Re: [PATCH 2/2] Initialize all of datahead structure in nscd [BZ #16791]

[Siddhesh Poyarekar <siddhesh at redhat dot com>]

On Tue, Apr 01, 2014 at 11:26:16AM -0700, Roland McGrath wrote:
> If you bzero before explicitly filling fields, does the compiler optimize
> to only zeroing the one unused field?  If so, that seems more futureproof.

It doesn't quite, but (now that I checked) it really only results in a
couple of additional instructions (as opposed to a function call that
I had implicitly assumed), so I'll add it anyway if you think it is
OK.  This obviously also needs the previous patch that consolidates
the head initialization code.

Siddhesh

	[BZ #16791]
	* nscd/nscd-client.h (datahead_init_common): Initialize entire
	structure.
	(datahead_init_pos): Call datahead_init_common early.
	(datahead_init_neg): Likewise.

diff --git a/nscd/nscd-client.h b/nscd/nscd-client.h
index c069bf6..e2d04f2 100644
--- a/nscd/nscd-client.h
+++ b/nscd/nscd-client.h
@@ -240,6 +240,10 @@ static inline time_t
 datahead_init_common (struct datahead *head, nscd_ssize_t allocsize,
 		      nscd_ssize_t recsize, uint32_t ttl)
 {
+  /* Initialize so that we don't write out junk in uninitialized data to the
+     cache.  */
+  bzero (head, sizeof (*head));
+
   head->allocsize = allocsize;
   head->recsize = recsize;
   head->usable = true;
@@ -253,18 +257,21 @@ static inline time_t
 datahead_init_pos (struct datahead *head, nscd_ssize_t allocsize,
 		   nscd_ssize_t recsize, uint8_t nreloads, uint32_t ttl)
 {
+  time_t ret = datahead_init_common (head, allocsize, recsize, ttl);
   head->notfound = false;
   head->nreloads = nreloads;
-  return datahead_init_common (head, allocsize, recsize, ttl);
+  return ret;
 }
 
 static inline time_t
 datahead_init_neg (struct datahead *head, nscd_ssize_t allocsize,
 		   nscd_ssize_t recsize, uint32_t ttl)
 {
+  time_t ret = datahead_init_common (head, allocsize, recsize, ttl);
+  /* We don't need to touch nreloads here since it is set to our desired value
+     (0) when we bzero the structure.  */
   head->notfound = true;
-  head->nreloads = 0;
-  return datahead_init_common (head, allocsize, recsize, ttl);
+  return ret;
 }
 
 /* Structure for one hash table entry.  */

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