gdb: check max-value-size when reading strings for printf

I noticed that the printf code for strings, printf_c_string and
printf_wide_c_string, don't take max-value-size into account, but do
load a complete string from the inferior into a GDB buffer.

As such it would be possible for an badly behaved inferior to cause
GDB to try and allocate an excessively large buffer, potentially
crashing GDB, or at least causing GDB to swap lots, which isn't
great.

We already have a setting to protect against this sort of thing, the
'max-value-size'.  So this commit updates the two function mentioned
above to check the max-value-size and give an error if the
max-value-size is exceeded.

If the max-value-size is exceeded, I chose to continue reading
inferior memory to figure out how long the string actually is, we just
don't store the results.  The benefit of this is that when we give the
user an error we can tell the user how big the string actually is,
which would allow them to correctly adjust max-value-size, if that's
what they choose to do.

The default for max-value-size is 64k so there should be no user
visible changes after this commit, unless the user was previously
printing very large strings.  If that is the case then the user will
now need to increase max-value-size.

gdb/NEWS

@@ -78,6 +78,12 @@
   functionality is also available for dprintf when dprintf-style is
   'gdb'.
 
+* When the printf command requires a string to be fetched from the
+  inferior, GDB now uses the existing 'max-value-size' setting to the
+  limit the memory allocated within GDB.  The default 'max-value-size'
+  is 64k.  To print longer strings you should increase
+  'max-value-size'.
+
 * New commands
 
 maintenance print record-instruction [ N ]

gdb/printcmd.c

@@ -2480,17 +2480,24 @@ printf_c_string (struct ui_file *stream, const char *format,
       /* This is a %s argument.  Build the string in STR which is
 	 currently empty.  */
       gdb_assert (str.size () == 0);
-      for (size_t len = 0;; len++)
+      size_t len;
+      for (len = 0;; len++)
 	{
 	  gdb_byte c;
 
 	  QUIT;
+
 	  read_memory (tem + len, &c, 1);
-	  str.push_back (c);
+	  if (!exceeds_max_value_size (len + 1))
+	    str.push_back (c);
 	  if (c == 0)
 	    break;
 	}
 
+      if (exceeds_max_value_size (len + 1))
+	error (_("printed string requires %s bytes, which is more than "
+		 "max-value-size"), plongest (len + 1));
+
       /* We will have passed through the above loop at least once, and will
 	 only exit the loop when we have pushed a zero byte onto the end of
 	 STR.  */
@@ -2547,13 +2554,37 @@ printf_wide_c_string (struct ui_file *stream, const char *format,
       for (len = 0;; len += wcwidth)
 	{
 	  QUIT;
-	  tem_str->resize (tem_str->size () + wcwidth);
-	  gdb_byte *dst = tem_str->data () + len;
+	  gdb_byte *dst;
+	  if (!exceeds_max_value_size (len + wcwidth))
+	    {
+	      tem_str->resize (tem_str->size () + wcwidth);
+	      dst = tem_str->data () + len;
+	    }
+	  else
+	    {
+	      /* We still need to check for the null-character, so we need
+		 somewhere to place the data read from the inferior.  We
+		 can't keep growing TEM_STR, it's gotten too big, so
+		 instead just read the new character into the start of
+		 TEMS_STR.  This will corrupt the previously read contents,
+		 but we're not going to print this string anyway, we just
+		 want to know how big it would have been so we can tell the
+		 user in the error message (see below).
+
+		 And we know there will be space in this buffer so long as
+		 WCWIDTH is smaller than our LONGEST type, the
+		 max-value-size can't be smaller than a LONGEST.  */
+	      dst = tem_str->data ();
+	    }
 	  read_memory (tem + len, dst, wcwidth);
 	  if (extract_unsigned_integer (dst, wcwidth, byte_order) == 0)
 	    break;
 	}
 
+      if (exceeds_max_value_size (len + wcwidth))
+	error (_("printed string requires %s bytes, which is more than "
+		 "max-value-size"), plongest (len + wcwidth));
+
       str = tem_str->data ();
     }
 

gdb/testsuite/gdb.base/printcmds.c

@@ -75,6 +75,8 @@ char *teststring = (char*)"teststring contents";
 typedef char *charptr;
 charptr teststring2 = "more contents";
 
+const char *teststring3 = "this is a longer test string that we can use";
+
 /* Test printing of a struct containing character arrays. */
 
 struct some_arrays {

gdb/testsuite/gdb.base/printcmds.exp

@@ -901,6 +901,11 @@ proc test_printf {} {
 
     # PR cli/14977.
     gdb_test "printf \"%s\\n\", 0" "\\(null\\)"
+
+    with_max_value_size 20 {
+	gdb_test {printf "%s", teststring3} \
+	    "^printed string requires 45 bytes, which is more than max-value-size"
+    }
 }
 
 #Test printing DFP values with printf

gdb/testsuite/gdb.base/printf-wchar_t.c

@@ -18,6 +18,8 @@
 #include <wchar.h>
 
 const wchar_t wide_str[] = L"wide string";
+const wchar_t long_wide_str[]
+  = L"this is a much longer wide string that we can use if needed";
 
 int
 main (void)

gdb/testsuite/gdb.base/printf-wchar_t.exp

@@ -24,3 +24,9 @@ if {![runto_main]} {
 }
 
 gdb_test {printf "%ls\n", wide_str} "^wide string"
+
+# Check that if the max-value-size will kick in when using printf on strings.
+with_max_value_size 20 {
+    gdb_test {printf "%ls\n", long_wide_str} \
+	"^printed string requires 240 bytes, which is more than max-value-size"
+}

gdb/testsuite/lib/gdb.exp

@@ -3192,6 +3192,36 @@ proc with_target_charset { target_charset body } {
     }
 }
 
+# Run tests in BODY with max-value-size set to SIZE.  When BODY is
+# finished restore max-value-size.
+
+proc with_max_value_size { size body } {
+    global gdb_prompt
+
+    set saved ""
+    gdb_test_multiple "show max-value-size" "" {
+	-re -wrap "Maximum value size is ($::decimal) bytes\\." {
+	    set saved $expect_out(1,string)
+	}
+	-re ".*$gdb_prompt " {
+	    fail "get max-value-size"
+	}
+    }
+
+    gdb_test_no_output -nopass "set max-value-size $size"
+
+    set code [catch {uplevel 1 $body} result]
+
+    gdb_test_no_output -nopass "set max-value-size $saved"
+
+    if {$code == 1} {
+	global errorInfo errorCode
+	return -code $code -errorinfo $errorInfo -errorcode $errorCode $result
+    } else {
+	return -code $code $result
+    }
+}
+
 # Switch the default spawn id to SPAWN_ID, so that gdb_test,
 # mi_gdb_test etc. default to using it.
 

gdb/value.c

@@ -804,7 +804,7 @@ check_type_length_before_alloc (const struct type *type)
 {
   ULONGEST length = type->length ();
 
-  if (max_value_size > -1 && length > max_value_size)
+  if (exceeds_max_value_size (length))
     {
       if (type->name () != NULL)
 	error (_("value of type `%s' requires %s bytes, which is more "
@@ -815,6 +815,14 @@ check_type_length_before_alloc (const struct type *type)
     }
 }
 
+/* See value.h.  */
+
+bool
+exceeds_max_value_size (ULONGEST length)
+{
+  return max_value_size > -1 && length > max_value_size;
+}
+
 /* When this has a value, it is used to limit the number of array elements
    of an array that are loaded into memory when an array value is made
    non-lazy.  */

gdb/value.h

@@ -1612,6 +1612,11 @@ extern void finalize_values ();
    of floating-point, fixed-point, or integer type.  */
 extern gdb_mpq value_to_gdb_mpq (struct value *value);
 
+/* Return true if LEN (in bytes) exceeds the max-value-size setting,
+   otherwise, return false.  If the user has disabled (set to unlimited)
+   the max-value-size setting then this function will always return false.  */
+extern bool exceeds_max_value_size (ULONGEST length);
+
 /* While an instance of this class is live, and array values that are
    created, that are larger than max_value_size, will be restricted in size
    to a particular number of elements.  */