Re: [PATCH] manual: remove an obsolete requirement on aligned_alloc() usage

[Paul Eggert <eggert@cs.ucla.edu>]

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On 10/25/21 08:45, Martin Sebor via Libc-alpha wrote:
> Based on my code inspection of _mid_memalign in malloc.c I think
> a change that would more accurately reflect the implementation
> is one that described that when alignment is not a power of two
> it's bumped up to next larger power of two, and adjusted
> the EINVAL condition appropriately.

Let's not document this implementation detail, as it might tie us down 
unnecessarily in the future.

> I believe the DR460 change went into C17.  Here's an early C2x
> draft from 2018 publicly available on the WG14 site that should
> still be very close to C17:
> 
>   http://www.open-std.org/jtc1/sc22/wg14/www/docs/n2310.pdf

This says that aligned_alloc "shall fail" if the alignment is not a 
valid alignment supported by the implementation. This means the current 
glibc alloc_aligned behavior does not conform to C17 and should be fixed 
so that it does. Something like the attached (untested) patch, say.

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diff --git a/malloc/malloc.c b/malloc/malloc.c
index 2ba1fee144..386fb5a4cb 100644
--- a/malloc/malloc.c
+++ b/malloc/malloc.c
@@ -3416,6 +3416,12 @@ _mid_memalign (size_t alignment, size_t bytes, void *address)
   mstate ar_ptr;
   void *p;
 
+  if (!powerof2 (alignment))
+    {
+      __set_errno (EINVAL);
+      return 0;
+    }
+
   /* If we need less alignment than we give anyway, just relay to malloc.  */
   if (alignment <= MALLOC_ALIGNMENT)
     return __libc_malloc (bytes);
@@ -3424,24 +3430,6 @@ _mid_memalign (size_t alignment, size_t bytes, void *address)
   if (alignment < MINSIZE)
     alignment = MINSIZE;
 
-  /* If the alignment is greater than SIZE_MAX / 2 + 1 it cannot be a
-     power of 2 and will cause overflow in the check below.  */
-  if (alignment > SIZE_MAX / 2 + 1)
-    {
-      __set_errno (EINVAL);
-      return 0;
-    }
-
-
-  /* Make sure alignment is power of 2.  */
-  if (!powerof2 (alignment))
-    {
-      size_t a = MALLOC_ALIGNMENT * 2;
-      while (a < alignment)
-        a <<= 1;
-      alignment = a;
-    }
-
   if (SINGLE_THREAD_P)
     {
       p = _int_memalign (&main_arena, alignment, bytes);