[v2,03/16] libceph: define ceph_decode_string()

Commit Message

Alex Elder July 11, 2012, 10:09 p.m. UTC

There is no string decoding function defined in <decode.h>, so this
defines one.

This function is a little different from the others in that the
length of the encoded string is not known a priori.  So the
interface is defined a bit like snprintf(), where the value returned
indicates the space required--even if it's more than the space
allotted.

The function also avoids overrunning the end of the memory range
being converted.

Signed-off-by: Alex Elder <[e-mail]>
---
v2: Made the function safe from overrunning the source memory

 include/linux/ceph/decode.h |   45
++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 45 insertions(+)


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Comments

Alex Elder July 12, 2012, 5:13 p.m. UTC | #1

OK, in a side discussion, Sage convinced me that since the only
user of this function would be ceph_extract_encoded_string()
(introduced in the next patch), and that under normal circumstances
that function most likely implements the most likely legitimate
use of this function, there is really no need for this function
to stand by itself.

So I'm retracting this patch, and am re-posting the next one,
which open-codes the functionality of ceph_decode_string()
directly inside ceph_extract_encoded_string().

					-Alex
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Patch

Index: b/include/linux/ceph/decode.h
===================================================================
--- a/include/linux/ceph/decode.h
+++ b/include/linux/ceph/decode.h
@@ -85,6 +85,51 @@  static inline int ceph_has_room(void **p
 	} while (0)

 /*
+ * Decode the wire-encoded string at *p into the buffer "s"
+ * provided, whose size is indicated by "size".  Note that "s" can
+ * be a null pointer if size is 0.  If it fits, the resulting string
+ * will always be terminated with '\0'; otherwise the buffer will
+ * be unchanged.
+ *
+ * Care is taken to ensure the result of decoding the string will
+ * not touch anything at or beyond the "end" address provided.  If
+ * it would, -ERANGE is returned.
+ *
+ * Otherwise, returns the length of the encoded string (which may be
+ * greater than or equal to the buffer size).  The return value does
+ * not include the terminating '\0'.
+ *
+ * If the the return value is not negative and is less than the size
+ * provided, *p will be advanced past the decoded data; otherwise it
+ * is unchanged.  This allows for a two call sequence to be used to
+ * allocate sufficient space for the string.
+ *
+ */
+static inline ssize_t ceph_decode_string(void **p, void *end,
+					char *s, size_t size)
+{
+	size_t len;
+	void *cp = *p;
+
+	ceph_decode_32_safe(&cp, end, len, bad);
+	if (!ceph_has_room(&cp, end, len))
+	    	goto bad;
+
+	if (size < len + 1)
+		return len;	/* Not enough room */
+
+	if (len)
+		memcpy(s, cp, len);
+	*(s + len) = '\0';
+
+	*p = (char *) *p + sizeof (u32) + len;
+
+    	return (ssize_t) len;
+bad:
+    	return (ssize_t) -ERANGE;
+}
+
+/*
  * struct ceph_timespec <-> struct timespec
  */
 static inline void ceph_decode_timespec(struct timespec *ts,