Bug 9741

Summary: 'cat' broken for kernel < 3.0
Product: Busybox Reporter: Grant Edwards <grant.b.edwards>
Component: OtherAssignee: unassigned
Status: NEW ---    
Severity: normal CC: busybox-cvs
Priority: P5    
Version: unspecified   
Target Milestone: ---   
Hardware: All   
OS: Linux   
Host: Target:
Build:

Description Grant Edwards 2017-03-13 21:01:47 UTC 
The 'cat' utility fails on older kernel versions with multiple files
when stdout is a file.

In older kernel versions, the sendfile() system call that 'cat' uses
does not update the output file position.  When 'cat' stdout is a
file, this results in each input file overwriting the previous one:

   # ls -l
   total 12
   -rw-r--r--    1 root     root            29 Mar 13 17:44 d1
   -rw-r--r--    1 root     root            29 Mar 13 17:44 d2
   -rw-r--r--    1 root     root            29 Mar 13 17:44 d3
   
   # cat d1 d2 d3
   Mon Mar 13 17:44:35 UTC 2017
   Mon Mar 13 17:44:40 UTC 2017
   Mon Mar 13 17:44:43 UTC 2017
   
   # cat d1 d2 d3 >foo
   
   # ls -l
   total 16
   -rw-r--r--    1 root     root            29 Mar 13 17:44 d1
   -rw-r--r--    1 root     root            29 Mar 13 17:44 d2
   -rw-r--r--    1 root     root            29 Mar 13 17:44 d3
   -rw-r--r--    1 root     root            29 Mar 13 20:56 foo
   
   # cat foo
   Mon Mar 13 17:44:43 UTC 2017
   
   # cat d1 d2 d3 | cat >foo
   
   # ls -l foo
   -rw-r--r--    1 root     root            87 Mar 13 20:56 foo
   
   # cat foo
   Mon Mar 13 17:44:35 UTC 2017
   Mon Mar 13 17:44:40 UTC 2017
   Mon Mar 13 17:44:43 UTC 2017
   
I'm not sure exactly which kernel version breaks this.  It fails with
kernel 2.6.33, but is OK with 3.8.13.
Comment 1 Grant Edwards 2017-03-13 21:57:37 UTC 
I've confirmed 'cat' that it works on a 3.2.86 kernel.

At the moment, I don't have means to build/test kernels between 2.6.33 and 3.2.
Comment 2 Grant Edwards 2017-03-13 22:07:03 UTC 
My apologies -- 

I finally stumbled across CONFIG_FEATURE_USE_SENDFILE