144 lines
5.7 KiB
Plaintext
144 lines
5.7 KiB
Plaintext
NOEXEC and NOFORK applets.
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Unix shells traditionally execute some commands internally in the attempt
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to dramatically speed up execution. It will be slow as hell if for every
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"echo blah" shell will fork and exec /bin/echo. To this end, shells
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have to _reimplement_ these commands internally.
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Busybox is unique in this regard because it already is a collection
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of reimplemented Unix commands, and we can do the same trick
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for speeding up busybox shells, and more. NOEXEC and NOFORK applets
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are exactly those applets which are eligible for these tricks.
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Applet will be subject to NOFORK/NOEXEC tricks only if it is marked
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as such in applets.src.h or in their inline "//applet:" directives.
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In C, if you want to call a program and wait for it, use
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spawn_and_wait(argv), BB_EXECVP(prog,argv) or BB_EXECLP(prog,argv0,...).
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They check whether program name is an applet name and optionally
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do NOFORK/NOEXEC thing depending on configuration.
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Relevant CONFIG options
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FEATURE_PREFER_APPLETS
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Globally enables NOFORK/NOEXEC tricks for such programs as xargs
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and find:
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BB_EXECVP(cmd, argv) will try to exec /proc/self/exe
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if command's name matches some applet name;
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spawn_and_wait(argv) will do NOFORK/NOEXEC tricks
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//TODO: the above two things probably should have separate options?
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FEATURE_SH_STANDALONE
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shells will try to exec /proc/self/exe if command's name matches
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some applet name; shells will do NOEXEC trick on NOEXEC applets
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//TODO: split (same as for PREFER_APPLETS)
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FEATURE_SH_NOFORK
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shells will do NOFORK trick on NOFORK applets
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NB: shell builtins use these tricks regardless of FEATURE_SH_STANDALONE,
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FEATURE_PREFER_APPLETS or FEATURE_SH_NOFORK. In effect, builtins
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are "always NOFORK".
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NOEXEC
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NOEXEC applet should work correctly if another applet forks and then
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executes exit(<applet>_main(argc,argv)) in the child. The rules
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roughly are:
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* do not expect shared global variables/buffers to be in their
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"initialized" state. Examples: xfunc_error_retval can be != 1,
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bb_common_bufsiz1 can be scribbled over, ...
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(although usually xfunc_error_retval's state is not a problem).
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* do not expect that stdio wasn't used before. Calling set[v]buf()
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can be disastrous.
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* ...
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NOEXEC applets save only one half of fork+exec overhead.
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NOEXEC trick is disabled for NOMMU build.
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NOFORK
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NOFORK applet should work correctly if another applet simply runs
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<applet>_main(argc,argv) and then continues with its business.
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xargs, find, shells do it (grep for "spawn_and_wait" and
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"run_nofork_applet" to find more users).
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This poses much more serious limitations on what applet can do:
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* all NOEXEC limitations apply.
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* do not run for a long time or wait for user input:
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hush shell only handles signals (like ^C) after you return
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from APPLET_main().
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* do not ever exit() or exec().
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- xfuncs are okay. They are using special trick to return
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to the caller applet instead of dying when they detect "x" condition.
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- you may "exit" to caller applet by calling xfunc_die(). Return value
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is taken from xfunc_error_retval.
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- fflush_stdout_and_exit(n) is ok to use.
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* do not use shared global data, or save/restore shared global data
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(e.g. bb_common_bufsiz1) prior to returning.
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- getopt32() is ok to use. You do not need to save/restore option_mask32,
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xfunc_error_retval, and logmode - it is already done by core code.
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* if you allocate memory, you can use xmalloc() only on the very first
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allocation. All other allocations should use malloc[_or_warn]().
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After first allocation, you cannot use any xfuncs.
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Otherwise, failing xfunc will return to caller applet
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without freeing malloced data!
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* the same applies to other resources, such as open fds: no xfuncs after
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acquiring them!
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* All allocated data, opened files, signal handlers, termios settings
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etc should be freed/closed/restored prior to return.
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Currently, ash shell signal handling is implemented in a way that signals
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have non-SA_RESTARTed handlers. This means that system calls can
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return EINTR. An example of such problem is "yes" applet:
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it is implemented so that it has a writing loop, this loop is exited on
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any write error, and in the case of user pressing ^C the error was EINTR.
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The problem is, the error causes stdout FILE* object to get into error
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state, needing clearerr() - or else subsequent shell output will also
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not work. ("yes" has been downgraded to NOEXEC, since hush signal handling
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does not have this problem - which makes "yes" to not exit on ^C (bug).
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But stray EINTRs can be seen in any NOFORK under ash, until ash is fixed).
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NOFORK applets give the most of speed advantage, but are trickiest
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to implement. In order to minimize amount of bugs and maintenance,
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prime candidates for NOFORK-ification are those applets which
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are small and easy to audit, and those which are more likely to be
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frequently executed from shell/find/xargs, particularly in shell
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script loops. Applets which mess with signal handlers, termios etc
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are probably not worth the effort.
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Applets which must be interruptible by ^C in shells can not be NOFORKs.
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Any NOFORK applet is also a NOEXEC applet.
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Calling NOFORK applets
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API to call NOFORK applets is two functions:
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run_nofork_applet(appno, argv)
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spawn_and_wait(argv) // only if FEATURE_PREFER_APPLETS=y
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First one is directly used by shells if FEATURE_SH_NOFORK=y.
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Second one is used by many applets, but main users are xargs and find.
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It itself calls run_nofork_applet(), if argv[0] is a name
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of a NOFORK applet.
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run_nofork_applet() saves/inits/restores option parsing, xfunc_error_retval,
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logmode, applet_name. Thus, for example, caller does not need to worry about
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option_mask32 getting trashed.
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Calling NOEXEC applets
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It's the same trusty spawn_and_wait(argv). If FEATURE_PREFER_APPLETS=y,
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it does NOEXEC trick. It resets xfunc_error_retval = 1 and
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logmode = LOGMODE_STDIO in the child.
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