strncpy and snprintf bounds checks

[dkg]

over in https://bugs.debian.org/891551, James Cowgill writes:

  I recently performed an (unofficial) archive rebuild with GCC 8 on
  mips64el. The main purpose of the rebuild was to discover mips toolchain
  regressions, however I noticed this error in the logs which might be
  interesting to you:

  > faketime.c: In function 'main':
  > faketime.c:289:45: error: '%s' directive output may be truncated writing up to 4095 bytes into a region of size between 0 and 4095 [-Werror=format-truncation=]
  [ 47 more citation lines. Click/Enter to hide. ]
  >      snprintf(shared_objs, PATH_BUFSIZE, "%s %s", sem_name, shm_name);
  >                                              ^~             ~~~~~~~~
  > faketime.c:289:5: note: 'snprintf' output between 2 and 8192 bytes into a destination of size 4096
  >      snprintf(shared_objs, PATH_BUFSIZE, "%s %s", sem_name, shm_name);
  >      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  > In file included from libfaketime.c:49:
  > libfaketime.c: In function 'fake_clock_gettime':
  > libfaketime.c:1986:24: error: cast between incompatible function types from 'int (*)(pthread_mutex_t *)' {aka 'int (*)(union <anonymous> *)'} to 'void (*)(void *)'
  > [-Werror=cast-function-type]
  >    pthread_cleanup_push((void (*)(void *))pthread_mutex_unlock, (void *)&time_mutex);
  >                         ^
  > cc1: all warnings being treated as errors
  > Makefile:98: recipe for target 'faketime' failed
  > make[2]: *** [faketime] Error 1
  > make[2]: *** Waiting for unfinished jobs....
  > libfaketime.c: In function 'fake_clock_gettime.part.4':
  > libfaketime.c:2081:7: error: 'strncpy' specified bound 256 equals destination size [-Werror=stringop-truncation]
  >        strncpy(user_faked_time, tmp_env, BUFFERLEN);
  >        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  > libfaketime.c:2081:7: error: 'strncpy' specified bound 256 equals destination size [-Werror=stringop-truncation]
  >        strncpy(user_faked_time, tmp_env, BUFFERLEN);
  >        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  > libfaketime.c: In function 'ftpl_init':
  > libfaketime.c:1831:12: error: 'strncpy' specified bound 1024 equals destination size [-Werror=stringop-truncation]
  >      (void) strncpy(ft_spawn_target, getenv("FAKETIME_SPAWN_TARGET"), 1024);
  >             ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  > libfaketime.c:1892:5: error: 'strncpy' specified bound 8192 equals destination size [-Werror=stringop-truncation]
  >      strncpy(user_faked_time_fmt, tmp_env, BUFSIZ);
  >      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  > libfaketime.c: In function 'ftpl_init':
  > libfaketime.c:1831:12: error: 'strncpy' specified bound 1024 equals destination size [-Werror=stringop-truncation]
  >      (void) strncpy(ft_spawn_target, getenv("FAKETIME_SPAWN_TARGET"), 1024);
  >             ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  > libfaketime.c:1892:5: error: 'strncpy' specified bound 8192 equals destination size [-Werror=stringop-truncation]
  >      strncpy(user_faked_time_fmt, tmp_env, BUFSIZ);
  >      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  > cc1: all warnings being treated as errors
  > Makefile:92: recipe for target 'libfaketime.o' failed
  > make[2]: *** [libfaketime.o] Error 1
  > cc1: all warnings being treated as errors
  > Makefile:92: recipe for target 'libfaketimeMT.o' failed
  > make[2]: *** [libfaketimeMT.o] Error 1
  > make[2]: Leaving directory '/<<PKGBUILDDIR>>/src'
  > Makefile:7: recipe for target 'all' failed
  > make[1]: *** [all] Error 2
  > make[1]: Leaving directory '/<<PKGBUILDDIR>>'
  > dh_auto_build: make -j3 "INSTALL=install --strip-program=true" returned exit code 2
  > debian/rules:22: recipe for target 'build-arch' failed
  > make: *** [build-arch] Error 2
  > dpkg-buildpackage: error: debian/rules build-arch subprocess returned exit status 2

  As you probably know, you need to be careful using strncpy.

  Instead of:
   char out[SIZE];
   strncpy(out, in, SIZE);

  You need to do:
   char out[SIZE]
   strncpy(out, in, SIZE - 1);
   out[SIZE - 1] = '\0';

  See strcpy(3)

[wolfcw]

fixed by 0d96436 , thanks