Jonas Holmberg from axis dot com writes:

This patch makes msh handle variable expansion within backticks more
correctly.

Current behaviour (wrong):
--------------------------

BusyBox v1.00-rc3 (2004.08.26-11:51+0000) Built-in shell (msh)
Enter 'help' for a list of built-in commands.

$ A='`echo hello`'
$ echo $A
`echo hello`
$ echo `echo $A`
hello
$


New behaviour (correct):
------------------------

BusyBox v1.00-rc3 (2004.08.26-11:51+0000) Built-in shell (msh)
Enter 'help' for a list of built-in commands.

$ A='`echo hello`'
$ echo $A
`echo hello`
$ echo `echo $A`
`echo hello`
$

The current behaviour (wrong according to standards) was actually my
fault. msh handles backticks by executing a subshell (which makes it
work on MMU-less systems). Executing a subshell makes it hard to only
expand variables once in the parent. Therefore I export all variables
that will be expanded within the backticks and let the subshell handle
the expansion instead.

The bug was found while searching for security leaks in CGI-scripts.
Current behaviour of msh makes it easy to expand backticks by mistake
in $QUERY_STRING. I recommend appling the patch before release of bb
1.00.

/Jonas

1 files changed, 46 insertions, 22 deletions

diff --git a/shell/msh.c b/shell/msh.c
index df4c3dab3..2fb0df739 100644
--- a/shell/msh.c
+++ b/shell/msh.c
@@ -299,7 +299,7 @@ static int rlookup(char *n);
 static struct wdblock *glob(char *cp, struct wdblock *wb);
 static int my_getc(int ec);
 static int subgetc(int ec, int quoted);
-static char **makenv(int all);
+static char **makenv(int all, struct wdblock *wb);
 static char **eval(char **ap, int f);
 static int setstatus(int s);
 static int waitfor(int lastpid, int canintr);
@@ -3032,7 +3032,7 @@ forkexec(REGISTER struct op *t, int *pin, int *pout, int act, char **wp)
         if (wp[0] == NULL)
                 _exit(0);
 
-        cp = rexecve(wp[0], wp, makenv(0));
+        cp = rexecve(wp[0], wp, makenv(0, NULL));
         prs(wp[0]);
         prs(": ");
         err(cp);
@@ -3486,7 +3486,7 @@ struct op *t;
                 signal(SIGINT, SIG_DFL);
                 signal(SIGQUIT, SIG_DFL);
         }
-        cp = rexecve(t->words[0], t->words, makenv(0));
+        cp = rexecve(t->words[0], t->words, makenv(0, NULL));
         prs(t->words[0]);
         prs(": ");
         err(cp);
@@ -3954,14 +3954,12 @@ static char **eval(char **ap, int f)
  * names in the dictionary. Keyword assignments
  * will already have been done.
  */
-static char **makenv(int all)
+static char **makenv(int all, struct wdblock *wb)
 {
-        REGISTER struct wdblock *wb;
         REGISTER struct var *vp;
 
         DBGPRINTF5(("MAKENV: enter, all=%d\n", all));
 
-        wb = NULL;
         for (vp = vlist; vp; vp = vp->next)
                 if (all || vp->status & EXPORT)
                         wb = addword(vp->name, wb);
@@ -4251,6 +4249,7 @@ int quoted;
         int ignore;
         int ignore_once;
         char *argument_list[4];
+        struct wdblock *wb = NULL;
 
 #if __GNUC__
         /* Avoid longjmp clobbering */
@@ -4323,22 +4322,47 @@ int quoted;
                                 src++;
                         }
 
-                        vp = lookup(var_name);
-                        if (vp->value != null)
-                                value = (operator == '+') ? alt_value : vp->value;
-                        else if (operator == '?') {
-                                err(alt_value);
-                                return (0);
-                        } else if (alt_index && (operator != '+')) {
-                                value = alt_value;
-                                if (operator == '=')
-                                        setval(vp, value);
-                        } else
-                                continue;
+                        if (isalpha(*var_name)) {
+                                /* let subshell handle it instead */
+
+                                char *namep = var_name;
 
-                        while (*value && (count < LINELIM)) {
-                                *dest++ = *value++;
-                                count++;
+                                *dest++ = '$';
+                                if (braces)
+                                        *dest++ = '{';
+                                while (*namep)
+                                        *dest++ = *namep++;
+                                if (operator) {
+                                        char *altp = alt_value;
+                                        *dest++ = operator;
+                                        while (*altp)
+                                                *dest++ = *altp++;
+                                }
+                                if (braces)
+                                        *dest++ = '}';
+
+                                wb = addword(lookup(var_name)->name, wb);
+                        } else {
+                                /* expand */
+
+                                vp = lookup(var_name);
+                                if (vp->value != null)
+                                        value = (operator == '+') ?
+                                                alt_value : vp->value;
+                                else if (operator == '?') {
+                                        err(alt_value);
+                                        return (0);
+                                } else if (alt_index && (operator != '+')) {
+                                        value = alt_value;
+                                        if (operator == '=')
+                                                setval(vp, value);
+                                } else
+                                        continue;
+
+                                while (*value && (count < LINELIM)) {
+                                        *dest++ = *value++;
+                                        count++;
+                                }
                         }
                 } else {
                         *dest++ = *src++;
@@ -4383,7 +4407,7 @@ int quoted;
         argument_list[2] = child_cmd;
         argument_list[3] = 0;
 
-        cp = rexecve(argument_list[0], argument_list, makenv(1));
+        cp = rexecve(argument_list[0], argument_list, makenv(1, wb));
         prs(argument_list[0]);
         prs(": ");
         err(cp);