ash: exec: Do not allocate stack string in padvance

Upstream commit:

    Date: Sat, 19 May 2018 02:39:48 +0800
    exec: Do not allocate stack string in padvance

    Many callers of padvance immediately free the allocated string
    so this patch moves the stalloc call to the caller.  Instead of
    returning the allocated string, padvance now returns the length
    to allocate (this may be longer than the actual string length,
    even including the NUL).  For the case where we would previously
    return NULL, we now return -1.

    Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

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

1 files changed, 38 insertions, 36 deletions

diff --git a/shell/ash.c b/shell/ash.c
index 6505f4984..e89cecf0b 100644
--- a/shell/ash.c
+++ b/shell/ash.c
@@ -2560,17 +2560,17 @@ listvars(int on, int off, struct strlist *lp, char ***end)
 /* ============ Path search helper
  *
  * The variable path (passed by reference) should be set to the start
- * of the path before the first call; path_advance will update
- * this value as it proceeds.  Successive calls to path_advance will return
+ * of the path before the first call; padvance will update
+ * this value as it proceeds.  Successive calls to padvance will return
  * the possible path expansions in sequence.  If an option (indicated by
  * a percent sign) appears in the path entry then the global variable
  * pathopt will be set to point to it; otherwise pathopt will be set to
  * NULL.
  */
-static const char *pathopt;     /* set by path_advance */
+static const char *pathopt;     /* set by padvance */
 
-static char *
-path_advance(const char **path, const char *name)
+static int
+padvance(const char **path, const char *name)
 {
         const char *p;
         char *q;
@@ -2578,7 +2578,7 @@ path_advance(const char **path, const char *name)
         size_t len;
 
         if (*path == NULL)
-                return NULL;
+                return -1;
         start = *path;
         for (p = start; *p && *p != ':' && *p != '%'; p++)
                 continue;
@@ -2599,7 +2599,7 @@ path_advance(const char **path, const char *name)
                 *path = p + 1;
         else
                 *path = NULL;
-        return stalloc(len);
+        return len;
 }
 
 
@@ -2840,6 +2840,7 @@ cdcmd(int argc UNUSED_PARAM, char **argv UNUSED_PARAM)
         char c;
         struct stat statb;
         int flags;
+        int len;
 
         flags = cdopt();
         dest = *argptr;
@@ -2869,9 +2870,10 @@ cdcmd(int argc UNUSED_PARAM, char **argv UNUSED_PARAM)
         if (!*dest)
                 dest = ".";
         path = bltinlookup("CDPATH");
-        while (path) {
-                c = *path;
-                p = path_advance(&path, dest);
+        while (p = path, (len = padvance(&path, dest)) >= 0) {
+                c = *p;
+                p = stalloc(len);
+
                 if (stat(p, &statb) >= 0 && S_ISDIR(statb.st_mode)) {
                         if (c && c != ':')
                                 flags |= CD_PRINT;
@@ -8169,13 +8171,13 @@ static void shellexec(char *prog, char **argv, const char *path, int idx)
         } else {
  try_PATH:
                 e = ENOENT;
-                while ((cmdname = path_advance(&path, prog)) != NULL) {
+                while (padvance(&path, argv[0]) >= 0) {
+                        cmdname = stackblock();
                         if (--idx < 0 && pathopt == NULL) {
                                 tryexec(IF_FEATURE_SH_STANDALONE(-1,) cmdname, argv, envp);
                                 if (errno != ENOENT && errno != ENOTDIR)
                                         e = errno;
                         }
-                        stunalloc(cmdname);
                 }
         }
 
@@ -8208,9 +8210,9 @@ printentry(struct tblentry *cmdp)
         idx = cmdp->param.index;
         path = pathval();
         do {
-                name = path_advance(&path, cmdp->cmdname);
-                stunalloc(name);
+                padvance(&path, cmdp->cmdname);
         } while (--idx >= 0);
+        name = stackblock();
         out1fmt("%s%s\n", name, (cmdp->rehash ? "*" : nullstr));
 }
 
@@ -8603,9 +8605,9 @@ describe_command(char *command, const char *path, int describe_command_verbose)
                         p = command;
                 } else {
                         do {
-                                p = path_advance(&path, command);
-                                stunalloc(p);
+                                padvance(&path, command);
                         } while (--j >= 0);
+                        p = stackblock();
                 }
                 if (describe_command_verbose) {
                         out1fmt(" is %s", p);
@@ -11020,8 +11022,12 @@ chkmail(void)
         mpath = mpathset() ? mpathval() : mailval();
         new_hash = 0;
         for (;;) {
-                p = path_advance(&mpath, nullstr);
-                if (p == NULL)
+                int len;
+
+                len = padvance(&mpath, nullstr);
+                if (!len)
+                        break;
+                p = stackblock();
                         break;
                 if (*p == '\0')
                         continue;
@@ -13341,33 +13347,30 @@ cmdloop(int top)
  * search for the file, which is necessary to find sub-commands.
  */
 static char *
-find_dot_file(char *name)
+find_dot_file(char *basename)
 {
         char *fullname;
         const char *path = pathval();
         struct stat statb;
+        int len;
 
         /* don't try this for absolute or relative paths */
-        if (strchr(name, '/'))
-                return name;
+        if (strchr(basename, '/'))
+                return basename;
 
-        while ((fullname = path_advance(&path, name)) != NULL) {
+        while ((len = padvance(&path, basename)) >= 0) {
+                fullname = stackblock();
                 if ((stat(fullname, &statb) == 0) && S_ISREG(statb.st_mode)) {
-                        /*
-                         * Don't bother freeing here, since it will
-                         * be freed by the caller.
-                         */
-                        return fullname;
+                        /* This will be freed by the caller. */
+                        return stalloc(len);
                 }
-                if (fullname != name)
-                        stunalloc(fullname);
         }
         /* not found in PATH */
 
 #if ENABLE_ASH_BASH_SOURCE_CURDIR
-        return name;
+        return basename;
 #else
-        ash_msg_and_raise_error("%s: not found", name);
+        ash_msg_and_raise_error("%s: not found", basename);
         /* NOTREACHED */
 #endif
 }
@@ -13470,6 +13473,7 @@ find_command(char *name, struct cmdentry *entry, int act, const char *path)
         int e;
         int updatetbl;
         struct builtincmd *bcmd;
+        int len;
 
         /* If name contains a slash, don't use PATH or hash table */
         if (strchr(name, '/') != NULL) {
@@ -13561,10 +13565,8 @@ find_command(char *name, struct cmdentry *entry, int act, const char *path)
         e = ENOENT;
         idx = -1;
  loop:
-        while ((fullname = path_advance(&path, name)) != NULL) {
-                stunalloc(fullname);
-                /* NB: code below will still use fullname
-                 * despite it being "unallocated" */
+        while ((len = padvance(&path, name)) >= 0) {
+                fullname = stackblock();
                 idx++;
                 if (pathopt) {
                         if (prefix(pathopt, "builtin")) {
@@ -13598,7 +13600,7 @@ find_command(char *name, struct cmdentry *entry, int act, const char *path)
                 if (!S_ISREG(statb.st_mode))
                         continue;
                 if (pathopt) {          /* this is a %func directory */
-                        stalloc(strlen(fullname) + 1);
+                        stalloc(len);
                         /* NB: stalloc will return space pointed by fullname
                          * (because we don't have any intervening allocations
                          * between stunalloc above and this stalloc) */