applet_tables: save space by removing applet name offsets

The array applet_nameofs consumes two bytes per applet.  It encodes

   nofork/noexec flags
   suid flags
   the offset of the applet name in the applet_name string

Change the applet_table build tool to store the flags in two separate
arrays (applet_flags and applet_suid).  Replace applet_nameofs[] with a
smaller version that only stores a limited number of offsets.

This requires changes to the macros APPLET_IS_NOFORK, APPLET_IS_NOEXEC
and APPLET_SUID.

   According to Valgrind the original find_applet_by_name required
   353 cycles per call, averaged over all names.  Adjusting the number
   of known offsets allows space to be traded off against execution time:

      KNOWN_OFFSETS     cycles     bytes (wrt KNOWN_OFFSETS = 0)

           0             9057        -
           2             4604       32
           4             2407       75
           8             1342       98
          16              908      130
          32              884      194

   This patch uses KNOWN_OFFSETS = 8.

v2:
   Remove some dead code from the applet_table tool;
   Treat the applet in the middle of the table as a special case.

v3:
   Use the middle applet to adjust the start of the linear search as
   well as the last applet found.

v4:
   Use an augmented linear search in find_applet_by_name.
   Drop the special treatment of the middle name from get_applet_name:
   most of the advantage now derives from the last stored value.

v5:
   Don't store index in applet_nameofs, it can be calculated.

v6:
   Tweaks by Denys

function                                             old     new   delta
find_applet_by_name                                   25     125    +100
applet_suid                                            -      92     +92
run_applet_no_and_exit                               452     460      +8
run_applet_and_exit                                  695     697      +2
applet_name_compare                                   31       -     -31
applet_nameofs                                       734      14    -720
------------------------------------------------------------------------------
(add/remove: 1/1 grow/shrink: 3/1 up/down: 202/-751)         Total: -549 bytes
   text       data        bss        dec        hex    filename
 925464        906      17160     943530      e65aa    busybox_old
 924915        906      17160     942981      e6385    busybox_unstripped

Signed-off-by: Ron Yorston <rmy@pobox.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

applets/applet_tables.c

@@ -41,8 +41,6 @@ struct bb_applet {
 
 enum { NUM_APPLETS = ARRAY_SIZE(applets) };
 
-static int offset[NUM_APPLETS];
-
 static int cmp_name(const void *a, const void *b)
 {
 	const struct bb_applet *aa = a;
@@ -60,22 +58,37 @@ static int str_isalnum_(const char *s)
 	return 1;
 }
 
+// Before linear search, narrow it down by looking at N "equidistant" names:
+// KNOWN_APPNAME_OFFSETS  cycles  code_size
+//                     0    9057
+//                     2    4604        +32
+//                     4    2407        +75
+//                     8    1342        +98
+//                    16     908       +130
+//                    32     884       +194
+// With 8, applet_nameofs[] table has 7 elements.
+#define KNOWN_APPNAME_OFFSETS 8
+
 int main(int argc, char **argv)
 {
-	int i;
+	int i, j;
-	int ofs;
+	int ofs, offset[KNOWN_APPNAME_OFFSETS], index[KNOWN_APPNAME_OFFSETS];
 //	unsigned MAX_APPLET_NAME_LEN = 1;
 
 	qsort(applets, NUM_APPLETS, sizeof(applets[0]), cmp_name);
 
+	for (i = 0; i < KNOWN_APPNAME_OFFSETS; i++)
+		index[i] = i * NUM_APPLETS / KNOWN_APPNAME_OFFSETS;
+
 	ofs = 0;
 	for (i = 0; i < NUM_APPLETS; i++) {
-		offset[i] = ofs;
+		for (j = 0; j < KNOWN_APPNAME_OFFSETS; j++)
+			if (i == index[j])
+				offset[j] = ofs;
 		ofs += strlen(applets[i].name) + 1;
 	}
-	/* We reuse 4 high-order bits of offset array for other purposes,
+	/* If the list of names is too long refuse to proceed */
-	 * so if they are indeed needed, refuse to proceed */
+	if (ofs > 0xffff)
-	if (ofs > 0xfff)
 		return 1;
 	if (!argv[1])
 		return 1;
@@ -94,7 +107,17 @@ int main(int argc, char **argv)
 		printf("#define SINGLE_APPLET_STR \"%s\"\n", applets[0].name);
 		printf("#define SINGLE_APPLET_MAIN %s_main\n", applets[0].main);
 	}
-	printf("\n");
+
+	if (KNOWN_APPNAME_OFFSETS > 0 && NUM_APPLETS > 2*KNOWN_APPNAME_OFFSETS) {
+		printf("#define KNOWN_APPNAME_OFFSETS %u\n\n", KNOWN_APPNAME_OFFSETS);
+		printf("const uint16_t applet_nameofs[] ALIGN2 = {\n");
+		for (i = 1; i < KNOWN_APPNAME_OFFSETS; i++)
+			printf("%d,\n", offset[i]);
+		printf("};\n\n");
+	}
+	else {
+		printf("#define KNOWN_APPNAME_OFFSETS 0\n\n");
+	}
 
 	//printf("#ifndef SKIP_definitions\n");
 	printf("const char applet_names[] ALIGN1 = \"\"\n");
@@ -119,20 +142,39 @@ int main(int argc, char **argv)
 	printf("};\n");
 	printf("#endif\n\n");
 
-	printf("const uint16_t applet_nameofs[] ALIGN2 = {\n");
-	for (i = 0; i < NUM_APPLETS; i++) {
-		printf("0x%04x,\n",
-			offset[i]
 #if ENABLE_FEATURE_PREFER_APPLETS
-			+ (applets[i].nofork << 12)
+	printf("const uint8_t applet_flags[] ALIGN1 = {\n");
-			+ (applets[i].noexec << 13)
+	i = 0;
-#endif
+	while (i < NUM_APPLETS) {
-#if ENABLE_FEATURE_SUID
+		int v = applets[i].nofork + (applets[i].noexec << 1);
-			+ (applets[i].need_suid << 14) /* 2 bits */
+		if (++i < NUM_APPLETS)
-#endif
+			v |= (applets[i].nofork + (applets[i].noexec << 1)) << 2;
-		);
+		if (++i < NUM_APPLETS)
+			v |= (applets[i].nofork + (applets[i].noexec << 1)) << 4;
+		if (++i < NUM_APPLETS)
+			v |= (applets[i].nofork + (applets[i].noexec << 1)) << 6;
+		printf("0x%02x,\n", v);
+		i++;
 	}
 	printf("};\n\n");
+#endif
+
+#if ENABLE_FEATURE_SUID
+	printf("const uint8_t applet_suid[] ALIGN1 = {\n");
+	i = 0;
+	while (i < NUM_APPLETS) {
+		int v = applets[i].need_suid; /* 2 bits */
+		if (++i < NUM_APPLETS)
+			v |= applets[i].need_suid << 2;
+		if (++i < NUM_APPLETS)
+			v |= applets[i].need_suid << 4;
+		if (++i < NUM_APPLETS)
+			v |= applets[i].need_suid << 6;
+		printf("0x%02x,\n", v);
+		i++;
+	}
+	printf("};\n\n");
+#endif
 
 #if ENABLE_FEATURE_INSTALLER
 	printf("const uint8_t applet_install_loc[] ALIGN1 = {\n");

include/busybox.h

@@ -15,25 +15,20 @@ PUSH_AND_SET_FUNCTION_VISIBILITY_TO_HIDDEN
 /* Keep in sync with applets/applet_tables.c! */
 extern const char applet_names[] ALIGN1;
 extern int (*const applet_main[])(int argc, char **argv);
-extern const uint16_t applet_nameofs[];
+extern const uint8_t applet_flags[] ALIGN1;
+extern const uint8_t applet_suid[] ALIGN1;
 extern const uint8_t applet_install_loc[] ALIGN1;
 
-#if ENABLE_FEATURE_SUID || ENABLE_FEATURE_PREFER_APPLETS
-# define APPLET_NAME(i) (applet_names + (applet_nameofs[i] & 0x0fff))
-#else
-# define APPLET_NAME(i) (applet_names + applet_nameofs[i])
-#endif
-
 #if ENABLE_FEATURE_PREFER_APPLETS
-# define APPLET_IS_NOFORK(i) (applet_nameofs[i] & (1 << 12))
+# define APPLET_IS_NOFORK(i) (applet_flags[(i)/4] & (1 << (2 * ((i)%4))))
-# define APPLET_IS_NOEXEC(i) (applet_nameofs[i] & (1 << 13))
+# define APPLET_IS_NOEXEC(i) (applet_flags[(i)/4] & (1 << ((2 * ((i)%4))+1)))
 #else
 # define APPLET_IS_NOFORK(i) 0
 # define APPLET_IS_NOEXEC(i) 0
 #endif
 
 #if ENABLE_FEATURE_SUID
-# define APPLET_SUID(i) ((applet_nameofs[i] >> 14) & 0x3)
+# define APPLET_SUID(i) ((applet_suid[(i)/4] >> (2 * ((i)%4)) & 3))
 #endif
 
 #if ENABLE_FEATURE_INSTALLER

libbb/appletlib.c

@@ -139,36 +139,56 @@ void FAST_FUNC bb_show_usage(void)
 	xfunc_die();
 }
 
-#if NUM_APPLETS > 8
-static int applet_name_compare(const void *name, const void *idx)
-{
-	int i = (int)(ptrdiff_t)idx - 1;
-	return strcmp(name, APPLET_NAME(i));
-}
-#endif
 int FAST_FUNC find_applet_by_name(const char *name)
 {
-#if NUM_APPLETS > 8
+	unsigned i, max;
-	/* Do a binary search to find the applet entry given the name. */
+	int j;
 	const char *p;
-	p = bsearch(name, (void*)(ptrdiff_t)1, ARRAY_SIZE(applet_main), 1, applet_name_compare);
+
-	/*
+	p = applet_names;
-	 * if (!p) return -1;
+	i = 0;
-	 * ^^^^^^^^^^^^^^^^^^ the code below will do this if p == NULL :)
+#if KNOWN_APPNAME_OFFSETS <= 0
-	 */
+	max = NUM_APPLETS;
-	return (int)(ptrdiff_t)p - 1;
 #else
-	/* A version which does not pull in bsearch */
+	max = NUM_APPLETS * KNOWN_APPNAME_OFFSETS;
-	int i = 0;
+	for (j = ARRAY_SIZE(applet_nameofs)-1; j >= 0; j--) {
-	const char *p = applet_names;
+		const char *pp = applet_names + applet_nameofs[j];
-	while (i < NUM_APPLETS) {
+		if (strcmp(name, pp) >= 0) {
-		if (strcmp(name, p) == 0)
+			//bb_error_msg("name:'%s' >= pp:'%s'", name, pp);
-			return i;
+			p = pp;
-		p += strlen(p) + 1;
+			i = max - NUM_APPLETS;
+			break;
+		}
+		max -= NUM_APPLETS;
+	}
+	max /= (unsigned)KNOWN_APPNAME_OFFSETS;
+	i /= (unsigned)KNOWN_APPNAME_OFFSETS;
+	//bb_error_msg("name:'%s' starting from:'%s' i:%u max:%u", name, p, i, max);
+#endif
+
+	/* Open-coding without strcmp/strlen calls for speed */
+	while (i < max) {
+		char ch;
+		j = 0;
+		/* Do we see "name\0" in applet_names[p] position? */
+		while ((ch = *p) == name[j]) {
+			if (ch == '\0') {
+				//bb_error_msg("found:'%s' i:%u", name, i);
+				return i; /* yes */
+			}
+			p++;
+			j++;
+		}
+		/* No.
+		 * p => 1st non-matching char in applet_names[],
+		 * skip to and including NUL.
+		 */
+		while (ch != '\0')
+			ch = *++p;
+		p++;
 		i++;
 	}
 	return -1;
-#endif
 }
 
 
@@ -583,6 +603,7 @@ static void install_links(const char *busybox, int use_symbolic_links,
 	 * busybox.h::bb_install_loc_t, or else... */
 	int (*lf)(const char *, const char *);
 	char *fpc;
+        const char *appname = applet_names;
 	unsigned i;
 	int rc;
 
@@ -593,7 +614,7 @@ static void install_links(const char *busybox, int use_symbolic_links,
 	for (i = 0; i < ARRAY_SIZE(applet_main); i++) {
 		fpc = concat_path_file(
 				custom_install_dir ? custom_install_dir : install_dir[APPLET_INSTALL_LOC(i)],
-				APPLET_NAME(i));
+				appname);
 		// debug: bb_error_msg("%slinking %s to busybox",
 		//		use_symbolic_links ? "sym" : "", fpc);
 		rc = lf(busybox, fpc);
@@ -601,6 +622,8 @@ static void install_links(const char *busybox, int use_symbolic_links,
 			bb_simple_perror_msg(fpc);
 		}
 		free(fpc);
+		while (*appname++ != '\0')
+			continue;
 	}
 }
 # else
@@ -754,7 +777,7 @@ void FAST_FUNC run_applet_no_and_exit(int applet_no, char **argv)
 
 	/* Reinit some shared global data */
 	xfunc_error_retval = EXIT_FAILURE;
-	applet_name = APPLET_NAME(applet_no);
+	applet_name = bb_get_last_path_component_nostrip(argv[0]);
 
 	/* Special case. POSIX says "test --help"
 	 * should be no different from e.g. "test --foo".
@@ -785,11 +808,14 @@ void FAST_FUNC run_applet_no_and_exit(int applet_no, char **argv)
 
 void FAST_FUNC run_applet_and_exit(const char *name, char **argv)
 {
-	int applet = find_applet_by_name(name);
+	int applet;
-	if (applet >= 0)
+
-		run_applet_no_and_exit(applet, argv);
 	if (is_prefixed_with(name, "busybox"))
 		exit(busybox_main(argv));
+	/* find_applet_by_name() search is more expensive, so goes second */
+	applet = find_applet_by_name(name);
+	if (applet >= 0)
+		run_applet_no_and_exit(applet, argv);
 }
 
 #endif /* !defined(SINGLE_APPLET_MAIN) */

libbb/vfork_daemon_rexec.c

@@ -116,8 +116,6 @@ int FAST_FUNC run_nofork_applet(int applet_no, char **argv)
 
 	save_nofork_data(&old);
 
-	applet_name = APPLET_NAME(applet_no);
-
 	xfunc_error_retval = EXIT_FAILURE;
 
 	/* In case getopt() or getopt32() was already called:
@@ -157,6 +155,7 @@ int FAST_FUNC run_nofork_applet(int applet_no, char **argv)
 		 * need argv untouched because they free argv[i]! */
 		char *tmp_argv[argc+1];
 		memcpy(tmp_argv, argv, (argc+1) * sizeof(tmp_argv[0]));
+		applet_name = tmp_argv[0];
 		/* Finally we can call NOFORK applet's main() */
 		rc = applet_main[applet_no](argc, tmp_argv);
 	} else {