| NOEXEC and NOFORK applets. |
| |
| Unix shells traditionally execute some commands internally in the attempt |
| to dramatically speed up execution. It will be slow as hell if for every |
| "echo blah" shell will fork and exec /bin/echo. To this end, shells |
| have to _reimplement_ these commands internally. |
| |
| Busybox is unique in this regard because it already is a collection |
| of reimplemented Unix commands, and we can do the same trick |
| for speeding up busybox shells, and more. NOEXEC and NOFORK applets |
| are exactly those applets which are eligible for these tricks. |
| |
| Applet will be subject to NOFORK/NOEXEC tricks if it is marked as such |
| in applets.h. FEATURE_PREFER_APPLETS is a config option which |
| globally enables usage of NOFORK/NOEXEC tricks. |
| If it is enabled, FEATURE_SH_STANDALONE can be enabled too, |
| and then shells will use NOFORK/NOEXEC tricks for ordinary commands. |
| NB: shell builtins use these tricks regardless of FEATURE_SH_STANDALONE |
| or FEATURE_PREFER_APPLETS. |
| |
| In C, if you want to call a program and wait for it, use |
| spawn_and_wait(argv), BB_EXECVP(prog,argv) or BB_EXECLP(prog,argv0,...). |
| They check whether program name is an applet name and optionally |
| do NOFORK/NOEXEC thing depending on configuration. |
| |
| |
| NOEXEC |
| |
| NOEXEC applet should work correctly if another applet forks and then |
| executes exit(<applet>_main(argc,argv)) in the child. The rules |
| roughly are: |
| |
| * do not expect shared global variables/buffers to be in their |
| "initialized" state. Examples: xfunc_error_retval can be != 1, |
| bb_common_bufsiz1 can be scribbled over, ... |
| (although usually xfunc_error_retval's state is not a problem). |
| * do not expect that stdio wasn't used before. Calling set[v]buf() |
| can be disastrous. |
| * ... |
| |
| NOEXEC applets save only one half of fork+exec overhead. |
| NOEXEC trick is disabled for NOMMU build. |
| |
| |
| NOFORK |
| |
| NOFORK applet should work correctly if another applet simply runs |
| <applet>_main(argc,argv) and then continues with its business. |
| xargs, find, shells do it (grep for "spawn_and_wait" and |
| "run_nofork_applet" to find more users). |
| |
| This poses much more serious limitations on what applet can do: |
| |
| * all NOEXEC limitations apply. |
| * do not ever exit() or exec(). |
| - xfuncs are okay. They are using special trick to return |
| to the caller applet instead of dying when they detect "x" condition. |
| - you may "exit" to caller applet by calling xfunc_die(). Return value |
| is taken from xfunc_error_retval. |
| - fflush_stdout_and_exit(n) is ok to use. |
| * do not use shared global data, or save/restore shared global data |
| (e.g. bb_common_bufsiz1) prior to returning. |
| - getopt32() is ok to use. You do not need to save/restore option_mask32, |
| it is already done by core code. |
| * if you allocate memory, you can use xmalloc() only on the very first |
| allocation. All other allocations should use malloc[_or_warn](). |
| After first allocation, you cannot use any xfuncs. |
| Otherwise, failing xfunc will return to caller applet |
| without freeing malloced data! |
| * All allocated data, opened files, signal handlers, termios settings, |
| O_NONBLOCK flags etc should be freed/closed/restored prior to return. |
| * ... |
| |
| NOFORK applets give the most of speed advantage, but are trickiest |
| to implement. In order to minimize amount of bugs and maintenance, |
| prime candidates for NOFORK-ification are those applets which |
| are small and easy to audit, and those which are more likely to be |
| frequently executed from shell/find/xargs, particularly in shell |
| script loops. Applets which mess with signal handlers, termios etc |
| are probably not worth the effort. |
| |
| Any NOFORK applet is also a NOEXEC applet. |
| |
| |
| Calling NOFORK applets |
| |
| API to call NOFORK applets is two functions: |
| |
| run_nofork_applet(appno, argv) |
| spawn_and_wait(argv) // only if FEATURE_PREFER_APPLETS=y |
| |
| First one is directly used by shells if FEATURE_SH_NOFORK=y. |
| Second one is used by many applets, but main users are xargs and find. |
| It itself calls run_nofork_applet(), if argv[0] turned out to be a name |
| of a NOFORK applet. |
| |
| run_nofork_applet() saves/inits/restores option parsing, xfunc_error_retval, |
| applet_name. Thus, for example, caller does not need to worry about |
| option_mask32 getting trashed. |
| |
| |
| Calling NOEXEC applets |
| |
| It's the same trusty spawn_and_wait(argv). If FEATURE_PREFER_APPLETS=y, |
| it does NOEXEC trick. It resets xfunc_error_retval = 1 and |
| logmode = LOGMODE_STDIO in the child. |
| |
| |
| Relevant CONFIG options |
| |
| FEATURE_PREFER_APPLETS |
| BB_EXECVP(cmd, argv) will try to exec /proc/self/exe |
| if command's name matches some applet name; |
| spawn_and_wait(argv) will do NOFORK/NOEXEC tricks |
| |
| //TODO: the above two things probably should have separate options? |
| |
| FEATURE_SH_STANDALONE |
| shells will try to exec /proc/self/exe if command's name matches |
| some applet name; shells will do NOEXEC trick on NOEXEC applets |
| |
| //TODO: split (same as for PREFER_APPLETS) |
| |
| FEATURE_SH_NOFORK |
| shells will do NOFORK trick on NOFORK applets |