Re: [PATCH 3/6] malloc: Use chunk2rawmem throughout

[DJ Delorie <dj@redhat.com>]

Szabolcs Nagy <szabolcs.nagy@arm.com> writes:
> The difference between chunk2mem and chunk2rawmem is that the latter
> does not get the memory tag for the returned pointer.  It turns out
> chunk2rawmem almost always works:

Given that these two macros are identical on non-aarch64 systems, I'm
going to gloss over the "is it tagged correctly" question since you
folks can just test it, and other targets won't care ;-)

LGTM
Reviewed-by: DJ Delorie <dj@redhat.com>

> sysmalloc: Returns untagged memory.
> _int_malloc: Returns untagged memory.
> _int_free: Takes untagged memory.
> _int_memalign: Returns untagged memory.
> _int_realloc: Takes and returns tagged memory.

We should probably put this information in comments at each function
implementation too, but at least it's in the source files :-)

> -        newmem = chunk2mem (newp);
> +        newmem = tag_at (chunk2rawmem (newp));

Ok.

>     headers have distinct tags.  Converting fully from one to the other
>     involves extracting the tag at the other address and creating a
>     suitable pointer using it.  That can be quite expensive.  There are
> -   many occasions, though when the pointer will not be dereferenced
> -   (for example, because we only want to assert that the pointer is
> -   correctly aligned).  In these cases it is more efficient not
> -   to extract the tag, since the answer will be the same either way.
> -   chunk2rawmem() can be used in these cases.
> - */
> +   cases when the pointers are not dereferenced (for example only used
> +   for alignment check) so the tags are not relevant, and there are
> +   cases when user data is not tagged distinctly from malloc headers
> +   (user data is untagged because tagging is done late in malloc and
> +   early in free).  User memory tagging across internal interfaces:
> +
> +      sysmalloc: Returns untagged memory.
> +      _int_malloc: Returns untagged memory.
> +      _int_free: Takes untagged memory.
> +      _int_memalign: Returns untagged memory.
> +      _int_memalign: Returns untagged memory.
> +      _mid_memalign: Returns tagged memory.
> +      _int_realloc: Takes and returns tagged memory.
> +*/

Ok.

> -/* Convert a user mem pointer to a chunk address without correcting
> +/* Convert a chunk address to a user mem pointer without correcting
>     the tag.  */
>  #define chunk2rawmem(p) ((void*)((char*)(p) + CHUNK_HDR_SZ))

Heh.  Ok.

>  #define misaligned_chunk(p) \
> -  ((uintptr_t)(MALLOC_ALIGNMENT == CHUNK_HDR_SZ ? (p) : chunk2mem (p)) \
> +  ((uintptr_t)(MALLOC_ALIGNMENT == CHUNK_HDR_SZ ? (p) : chunk2rawmem (p)) \
>     & MALLOC_ALIGN_MASK)

Ok.

> -              return chunk2mem (p);
> +              return chunk2rawmem (p);

Ok.

> -      return chunk2mem (p);
> +      return chunk2rawmem (p);

Ok.

> -  uintptr_t mem = (uintptr_t) chunk2mem(p);
> +  uintptr_t mem = (uintptr_t) chunk2rawmem(p);

Ok.

> -  tcache_entry *e = (tcache_entry *) chunk2mem (chunk);
> +  tcache_entry *e = (tcache_entry *) chunk2rawmem (chunk);

Ok.

> -	  void *newmem = chunk2mem (newp);
> +	  void *newmem = tag_at (chunk2rawmem (newp));

Ok.

> -	      void *p = chunk2mem (victim);
> +	      void *p = chunk2rawmem (victim);

Ok.

> -          void *p = chunk2mem (victim);
> +          void *p = chunk2rawmem (victim);

Ok.

> -              void *p = chunk2mem (victim);
> +              void *p = chunk2rawmem (victim);

Ok.

> -              void *p = chunk2mem (victim);
> +              void *p = chunk2rawmem (victim);

Ok.

> -              void *p = chunk2mem (victim);
> +              void *p = chunk2rawmem (victim);

Ok.

> -              void *p = chunk2mem (victim);
> +              void *p = chunk2rawmem (victim);

Ok.

> -          void *p = chunk2mem (victim);
> +          void *p = chunk2rawmem (victim);

Ok.

> -	tcache_entry *e = (tcache_entry *) chunk2mem (p);
> +	tcache_entry *e = (tcache_entry *) chunk2rawmem (p);

Ok.

> -    free_perturb (chunk2mem(p), size - CHUNK_HDR_SZ);
> +    free_perturb (chunk2rawmem(p), size - CHUNK_HDR_SZ);

Ok.

> -    free_perturb (chunk2mem(p), size - CHUNK_HDR_SZ);
> +    free_perturb (chunk2rawmem(p), size - CHUNK_HDR_SZ);

Ok.

> -          return chunk2mem (newp);
> +          return chunk2rawmem (newp);

Ok.

> -  return chunk2mem (p);
> +  return chunk2rawmem (p);

Ok.