ash: fix quadratic matching slowdown is ${v/*foo*/repl} (really bad one)

It is especially bad with patterns starting with "*".

With ASH_OPTIMIZE_FOR_SIZE=y, only those are optimized, +few bytes:
   text    data     bss     dec     hex filename
 836337     441    7564  844342   ce236 busybox_old
 836341     441    7564  844346   ce23a busybox_unstripped

With ASH_OPTIMIZE_FOR_SIZE off, we also optimize patterns _ending_ with "*",
which costs about 80 bytes:
   text    data     bss     dec     hex filename
 836656     441    7564  844661   ce375 busybox_old
 836732     441    7564  844737   ce3c1 busybox_unstripped

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

1 files changed, 59 insertions, 12 deletions

diff --git a/shell/ash.c b/shell/ash.c
index 0a8b6c0f2..ce82a965c 100644
--- a/shell/ash.c
+++ b/shell/ash.c
@@ -6040,25 +6040,61 @@ scanleft(char *startp, char *rmesc, char *rmescend UNUSED_PARAM, char *str, int
 }
 
 static char *
-scanright(char *startp, char *rmesc, char *rmescend, char *str, int quotes,
-        int zero)
+scanright(char *startp, char *rmesc, char *rmescend, char *pattern, int quotes, int match_at_start)
 {
+#if !ENABLE_ASH_OPTIMIZE_FOR_SIZE
+        int try2optimize = match_at_start;
+#endif
         int esc = 0;
         char *loc;
         char *loc2;
 
-        for (loc = str - 1, loc2 = rmescend; loc >= startp; loc2--) {
+        /* If we called by "${v/pattern/repl}" or "${v//pattern/repl}":
+         * startp="escaped_value_of_v" rmesc="raw_value_of_v"
+         * rmescend=""(ptr to NUL in rmesc) pattern="pattern" quotes=match_at_start=1
+         * Logic:
+         * loc starts at NUL at the end of startp, loc2 starts at the end of rmesc,
+         * and on each iteration they go back two/one char until they reach the beginning.
+         * We try to find a match in "raw_value_of_v", "raw_value_of_", "raw_value_of" etc.
+         */
+        /* TODO: document in what other circumstances we are called. */
+
+        for (loc = pattern - 1, loc2 = rmescend; loc >= startp; loc2--) {
                 int match;
                 char c = *loc2;
                 const char *s = loc2;
-                if (zero) {
+                if (match_at_start) {
                         *loc2 = '\0';
                         s = rmesc;
                 }
-                match = pmatch(str, s);
+                match = pmatch(pattern, s);
+                //bb_error_msg("pmatch(pattern:'%s',s:'%s'):%d", pattern, s, match);
                 *loc2 = c;
                 if (match)
                         return loc;
+#if !ENABLE_ASH_OPTIMIZE_FOR_SIZE
+                if (try2optimize) {
+                        /* Maybe we can optimize this:
+                         * if pattern ends with unescaped *, we can avoid checking
+                         * shorter strings: if "foo*" doesnt match "raw_value_of_v",
+                         * it wont match truncated "raw_value_of_" strings too.
+                         */
+                        unsigned plen = strlen(pattern);
+                        /* Does it end with "*"? */
+                        if (plen != 0 && pattern[--plen] == '*') {
+                                /* "xxxx*" is not escaped */
+                                /* "xxx\*" is escaped */
+                                /* "xx\\*" is not escaped */
+                                /* "x\\\*" is escaped */
+                                int slashes = 0;
+                                while (plen != 0 && pattern[--plen] == '\\')
+                                        slashes++;
+                                if (!(slashes & 1))
+                                        break; /* ends with unescaped "*" */
+                        }
+                        try2optimize = 0;
+                }
+#endif
                 loc--;
                 if (quotes) {
                         if (--esc < 0) {
@@ -6248,7 +6284,7 @@ subevalvar(char *p, char *str, int strloc, int subtype,
 
 #if ENABLE_ASH_BASH_COMPAT
         if (subtype == VSREPLACE || subtype == VSREPLACEALL) {
-                char *idx, *end, *restart_detect;
+                char *idx, *end;
 
                 if (!repl) {
                         repl = parse_sub_pattern(str, varflags & VSQUOTE);
@@ -6257,17 +6293,19 @@ subevalvar(char *p, char *str, int strloc, int subtype,
                 }
 
                 /* If there's no pattern to match, return the expansion unmolested */
-                if (*str == '\0')
+                if (str[0] == '\0')
                         return 0;
 
                 len = 0;
                 idx = startp;
                 end = str - 1;
                 while (idx < end) {
+ try_to_match:
                         loc = scanright(idx, rmesc, rmescend, str, quotes, 1);
                         if (!loc) {
                                 /* No match, advance */
-                                restart_detect = stackblock();
+                                char *restart_detect = stackblock();
+ skip_matching:
                                 STPUTC(*idx, expdest);
                                 if (quotes && (unsigned char)*idx == CTLESC) {
                                         idx++;
@@ -6279,7 +6317,16 @@ subevalvar(char *p, char *str, int strloc, int subtype,
                                 idx++;
                                 len++;
                                 rmesc++;
-                                continue;
+                                /* continue; - prone to quadratic behavior, smarter code: */
+                                if (idx >= end)
+                                        break;
+                                if (str[0] == '*') {
+                                        /* Pattern is "*foo". If "*foo" does not match "long_string",
+                                         * it would never match "ong_string" etc, no point in trying.
+                                         */
+                                        goto skip_matching;
+                                }
+                                goto try_to_match;
                         }
 
                         if (subtype == VSREPLACEALL) {
@@ -6294,7 +6341,7 @@ subevalvar(char *p, char *str, int strloc, int subtype,
                         }
 
                         for (loc = repl; *loc; loc++) {
-                                restart_detect = stackblock();
+                                char *restart_detect = stackblock();
                                 STPUTC(*loc, expdest);
                                 if (stackblock() != restart_detect)
                                         goto restart;
@@ -6303,7 +6350,7 @@ subevalvar(char *p, char *str, int strloc, int subtype,
 
                         if (subtype == VSREPLACE) {
                                 while (*idx) {
-                                        restart_detect = stackblock();
+                                        char *restart_detect = stackblock();
                                         STPUTC(*idx, expdest);
                                         if (stackblock() != restart_detect)
                                                 goto restart;
@@ -6332,7 +6379,7 @@ subevalvar(char *p, char *str, int strloc, int subtype,
         if (subtype < 0 || subtype > 7)
                 abort();
 #endif
-        /* zero = subtype == VSTRIMLEFT || subtype == VSTRIMLEFTMAX */
+        /* zero = (subtype == VSTRIMLEFT || subtype == VSTRIMLEFTMAX) */
         zero = subtype >> 1;
         /* VSTRIMLEFT/VSTRIMRIGHTMAX -> scanleft */
         scan = (subtype & 1) ^ zero ? scanleft : scanright;