[syslinux] Mismatch between code comments and reality in malloc.h

[H. Peter Anvin]

On 02/03/18 04:28, Brett Walker via Syslinux wrote:
> Hi Devs,
> 
> I've been doing some deep code diving and I seem to have found a discrepancy between code comments and reality. This difference could hint at a bug or performance degradation, but I doubt it. I thought to ask to get clarification.
> 
> In syslinux/core/mem/malloc.h, the comments and definition for arena_header, ARENA_PADDING and free_arena_header are:
> 
> 
> /*
>  * This structure should be a power of two.  This becomes the
>  * alignment unit.
>  */
> struct arena_header {
>     malloc_tag_t tag;
>     size_t attrs; /* Bits 0..1:  Type
>         2..3:  Heap,
> 4..31: MSB of the size  */
>     struct free_arena_header *next, *prev;
> 
> #ifdef DEBUG_MALLOC
>     unsigned long _pad[3];
>     unsigned int magic;
> #endif
> };
> 
> /* Pad to 2*sizeof(struct arena_header) */
> #define ARENA_PADDING ((2 * sizeof(struct arena_header)) - \
>        (sizeof(struct arena_header) + \
> sizeof(struct free_arena_header *) + \
> sizeof(struct free_arena_header *)))
> 
> /*
>  * This structure should be no more than twice the size of the
>  * previous structure.
>  */
> struct free_arena_header {
>     struct arena_header a;
>     struct free_arena_header *next_free, *prev_free;
>     size_t _pad[ARENA_PADDING];
> };
> 
> In my environment, the size of int and pointer are both 32-bits (x86 32-bits). When compiled with DEBUG_MALLOC undefined; I get
> 
> sizeof(arena_header) == 16
> sizeof(free_arena_header) == 56
> 
> Which disagrees with the comments. The comments do make a lot of sense to ensure good memory block alignment. To make the code agree with the comments (the easier option); the definition of ARENA_PADDING needs to be altered to:
> 
> #define ARENA_PADDING (((2 * sizeof(struct arena_header)) - \
>        (sizeof(struct arena_header) + \
> sizeof(struct free_arena_header *) + \
> sizeof(struct free_arena_header *))) / sizeof(size_t))
> 
> This includes a reduction in the size of the padding by the unit size of the padding, i.e sizeof(size_t). Given that this is the memory sub-system, this small change could have a significant impact.
> 

Right, either ARENA_PADDING needs to be changed as you say, or _pad
needs to be defined as char (or uint8_t) instead of size_t.  The latter
might be better.

	-hpa