: 7               Blocks: 8          IO Block: 4096   regular file
Device: 803h/2051d      Inode: 531866      Links: 1
Access: (0644/-rw-r--r--)  Uid: (16894/bwiedemann)   Gid: (   50/    suse)
Access: 2019-01-30 15:26:01.572086689 +0100
Modify: 2019-01-30 15:26:01.572086689 +0100
Change: 2019-01-30 15:26:01.572086689 +0100
 Birth: -
0

  File: /tmp/x
  Size: 7               Blocks: 8          IO Block: 4096   regular file
Device: 803h/2051d      Inode: 531866      Links: 1
Access: (0644/-rw-r--r--)  Uid: (16894/bwiedemann)   Gid: (   50/    suse)
Access: 2019-01-30 15:26:01.572086689 +0100
Modify: 2019-01-30 15:26:01.572086689 +0100
Change: 2019-01-30 15:26:01.572086689 +0100
 Birth: -
1
1


> faketime -f '+0y i1' bash -c "$cmd"
32
  File: /tmp/x
  Size: 7               Blocks: 8          IO Block: 4096   regular file
Device: 803h/2051d      Inode: 531866      Links: 1
Access: (0644/-rw-r--r--)  Uid: (16894/bwiedemann)   Gid: (   50/    suse)
Access: 2019-01-30 15:27:02.605862595 +0100
Modify: 2019-01-30 15:27:03.605862595 +0100
Change: 2019-01-30 15:27:01.605862595 +0100
 Birth: -
62
  File: /tmp/x
  Size: 7               Blocks: 8          IO Block: 4096   regular file
Device: 803h/2051d      Inode: 531866      Links: 1
Access: (0644/-rw-r--r--)  Uid: (16894/bwiedemann)   Gid: (   50/    suse)
Access: 2019-01-30 15:27:15.605862595 +0100
Modify: 2019-01-30 15:27:16.605862595 +0100
Change: 2019-01-30 15:27:14.605862595 +0100
 Birth: -
75
76

With faketime, the stat results are shifting, when nothing touched the file, which can confuse programs like make.

[wolfcw]

Yes, I can reproduce that problem.

Could you please try LD_PRELOAD'ing libfaketime.so.1 directly instead of using the faketime wrapper?

LD_PRELOAD=/path/to/libfaketime.so.1 FAKETIME='+0y i1' bash -c "$cmd"

produces the desired output here, and using libfaketime[MT].so.1 directly instead of the wrapper is generally recommended for more advanced use cases.

[bmwiedemann]

Using plain LD_PRELOAD lacks the other variable for shared memory required for the synchronized counter, so the two echo $SECONDS at the end print the same value