Temporary Files, Shared Libraries
Petr Ročkai
Temporary Files, Shared Libraries 1/9 December 7, 2017
Temporary Files
• useful for communication
• and/or co-opting external tools
• to a lesser degree to store large data
• should be removed upon exit
• usually created in a special, system-wide directory
− like /tmp and /var/tmp on most UNIX systems
Temporary Files, Shared Libraries 2/9 December 7, 2017
POSIX APIs
• mkstemp and mkdtemp
• temporary iles and directories respectively
• details in man pages: man mkdtemp
How to use:
char fn[] = "/tmp/mytmpdir.XXXXXX";
if ( !mkdtemp(fn) )
return ERROR;
/* ... */
rmdir( fn );
Temporary Files, Shared Libraries 3/9 December 7, 2017
Exercise 1
• create a temporary directory in /tmp
− create a named ifo in the directory (man 3 mkfifo)
− print the path of the ifo on stderr
• open the pipe for reading
− anything you read, print on stdout
− use the POSIX API for reading: man read
• in another terminal, run echo hello > fifo
− substitue the path the program printed for fifo
− run another copy of your program
Temporary Files, Shared Libraries 4/9 December 7, 2017
Exercise 1 Bonus
• the program is stopped when the user hits ^C
• ensure that the ifo and the directory are removed
− you can use the signal function for this
− see man signal
− the ifo must be unlink-ed
− the remaining empty directory can be rmdir’d
Temporary Files, Shared Libraries 5/9 December 7, 2017
Dynamic Linking Redux
• an application consists from multiple modules
• upon load, the modules are linked by the runtime linker
• on UNIX, the linker is known as ld.so
− see the manual page: man ld.so
− look for the description of LD_PRELOAD
Temporary Files, Shared Libraries 6/9 December 7, 2017
Hooking Calls
• call targets are resolved by ld.so at load/run time
• procedures are referenced as symbols (ASCII names)
• multiple libraries can provide the same symbol
• ld.so has rules to decide which function to use
• preloaded libraries come before all the others
Temporary Files, Shared Libraries 7/9 December 7, 2017
Exercise 2
• get ld_preload.tar.gz from study materials
• preload.c is a small C library that implements open
• the script preload.sh runs a command given as argument
− it irst builds preload.so from preload.c
− then preloads it into the application
• try for instance ./preload.sh cat /dev/null
− compare to just cat /dev/null
Temporary Files, Shared Libraries 8/9 December 7, 2017
Exercise 3
• modify preload.c to hijack read calls
• create a ile like /tmp/.exfiltrate.XXXXXX
− you should use mkstemp for this
− do not unlink the ile when the program exits
• print all the captured read data into the ile
• run your program from Ex. 1 using preload.sh
− verify that this behaves as expected
Temporary Files, Shared Libraries 9/9 December 7, 2017
Homework
• there is no homework
Next Week
• no lecture on Monday
• project defences on Thursday