ash: optimise running of scripts

The BusyBox shell detects certain cases where forking a command is
unnecessary (last command in a script or subshell, for example) and
calls execve(2) instead.  This doesn't help in the Windows port
because execve(2) is implemented by creating a process.

There is one case where it is possible to apply this optimisation:
if the command is a script and the script interpreter is an applet.

- Have evalcommand() pass a flag to indicate this situation to
  shellexec().  Also, allocate two spare elements before the start
  of the argv array.

- If the flag is TRUE shellexec() passes the shell's PATH variable
  down to tryexec() so it can perform a test for applet override.

- If tryexec() finds that all the necessary conditions apply it
  can run a script by directly invoking the interpreter's main().

Adds 192-224 bytes.

1 files changed, 59 insertions, 7 deletions

diff --git a/shell/ash.c b/shell/ash.c
index aa291b99d..2e4181b3f 100644
--- a/shell/ash.c
+++ b/shell/ash.c
@@ -9091,8 +9091,16 @@ static int builtinloc = -1;     /* index in path of %builtin, or -1 */
 
 
 static void
+#if ENABLE_PLATFORM_MINGW32
+tryexec(IF_FEATURE_SH_STANDALONE(int applet_no, const char *nfpath,)
+                        const char *cmd, char **argv, char **envp)
+#else
 tryexec(IF_FEATURE_SH_STANDALONE(int applet_no,) const char *cmd, char **argv, char **envp)
+#endif
 {
+#if ENABLE_PLATFORM_MINGW32 && ENABLE_FEATURE_SH_STANDALONE
+        interp_t interp;
+#endif
 #if ENABLE_FEATURE_SH_STANDALONE
         if (applet_no >= 0) {
 # if ENABLE_PLATFORM_MINGW32
@@ -9101,6 +9109,7 @@ tryexec(IF_FEATURE_SH_STANDALONE(int applet_no,) const char *cmd, char **argv, c
                 struct forkshell *fs = (struct forkshell *)sticky_mem_start;
                 if (applet_main[applet_no] != ash_main ||
                                 (fs && fs->fpid == FS_SHELLEXEC)) {
+ run_noexec:
                         /* mingw-w64's getopt() uses __argv[0] as the program name */
                         __argv[0] = (char *)cmd;
                         /* 'which' wants to know if it was invoked from a standalone
@@ -9141,6 +9150,28 @@ tryexec(IF_FEATURE_SH_STANDALONE(int applet_no,) const char *cmd, char **argv, c
 
         /* cmd was allocated on the stack with room for an extension */
         add_win32_extension((char *)cmd);
+
+# if ENABLE_FEATURE_SH_STANDALONE
+        /* If nfpath is non-NULL, evalcommand() has determined this
+         * command doesn't need a fork().  If the command is a script
+         * with an interpreter which is an applet we can run it as if
+         * it were a noexec applet. */
+        if (nfpath && parse_interpreter(cmd, &interp)) {
+                applet_no = find_applet_by_name_for_sh(interp.name, nfpath);
+                if (applet_no >= 0) {
+                        argv[0] = (char *)cmd;
+                        /* evalcommand() has added two elements before argv */
+                        if (interp.opts) {
+                                argv--;
+                                argv[0] = (char *)interp.opts;
+                        }
+                        argv--;
+                        cmd = argv[0] = (char *)interp.name;
+                        goto run_noexec;
+                }
+        }
+# endif
+
         execve(cmd, argv, envp);
         /* skip POSIX-mandated retry on ENOEXEC */
 #else /* !ENABLE_PLATFORM_MINGW32 */
@@ -9182,20 +9213,29 @@ tryexec(IF_FEATURE_SH_STANDALONE(int applet_no,) const char *cmd, char **argv, c
 #endif /* !ENABLE_PLATFORM_MINGW32 */
 }
 
+#if !ENABLE_PLATFORM_MINGW32 || !ENABLE_FEATURE_SH_STANDALONE
+# define shellexec(prg, a, pth, i, n) shellexec(prg, a, pth, i)
+#endif
+
 /*
  * Exec a program.  Never returns.  If you change this routine, you may
  * have to change the find_command routine as well.
  * argv[-1] must exist and be writable! See tryexec() for why.
  */
 static struct builtincmd *find_builtin(const char *name);
-static void shellexec(char *prog, char **argv, const char *path, int idx) NORETURN;
-static void shellexec(char *prog, char **argv, const char *path, int idx)
+static void shellexec(char *prog, char **argv, const char *path, int idx,
+                                                int nofork) NORETURN;
+static void shellexec(char *prog, char **argv, const char *path, int idx,
+                                                int nofork)
 {
         char *cmdname;
         int e;
         char **envp;
         int exerrno;
         int applet_no = -1; /* used only by FEATURE_SH_STANDALONE */
+#if ENABLE_PLATFORM_MINGW32 && ENABLE_FEATURE_SH_STANDALONE
+        const char *nfpath = nofork ? path : NULL;
+#endif
 
         envp = listvars(VEXPORT, VUNSET, /*strlist:*/ NULL, /*end:*/ NULL);
 #if ENABLE_FEATURE_SH_STANDALONE && ENABLE_PLATFORM_MINGW32 && defined(_UCRT)
@@ -9217,7 +9257,8 @@ static void shellexec(char *prog, char **argv, const char *path, int idx)
         ) {
 #if ENABLE_PLATFORM_MINGW32
                 char *progext = stack_add_ext_space(prog);
-                tryexec(IF_FEATURE_SH_STANDALONE(applet_no,) progext, argv, envp);
+                tryexec(IF_FEATURE_SH_STANDALONE(applet_no, nfpath,)
+                                progext, argv, envp);
 #else
                 tryexec(IF_FEATURE_SH_STANDALONE(applet_no,) prog, argv, envp);
 #endif
@@ -9234,7 +9275,7 @@ static void shellexec(char *prog, char **argv, const char *path, int idx)
                         const char *name = bb_basename(prog);
 # if ENABLE_FEATURE_SH_STANDALONE
                         if ((applet_no = find_applet_by_name_for_sh(name, path)) >= 0) {
-                                tryexec(applet_no, name, argv, envp);
+                                tryexec(applet_no, nfpath, name, argv, envp);
                                 e = errno;
                         }
 # endif
@@ -9250,7 +9291,12 @@ static void shellexec(char *prog, char **argv, const char *path, int idx)
                 while (padvance(&path, argv[0]) >= 0) {
                         cmdname = stackblock();
                         if (--idx < 0 && pathopt == NULL) {
+#if ENABLE_PLATFORM_MINGW32
+                                tryexec(IF_FEATURE_SH_STANDALONE(-1, nfpath,)
+                                                cmdname, argv, envp);
+#else
                                 tryexec(IF_FEATURE_SH_STANDALONE(-1,) cmdname, argv, envp);
+#endif
                                 if (errno != ENOENT && errno != ENOTDIR)
                                         e = errno;
                         }
@@ -11223,7 +11269,7 @@ execcmd(int argc UNUSED_PARAM, char **argv)
                 prog = argv[0];
                 if (optionarg)
                         argv[0] = optionarg;
-                shellexec(prog, argv, pathval(), 0);
+                shellexec(prog, argv, pathval(), 0, FALSE);
                 /* NOTREACHED */
         }
         return 0;
@@ -11581,7 +11627,13 @@ evalcommand(union node *cmd, int flags)
         localvar_stop = pushlocalvars(vlocal);
 
 #if ENABLE_PLATFORM_MINGW32
+# if ENABLE_FEATURE_SH_STANDALONE
+        /* Reserve two extra spots at the front for shellexec. */
+        nargv = stalloc(sizeof(char *) * (argc + 3));
+        argv = nargv = nargv + 2;
+# else
         argv = nargv = stalloc(sizeof(char *) * (argc + 1));
+# endif
 #else
         /* Reserve one extra spot at the front for shellexec. */
         nargv = stalloc(sizeof(char *) * (argc + 2));
@@ -11773,7 +11825,7 @@ evalcommand(union node *cmd, int flags)
                         /* fall through to exec'ing external program */
                 }
 #endif
-                shellexec(argv[0], argv, path, cmdentry.u.index);
+                shellexec(argv[0], argv, path, cmdentry.u.index, TRUE);
                 /* NOTREACHED */
         } /* default */
         case CMDBUILTIN:
@@ -16492,7 +16544,7 @@ forkshell_shellexec(struct forkshell *fs)
         char *path = fs->path;
 
         FORCE_INT_ON;
-        shellexec(argv[0], argv, path, idx);
+        shellexec(argv[0], argv, path, idx, FALSE);
 }
 
 static void