[glibc] malloc: Fix missing accounting of top chunk in malloc_info [BZ #24026]

[glibc] malloc: Fix missing accounting of top chunk in malloc_info [BZ #24026]

[Florian Weimer]

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https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=b6d2c4475d5abc05dd009575b90556bdd3c78ad0

commit b6d2c4475d5abc05dd009575b90556bdd3c78ad0
Author: Niklas Hambüchen <mail@nh2.me>
Date:   Thu Aug 8 22:02:27 2019 +0200

    malloc: Fix missing accounting of top chunk in malloc_info [BZ #24026]
    
    Fixes `<total type="rest" size="..."> incorrectly showing as 0 most
    of the time.
    
    The rest value being wrong is significant because to compute the
    actual amount of memory handed out via malloc, the user must subtract
    it from <system type="current" size="...">. That result being wrong
    makes investigating memory fragmentation issues like
    <https://bugzilla.redhat.com/show_bug.cgi?id=843478> close to
    impossible.

Diff:
---
 ChangeLog       | 6 ++++++
 malloc/malloc.c | 6 ++++++
 2 files changed, 12 insertions(+)

diff --git a/ChangeLog b/ChangeLog
index 760698b..d094614 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,9 @@
+2019-08-08  Niklas Hambüchen  <mail@nh2.me>
+	    Carlos O'Donell  <carlos@redhat.com>
+
+	[BZ #24026]
+	* malloc/malloc.c (__malloc_info): Account for top chunk.
+
 2019-08-07  Joseph Myers  <joseph@codesourcery.com>
 
 	* sysdeps/unix/sysv/linux/bits/fcntl-linux.h [__USE_GNU]
diff --git a/malloc/malloc.c b/malloc/malloc.c
index 343d89f..0e65d63 100644
--- a/malloc/malloc.c
+++ b/malloc/malloc.c
@@ -5406,6 +5406,12 @@ __malloc_info (int options, FILE *fp)
 
       __libc_lock_lock (ar_ptr->mutex);
 
+      /* Account for top chunk.  The top-most available chunk is
+	 treated specially and is never in any bin. See "initial_top"
+	 comments.  */
+      avail = chunksize (ar_ptr->top);
+      nblocks = 1;  /* Top always exists.  */
+
       for (size_t i = 0; i < NFASTBINS; ++i)
 	{
 	  mchunkptr p = fastbin (ar_ptr, i);