Re: [PATCH] Check \0-termination of string in c_getstr

["Martin Liška" <mliska@suse.cz>]

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On 10/11/2016 12:28 PM, Richard Biener wrote:
> On Tue, Oct 11, 2016 at 11:27 AM, Martin Liška <mliska@suse.cz> wrote:
>> As mentioned in the email that I reply to, c_getstr should check
>> null termination of string constants.
>>
>> Tests of the whole series have been running.
> 
> Looks ok to me (if testing passes).

Thanks for the review, however I decided to enhance the API to support
a requested length argument (if a string is not null terminated)
and to return length of the string(usable for strn* functions folding).

Patch can bootstrap on ppc64le-redhat-linux and survives regression tests as
a whole series.

Martin

> 
> Thanks,
> Richard.
> 
>> Thanks,
>> Martin


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From e6c16ea038104ef15b087ff9fca23d9b2406e65e Mon Sep 17 00:00:00 2001
From: marxin <mliska@suse.cz>
Date: Mon, 10 Oct 2016 12:13:12 +0200
Subject: [PATCH] Enhance c_getstr API

gcc/ChangeLog:

2016-10-10  Martin Liska  <mliska@suse.cz>

	* fold-const.c (c_getstr): Guard string termination, or validate
	that requested length is within a string constant.
	* fold-const.h (c_getstr): Set default value for the new arg.
---
 gcc/fold-const.c | 44 +++++++++++++++++++++++++++++++++++---------
 gcc/fold-const.h |  3 ++-
 2 files changed, 37 insertions(+), 10 deletions(-)

diff --git a/gcc/fold-const.c b/gcc/fold-const.c
index 02aa484..eb53e84 100644
--- a/gcc/fold-const.c
+++ b/gcc/fold-const.c
@@ -14440,24 +14440,50 @@ fold_build_pointer_plus_hwi_loc (location_t loc, tree ptr, HOST_WIDE_INT off)
 }
 
 /* Return a char pointer for a C string if it is a string constant
-   or sum of string constant and integer constant.  */
+   or sum of string constant and integer constant.
+   If the string constant is properly zero-terminated, the constant is returned.
+   Otherwise, if REQ_LENGTH is a non-negative number, the constant
+   is returned if the string length is greater or equal to REQ_LENGTH.
+   If STRLEN is a valid pointer, length (including terminatinch character)
+   of returned string is stored to the argument.  */
 
 const char *
-c_getstr (tree src)
+c_getstr (tree src, HOST_WIDE_INT req_length, unsigned HOST_WIDE_INT *strlen)
 {
   tree offset_node;
 
+  if (strlen)
+    *strlen = 0;
+
   src = string_constant (src, &offset_node);
   if (src == 0)
-    return 0;
+    return NULL;
 
-  if (offset_node == 0)
-    return TREE_STRING_POINTER (src);
-  else if (!tree_fits_uhwi_p (offset_node)
-	   || compare_tree_int (offset_node, TREE_STRING_LENGTH (src) - 1) > 0)
-    return 0;
+  unsigned HOST_WIDE_INT offset = 0;
+  if (offset_node != NULL_TREE)
+    {
+      if (!tree_fits_uhwi_p (offset_node))
+	return NULL;
+      else
+	offset = tree_to_uhwi (offset_node);
+    }
 
-  return TREE_STRING_POINTER (src) + tree_to_uhwi (offset_node);
+  unsigned HOST_WIDE_INT string_length = TREE_STRING_LENGTH (src);
+  const char *string = TREE_STRING_POINTER (src);
+  if (offset > string_length)
+    return NULL;
+
+  /* If the string is properly '\0' character terminated, return it.  */
+  if ((string_length > 0 && string[string_length - 1] == 0)
+      || (req_length != -1
+	  && (unsigned HOST_WIDE_INT)req_length <= string_length - offset))
+    {
+      if (strlen)
+	*strlen = string_length - offset;
+      return string + offset;
+    }
+  else
+    return NULL;
 }
 
 #if CHECKING_P
diff --git a/gcc/fold-const.h b/gcc/fold-const.h
index 637e46b..bbf831a 100644
--- a/gcc/fold-const.h
+++ b/gcc/fold-const.h
@@ -182,7 +182,8 @@ extern bool expr_not_equal_to (tree t, const wide_int &);
 extern tree const_unop (enum tree_code, tree, tree);
 extern tree const_binop (enum tree_code, tree, tree, tree);
 extern bool negate_mathfn_p (combined_fn);
-extern const char *c_getstr (tree);
+extern const char *c_getstr (tree src, HOST_WIDE_INT req_length = -1,
+			     unsigned HOST_WIDE_INT *strlen = NULL);
 
 /* Return OFF converted to a pointer offset type suitable as offset for
    POINTER_PLUS_EXPR.  Use location LOC for this conversion.  */
-- 
2.9.2