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

[Brett Walker]

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.

What are your thoughts?

Feedback most welcome,
Brett