win32: fix quoting of arguments with trailing backslashes

Correctly handle the case where an argument needs to be quoted
and has a backslash as its last character.

The issue was detected by the test "sed a cmd ended by double backslash"
in the BusyBox test suite.

1 files changed, 48 insertions, 15 deletions

diff --git a/win32/process.c b/win32/process.c
index 2206a0a5e..951519e59 100644
--- a/win32/process.c
+++ b/win32/process.c
@@ -119,58 +119,91 @@ parse_interpreter(const char *cmd, char ***opts, int *nopts)
 static char *
 quote_arg(const char *arg)
 {
-        /* count chars to quote */
         int len = 0, n = 0;
         int force_quotes = 0;
         char *q, *d;
         const char *p = arg;
-        if (!*p) force_quotes = 1;
+
+        /* empty arguments must be quoted */
+        if (!*p) {
+                force_quotes = 1;
+        }
+
         while (*p) {
-                if (isspace(*p))
+                if (isspace(*p)) {
+                        /* arguments containing whitespace must be quoted */
                         force_quotes = 1;
-                else if (*p == '"')
+                }
+                else if (*p == '"') {
+                        /* double quotes in arguments need to be escaped */
                         n++;
+                }
                 else if (*p == '\\') {
+                        /* count contiguous backslashes */
                         int count = 0;
                         while (*p == '\\') {
                                 count++;
                                 p++;
                                 len++;
                         }
-                        if (*p == '"')
+
+                        /*
+                         * Only escape backslashes before explicit double quotes or
+                         * or where the backslashes are at the end of an argument
+                         * that is scheduled to be quoted.
+                         */
+                        if (*p == '"' || (force_quotes && *p == '\0')) {
                                 n += count*2 + 1;
+                        }
+
+                        if (*p == '\0') {
+                                break;
+                        }
                         continue;
                 }
                 len++;
                 p++;
         }
-        if (!force_quotes && n == 0)
+
+        if (!force_quotes && n == 0) {
                 return (char*)arg;
+        }
 
-        /* insert \ where necessary */
+        /* insert double quotes and backslashes where necessary */
         d = q = xmalloc(len+n+3);
-        if (force_quotes)
+        if (force_quotes) {
                 *d++ = '"';
+        }
+
         while (*arg) {
-                if (*arg == '"')
+                if (*arg == '"') {
                         *d++ = '\\';
+                }
                 else if (*arg == '\\') {
                         int count = 0;
                         while (*arg == '\\') {
                                 count++;
                                 *d++ = *arg++;
                         }
-                        if (*arg == '"') {
-                                while (count-- > 0)
+
+                        if (*arg == '"' || (force_quotes && *arg == '\0')) {
+                                while (count-- > 0) {
                                         *d++ = '\\';
-                                *d++ = '\\';
+                                }
+                                if (*arg == '"') {
+                                        *d++ = '\\';
+                                }
                         }
                 }
-                *d++ = *arg++;
+                if (*arg != '\0') {
+                        *d++ = *arg++;
+                }
         }
-        if (force_quotes)
+        if (force_quotes) {
                 *d++ = '"';
-        *d++ = 0;
+        }
+        *d = '\0';
+
         return q;
 }