1 | NOEXEC and NOFORK applets.
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2 |
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3 | Unix shells traditionally execute some commands internally in the attempt
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4 | to dramatically speed up execution. It will be slow as hell if for every
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5 | "echo blah" shell will fork and exec /bin/echo. To this end, shells
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6 | have to _reimplement_ these commands internally.
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7 |
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8 | Busybox is unique in this regard because it already is a collection
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9 | of reimplemented Unix commands, and we can do the same trick
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10 | for speeding up busybox shells, and more. NOEXEC and NOFORK applets
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11 | are exactly those applets which are eligible for these tricks.
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12 |
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13 | Applet will be subject to NOFORK/NOEXEC tricks if it is marked as such
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14 | in applets.h. FEATURE_PREFER_APPLETS is a config option which
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15 | globally enables usage of NOFORK/NOEXEC tricks.
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16 | If it is enabled, FEATURE_SH_STANDALONE can be enabled too,
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17 | and then shells will use NOFORK/NOEXEC tricks for ordinary commands.
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18 | NB: shell builtins use these tricks regardless of FEATURE_SH_STANDALONE
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19 | or FEATURE_PREFER_APPLETS.
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20 |
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21 | In C, if you want to call a program and wait for it, use
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22 | spawn_and_wait(argv), BB_EXECVP(prog,argv) or BB_EXECLP(prog,argv0,...).
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23 | They check whether program name is an applet name and optionally
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24 | do NOFORK/NOEXEC thing depending on configuration.
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25 |
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26 |
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27 | NOEXEC
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28 |
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29 | NOEXEC applet should work correctly if another applet forks and then
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30 | executes exit(<applet>_main(argc,argv)) in the child. The rules
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31 | roughly are:
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32 |
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33 | * do not expect shared global variables/buffers to be in their
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34 | "initialized" state. Examples: xfunc_error_retval can be != 1,
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35 | bb_common_bufsiz1 can be scribbled over, ...
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36 | * do not expect that stdio wasn't used before. Calling set[v]buf()
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37 | can be disastrous.
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38 | * ...
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39 |
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40 | NOEXEC applets save only one half of fork+exec overhead.
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41 | NOEXEC trick is disabled for NOMMU build.
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42 |
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43 |
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44 | NOFORK
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45 |
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46 | NOFORK applet should work correctly if another applet simply runs
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47 | <applet>_main(argc,argv) and then continues with its business (xargs,
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48 | find, shells can do it). This poses much more serious limitations
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49 | on what applet can/cannot do:
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50 |
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51 | * all NOEXEC limitations apply.
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52 | * do not ever exit() or exec().
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53 | - xfuncs are okay. They are using special trick to return
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54 | to the caller applet instead of dying when they detect "x" condition.
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55 | - you may "exit" to caller applet by calling xfunc_die(). Return value
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56 | is taken from xfunc_error_retval.
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57 | - fflush_stdout_and_exit(n) is ok to use.
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58 | * do not use shared global data, or save/restore shared global data
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59 | prior to returning. (e.g. bb_common_bufsiz1 is off-limits).
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60 | - getopt32() is ok to use. You do not need to save/restore option_mask32,
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61 | it is already done by core code.
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62 | * if you allocate memory, you can use xmalloc() only on the very first
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63 | allocation. All other allocations should use malloc[_or_warn]().
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64 | After first allocation, you cannot use any xfuncs.
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65 | Otherwise, failing xfunc will return to caller applet
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66 | without freeing malloced data!
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67 | * All allocated data, opened files, signal handlers, termios settings,
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68 | O_NONBLOCK flags etc should be freed/closed/restored prior to return.
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69 | * ...
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70 |
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71 | NOFORK applets give the most of speed advantage, but are trickiest
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72 | to implement. In order to minimize amount of bugs and maintenance,
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73 | prime candidates for NOFORK-ification are those applets which
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74 | are small and easy to audit, and those which are more likely to be
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75 | frequently executed from shell/find/xargs, particularly in shell
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76 | script loops. Applets which mess with signal handlers, termios etc
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77 | are probably not worth the effort.
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78 |
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79 | Any NOFORK applet is also a NOEXEC applet.
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