On 21.08.19 18:47, Paul Durrant wrote:
Hi
You should also check whether the new requested alignment is compatible with
the existing block
alignment
I am wondering:
In case when we use only type-safe construction for the basic allocation
(xmalloc) and type-safe construction for the re-allocation
(xrealloc_flex_struct) over the same pointer of the correct type, will
we get a possible alignment incompatibility?
You shouldn't but you're trusting that no-one will want to call the function
directly with a higher alignment.
Yes, it is best to be on the safe side.
This is how I see it:
diff --git a/xen/common/xmalloc_tlsf.c b/xen/common/xmalloc_tlsf.c
index eecae2e..f90f453 100644
--- a/xen/common/xmalloc_tlsf.c
+++ b/xen/common/xmalloc_tlsf.c
@@ -616,8 +616,15 @@ void *_xrealloc(void *ptr, unsigned long size,
unsigned long align)
curr_size = PFN_ORDER(virt_to_page(ptr)) << PAGE_SHIFT;
else
{
- struct bhdr *b;
- b = (struct bhdr *)((char *)ptr - BHDR_OVERHEAD);
+ struct bhdr *b = (struct bhdr *)((char *)ptr - BHDR_OVERHEAD);
+
+ if ( b->size & FREE_BLOCK )
+ {
+ p = (char *)ptr - (b->size & ~FREE_BLOCK);
+ b = (struct bhdr *)((char *)p - BHDR_OVERHEAD);
+ ASSERT(!(b->size & FREE_BLOCK));
+ }
+
curr_size = b->size & BLOCK_SIZE_MASK;
Yes, that looks ok for getting the size right...
}
@@ -630,8 +637,8 @@ void *_xrealloc(void *ptr, unsigned long size,
unsigned long align)
tmp_size = ( tmp_size < MIN_BLOCK_SIZE ) ? MIN_BLOCK_SIZE :
ROUNDUP_SIZE(tmp_size);
- if ( tmp_size <= curr_size ) /* fits in current block */
- return ptr;
+ if ( tmp_size <= curr_size && ((unsigned long)ptr & (align - 1)) == 0 )
+ return ptr; /* fits in current block */
...and that should deal with the alignment too (although you may want to
mention the alignment part in the comment too)
p = _xmalloc(size, align);
if ( p )
(END)
Did I get your point correct?
Yes, with those changes I think it will look ok.
Thank you.
--
Regards,
Oleksandr Tyshchenko
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