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.

diff --git a/gdb/NEWS b/gdb/NEWS
index fd42864c692a98c419985d4d9e15d75aa0e5a229..b924834d3d730ae578b7d7cb8174c14b7a0bb3ad 100644 (file)
--- a/gdb/NEWS
+++ b/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 ]

diff --git a/gdb/printcmd.c b/gdb/printcmd.c
index 0535128c63f9ed936b67e8f2ef1d719b99b2b7c7..8d7d04231feb6e876e4a4c5713d72bc3ea04fda3 100644 (file)
--- a/gdb/printcmd.c
+++ b/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 ();
     }
 

diff --git a/gdb/testsuite/gdb.base/printcmds.c b/gdb/testsuite/gdb.base/printcmds.c
index fa3a62d6cdd729f258b18777afc5e9bbd9a6b766..8445fcc1aa2a67768c9e022a12e219b787bdc5d0 100644 (file)
--- a/gdb/testsuite/gdb.base/printcmds.c
+++ b/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 {

diff --git a/gdb/testsuite/gdb.base/printcmds.exp b/gdb/testsuite/gdb.base/printcmds.exp
index 73f145c9586fe970890ebf2a43ad51d1bbb802d1..eab0264af6350604c3b05ed20cf14e672e743cfa 100644 (file)
--- a/gdb/testsuite/gdb.base/printcmds.exp
+++ b/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

diff --git a/gdb/testsuite/gdb.base/printf-wchar_t.c b/gdb/testsuite/gdb.base/printf-wchar_t.c
index 4d13fd3c961b82e34940719d08bd1798e7a2cf12..2635932c780803ebc9518ce19c09bc4451caf3df 100644 (file)
--- a/gdb/testsuite/gdb.base/printf-wchar_t.c
+++ b/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)

diff --git a/gdb/testsuite/gdb.base/printf-wchar_t.exp b/gdb/testsuite/gdb.base/printf-wchar_t.exp
index 85c6edf292c6343c758fe2c003cc262d14fa5c57..8a6003d57858f9725df2605b150d27422dea27ad 100644 (file)
--- a/gdb/testsuite/gdb.base/printf-wchar_t.exp
+++ b/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"
+}

diff --git a/gdb/testsuite/lib/gdb.exp b/gdb/testsuite/lib/gdb.exp
index b4900ae25a66dbdbca7d4efa553f70b998c654c4..bf8cdbd750c2538c6e67b0f36e3a55926b4d4dcc 100644 (file)
--- a/gdb/testsuite/lib/gdb.exp
+++ b/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.
 

diff --git a/gdb/value.c b/gdb/value.c
index 1b31f91b4dcfee232286129565721154a4fc3f8a..4226e793ac957b1497d3cd43bca899e7ddd55d71 100644 (file)
--- a/gdb/value.c
+++ b/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.  */

diff --git a/gdb/value.h b/gdb/value.h
index 512d05eac1be7eeee7754aa4b6a60eb62e8176ae..e5c63dccbe21c41bdf6fa72a78c2a43b892978ad 100644 (file)
--- a/gdb/value.h
+++ b/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.  */