elf: Do not use memalign for TCB/TLS blocks allocation [BZ #17730]

Instead, call malloc and explicitly align the pointer.

There is no external location to store the original (unaligned)
pointer, and this commit increases the allocation size to store
the pointer at a fixed location relative to the TCB pointer.

The manual alignment means that some space goes unused which
was previously made available for subsequent allocations.
However, in the TLS_DTV_AT_TP case, the manual alignment code
avoids aligning the pre-TCB to the TLS block alignment.  (Even
while using memalign, the allocation had some unused padding
in front.)

This concludes the removal of memalign calls from the TLS code,
and the new tst-tls3-malloc test verifies that only core malloc
routines are used.

1 files changed, 53 insertions, 36 deletions

diff --git a/elf/dl-tls.c b/elf/dl-tls.c
index be6a3c7..17567ad 100644
--- a/elf/dl-tls.c
+++ b/elf/dl-tls.c
@@ -347,6 +347,22 @@ _dl_get_tls_static_info (size_t *sizep, size_t *alignp)
   *alignp = GL(dl_tls_static_align);
 }
 
+/* Derive the location of the pointer to the start of the original
+   allocation (before alignment) from the pointer to the TCB.  */
+static inline void **
+tcb_to_pointer_to_free_location (void *tcb)
+{
+#if TLS_TCB_AT_TP
+  /* The TCB follows the TLS blocks, and the pointer to the front
+     follows the TCB.  */
+  void **original_pointer_location = tcb + TLS_TCB_SIZE;
+#elif TLS_DTV_AT_TP
+  /* The TCB comes first, preceded by the pre-TCB, and the pointer is
+     before that.  */
+  void **original_pointer_location = tcb - TLS_PRE_TCB_SIZE - sizeof (void *);
+#endif
+  return original_pointer_location;
+}
 
 void *
 internal_function
@@ -359,39 +375,50 @@ _dl_allocate_tls_storage (void)
   /* Memory layout is:
      [ TLS_PRE_TCB_SIZE ] [ TLS_TCB_SIZE ] [ TLS blocks ]
 			  ^ This should be returned.  */
-  size += (TLS_PRE_TCB_SIZE + GL(dl_tls_static_align) - 1)
-	  & ~(GL(dl_tls_static_align) - 1);
+  size += TLS_PRE_TCB_SIZE;
 #endif
 
-  /* Allocate a correctly aligned chunk of memory.  */
-  result = __libc_memalign (GL(dl_tls_static_align), size);
-  if (__builtin_expect (result != NULL, 1))
-    {
-      /* Allocate the DTV.  */
-      void *allocated = result;
+  /* Perform the allocation.  Reserve space for the required alignment
+     and the pointer to the original allocation.  */
+  size_t alignment = GL(dl_tls_static_align);
+  void *allocated = malloc (size + alignment + sizeof (void *));
+  if (__glibc_unlikely (allocated == NULL))
+    return NULL;
 
+  /* Perform alignment and allocate the DTV.  */
 #if TLS_TCB_AT_TP
-      /* The TCB follows the TLS blocks.  */
-      result = (char *) result + size - TLS_TCB_SIZE;
-
-      /* Clear the TCB data structure.  We can't ask the caller (i.e.
-	 libpthread) to do it, because we will initialize the DTV et al.  */
-      memset (result, '\0', TLS_TCB_SIZE);
+  /* The TCB follows the TLS blocks, which determine the alignment.
+     (TCB alignment requirements have been taken into account when
+     calculating GL(dl_tls_static_align).)  */
+  void *aligned = (void *) roundup ((uintptr_t) allocated, alignment);
+  result = aligned + size - TLS_TCB_SIZE;
+
+  /* Clear the TCB data structure.  We can't ask the caller (i.e.
+     libpthread) to do it, because we will initialize the DTV et al.  */
+  memset (result, '\0', TLS_TCB_SIZE);
 #elif TLS_DTV_AT_TP
-      result = (char *) result + size - GL(dl_tls_static_size);
-
-      /* Clear the TCB data structure and TLS_PRE_TCB_SIZE bytes before it.
-	 We can't ask the caller (i.e. libpthread) to do it, because we will
-	 initialize the DTV et al.  */
-      memset ((char *) result - TLS_PRE_TCB_SIZE, '\0',
-	      TLS_PRE_TCB_SIZE + TLS_TCB_SIZE);
+  /* Pre-TCB and TCB come before the TLS blocks.  The layout computed
+     in _dl_determine_tlsoffset assumes that the TCB is aligned to the
+     TLS block alignment, and not just the TLS blocks after it.  This
+     can leave an unused alignment gap between the TCB and the TLS
+     blocks.  */
+  result = (void *) roundup
+    (sizeof (void *) + TLS_PRE_TCB_SIZE + (uintptr_t) allocated,
+     alignment);
+
+  /* Clear the TCB data structure and TLS_PRE_TCB_SIZE bytes before
+     it.  We can't ask the caller (i.e. libpthread) to do it, because
+     we will initialize the DTV et al.  */
+  memset (result - TLS_PRE_TCB_SIZE, '\0', TLS_PRE_TCB_SIZE + TLS_TCB_SIZE);
 #endif
 
-      result = allocate_dtv (result);
-      if (result == NULL)
-	free (allocated);
-    }
+  /* Record the value of the original pointer for later
+     deallocation.  */
+  *tcb_to_pointer_to_free_location (result) = allocated;
 
+  result = allocate_dtv (result);
+  if (result == NULL)
+    free (allocated);
   return result;
 }
 
@@ -558,17 +585,7 @@ _dl_deallocate_tls (void *tcb, bool dealloc_tcb)
     free (dtv - 1);
 
   if (dealloc_tcb)
-    {
-#if TLS_TCB_AT_TP
-      /* The TCB follows the TLS blocks.  Back up to free the whole block.  */
-      tcb -= GL(dl_tls_static_size) - TLS_TCB_SIZE;
-#elif TLS_DTV_AT_TP
-      /* Back up the TLS_PRE_TCB_SIZE bytes.  */
-      tcb -= (TLS_PRE_TCB_SIZE + GL(dl_tls_static_align) - 1)
-	     & ~(GL(dl_tls_static_align) - 1);
-#endif
-      free (tcb);
-    }
+    free (*tcb_to_pointer_to_free_location (tcb));
 }
 rtld_hidden_def (_dl_deallocate_tls)