This is the mail archive of the
libc-alpha@sourceware.org
mailing list for the glibc project.
Re: [PATCH] malloc: Use accessors for chunk metadata access
On 10/28/2016 05:07 PM, Carlos O'Donell wrote:
On 10/28/2016 10:52 AM, Florian Weimer wrote:
/* check for chunk from non-main arena */
-#define chunk_non_main_arena(p) ((p)->size & NON_MAIN_ARENA)
+#define chunk_main_arena(p) (((p)->mchunk_size & NON_MAIN_ARENA) == 0)
Comment is wrong now :}
Hmph.
I've taken the opportunity to adjust the chunk layout comment as well,
in preparation for documenting where the encryption happens.
Florian
malloc: Update comments about chunk layout
2016-10-28 Florian Weimer <fweimer@redhat.com>
* malloc/malloc.c: Update chunk layout comments.
(chunk_main_arena): Update comment.
diff --git a/malloc/malloc.c b/malloc/malloc.c
index a10477e..584edbf 100644
--- a/malloc/malloc.c
+++ b/malloc/malloc.c
@@ -1070,18 +1070,19 @@ struct malloc_chunk {
chunk-> +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
- | Size of previous chunk, if allocated | |
+ | Size of previous chunk, if unallocated (P clear) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
- | Size of chunk, in bytes |M|P|
+ | Size of chunk, in bytes |A|M|P|
mem-> +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| User data starts here... .
. .
. (malloc_usable_size() bytes) .
. |
nextchunk-> +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
- | Size of chunk |
+ | (size of chunk, but used for application data) |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ | Size of next chunk, in bytes |A|0|1|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
-
Where "chunk" is the front of the chunk for the purpose of most of
the malloc code, but "mem" is the pointer that is returned to the
@@ -1094,9 +1095,9 @@ nextchunk-> +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Free chunks are stored in circular doubly-linked lists, and look like this:
chunk-> +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
- | Size of previous chunk |
+ | Size of previous chunk, if unallocated (P clear) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
- `head:' | Size of chunk, in bytes |P|
+ `head:' | Size of chunk, in bytes |A|0|P|
mem-> +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Forward pointer to next chunk in list |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
@@ -1108,6 +1109,8 @@ nextchunk-> +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
nextchunk-> +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
`foot:' | Size of chunk, in bytes |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ | Size of next chunk, in bytes |A|0|0|
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The P (PREV_INUSE) bit, stored in the unused low-order bit of the
chunk size (which is always a multiple of two words), is an in-use
@@ -1120,12 +1123,21 @@ nextchunk-> +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
the size of the previous chunk, and might even get a memory
addressing fault when trying to do so.
+ The A (NON_MAIN_ARENA) bit is cleared for chunks on the initial,
+ main arena, described by the main_arena variable. When additional
+ threads are spawned, each thread receives its own arena (up to a
+ configurable limit, after which arenas are reused for multiple
+ threads), and the chunks in these arenas have the A bit set. To
+ find the arena for a chunk on such a non-main arena, heap_for_ptr
+ performs a bit mask operation and indirection through the ar_ptr
+ member of the per-heap header heap_info (see arena.c).
+
Note that the `foot' of the current chunk is actually represented
as the prev_size of the NEXT chunk. This makes it easier to
deal with alignments etc but can be very confusing when trying
to extend or adapt this code.
- The two exceptions to all this are
+ The three exceptions to all this are:
1. The special chunk `top' doesn't bother using the
trailing size field since there is no next contiguous chunk
@@ -1135,8 +1147,16 @@ nextchunk-> +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
2. Chunks allocated via mmap, which have the second-lowest-order
bit M (IS_MMAPPED) set in their size fields. Because they are
- allocated one-by-one, each must contain its own trailing size field.
+ allocated one-by-one, each must contain its own trailing size
+ field. If the M bit is set, the other bits are ignored
+ (because mmapped chunks are neither in an arena, nor adjacent
+ to a freed chunk). The M bit is also used for chunks which
+ originally came from a dumped heap via malloc_set_state in
+ hooks.c.
+ 3. Chunks in fastbins are treated as allocated chunks from the
+ point of view of the chunk allocator. They are consolidated
+ with their neighbors only in bulk, in malloc_consolidate.
*/
/*
@@ -1215,7 +1235,7 @@ nextchunk-> +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
the chunk to the user, if necessary. */
#define NON_MAIN_ARENA 0x4
-/* check for chunk from non-main arena */
+/* Check for chunk from main arena. */
#define chunk_main_arena(p) (((p)->mchunk_size & NON_MAIN_ARENA) == 0)
/* Mark a chunk as not being on the main arena. */