[glibc] mcheck Fix malloc_usable_size [BZ #22057]

[Siddhesh Poyarekar <siddhesh@sourceware.org>]

https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=6856975ed4ce79355752d08938f066f2ed9fcfb5

commit 6856975ed4ce79355752d08938f066f2ed9fcfb5
Author: Siddhesh Poyarekar <siddhesh@sourceware.org>
Date:   Thu Jul 22 18:38:16 2021 +0530

    mcheck Fix malloc_usable_size [BZ #22057]
    
    Interpose malloc_usable_size to return the correct mcheck value for
    malloc_usable_size.
    
    Reviewed-by: Carlos O'Donell <carlos@redhat.com>
    Tested-by: Carlos O'Donell <carlos@redhat.com>

Diff:
---
 malloc/Makefile       | 4 +---
 malloc/malloc-debug.c | 2 ++
 malloc/mcheck-impl.c  | 6 ++++++
 3 files changed, 9 insertions(+), 3 deletions(-)

diff --git a/malloc/Makefile b/malloc/Makefile
index 96328da247..d54285192a 100644
--- a/malloc/Makefile
+++ b/malloc/Makefile
@@ -83,7 +83,7 @@ ifeq ($(have-GLIBC_2.23)$(build-shared),yesyes)
 # the tests expect specific internal behavior that is changed due to linking to
 # libmcheck.a.
 tests-exclude-mcheck = tst-mallocstate \
-	tst-safe-linking tst-malloc-usable \
+	tst-safe-linking \
 	tst-malloc-backtrace \
 	tst-malloc-fork-deadlock \
 	tst-malloc-stats-cancellation \
@@ -92,8 +92,6 @@ tests-exclude-mcheck = tst-mallocstate \
 	tst-malloc-thread-fail \
 	tst-malloc-usable-tunables \
 	tst-malloc_info \
-	tst-pvalloc-fortify \
-	tst-reallocarray \
 	tst-compathooks-off tst-compathooks-on
 
 tests-mcheck = $(filter-out $(tests-exclude-mcheck), $(tests))
diff --git a/malloc/malloc-debug.c b/malloc/malloc-debug.c
index 34523b0cc3..9922ef5f25 100644
--- a/malloc/malloc-debug.c
+++ b/malloc/malloc-debug.c
@@ -399,6 +399,8 @@ strong_alias (__debug_calloc, calloc)
 size_t
 malloc_usable_size (void *mem)
 {
+  if (__is_malloc_debug_enabled (MALLOC_MCHECK_HOOK))
+    return mcheck_usable_size (mem);
   if (__is_malloc_debug_enabled (MALLOC_CHECK_HOOK))
     return malloc_check_get_size (mem);
 
diff --git a/malloc/mcheck-impl.c b/malloc/mcheck-impl.c
index 8857e6b179..6597a290a6 100644
--- a/malloc/mcheck-impl.c
+++ b/malloc/mcheck-impl.c
@@ -404,3 +404,9 @@ __mcheck_initialize (void (*func) (enum mcheck_status), bool in_pedantic)
   pedantic = in_pedantic;
   return 0;
 }
+
+static int
+mcheck_usable_size (struct hdr *h)
+{
+  return (h - 1)->size;
+}